Jean-Luc Miquel, Jean-Yves Boutin and Dominique Gontier- Extreme measurement in laser-generated...

8/3/2019 Jean-Luc Miquel, Jean-Yves Boutin and Dominique Gontier- Extreme measurement in laser-generated plasmas

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Extreme measurement

in laser-generated plasmasExtreme temperature and pressure conditions prevailin inertial-confinement thermonuclear fusion installations, such asthe Laser Integration Line (LIL), and more emphatically yetin the Megajoule Laser (LMJ), providing major tools in CEAs DefenseProgram. These require the development of measuring devices providingparticularly high performance, in terms of precision, measurementdynamics, and spectral range, and having the ability to operate

in a particularly harsh neutron and radiation environment.

Of the four states of matter, the plasma state isundoubtedly the most plentiful. Whereas solids,liquids and gases are defined in terms of restrictivetemperature, pressure or density conditions, the condi-tions under which a plasma may exist are far moreextensive, making plasmas ubiquitous in our daily life(fluorescent lighting, television sets, lightning), andright out to the confines of the Universe (stars, nebu-lae). In that range of conditions of existence, hotplasmas generated through lasermatter interaction,for the purposes ofinertial-confinement fusion (ICF)(see Box C, The principle of thermonuclear fusion),are those exhibiting the most extreme characteristics,

akin to conditions prevailing in star cores (see Figure 1).

The purpose of hot plasma experimentation

Their ability to bring matter to extreme temperatureand pressure conditions (10 million degrees,

100 million bars) means high-energy laser installa-tions, such as the Laser Integration Line (LIL), or theMegajoule Laser (LMJ: Laser mgajoule), two instal-lations accommodated at CEAs CESTA center, in theGironde dpartement(southwestern France), providetools allowing a widely diverse range of experimentsto be embarked upon. These relate, first of all, to theSimulation Program of the Military ApplicationsDivision (DAM) at CEA (radiation hydrodynamics,behavior of matter at high temperatures and/or highpressures), taking in the entire ICF-related approach,but equally to more fundamental investigations (spec-tral opacities, equations of state, astrophysics).

ICF consists in effecting the thermonuclear fusionof a deuteriumtritium (DT) mix, held in a cap-sule, by bringing it to extreme temperature and pres-sure conditions (see Box, on the following page, andFigure). Achieving such conditions presents a chal-lenge that may only be met through mastery of the

Checking diagnosticsinside the LaserIntegration Line (LIL)experiment chamber,the prototype installationfor the Megajoule Laser(LMJ), consisting in oneelementary laser line,comprising four beamsP.

Stroppa/CEA

CLEFS CEA - No. 54 - AUTUMN 2006

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Inertial-confinement fusion

For the purposes ofinertial-confinementfusion (ICF), the deuteriumtritium (DT)mix to be brought to the conditions requi-red (see Box C, The principle of thermo-nuclear fusion) is held in solid form, at 250 C, in a spherical capsule of milli-metric size. This is brought to conditionsof temperature and density of some10 million degrees, and about 100 g/cm3,by vaporizing suddenly (in a few nanose-conds) the outer shell of the capsule: dueto conservation of momentum, the innercapsule implodes, compressing the DTand heating it sufficiently to cause itsinflammation, spreading from a hotspot, and set off fusion reactions ( igni-tion).

Such heating requires considerablepower, that only lasers may supply. Thisis achieved by illuminating the capsule byintense laser beams (direct drive), or by

placing the capsule inside a hohlraum,used to convert the laser beams into ther-mal radiation (indirect drive). In the lat-ter case, one constructs, in effect, an X-radiation oven: it is the radiationgenerated as the laser energy is deposi-

ted onto the hohlraum walls that heatsthe capsule, the inner part of which implo-des with a velocity of about 100 km/s, set-ting up in the DT the conditions for igni-tion (see Figure).

Figure.ICF target, comprising a gold hohlraum,filled with a gas mixand holding a germanium-dopedplastic capsule, positioned at its center,holding the fusible mix in cryogenicform. The laser beams, groupedinto a number of cones, on either sideof the hohlraum, enter it throughtwo axial apertures, covered withthin polyimide membranes, to containthe gas. The various physical processes

involved may be subsumed underlaserplasma interaction, hohlraumradiation hydrodynamics, and capsulehydrodynamics and combustion.

multiplicity of physical processes involved in brin-

ging the thermonuclear fuel to the required condi-tion: coupling of the laser wave with the ion andelec-tron waves in the plasma inside the hohlraum(interaction physics); conversion of the laser illumi-nation into X-radiation, hydrodynamics of the plasmaheld in the hohlraum, control of capsule illumina-

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Measuring the very hot and very dense

tion symmetry throughout the laser pulse (hohlraum

physics); capsule ablation by the radiation flux, controlof hydrodynamic instabilities, acceleration, and com-bustion (capsule physics). All of these processes arebeing addressed by research themes, calling for dedi-cated experiments, to advance simulation, and unders-tanding of the phenomena.

Figure 1.Existence domains

for the various plasmas,as a function of density

and temperature.The plasmas found

in inertial-confinementfusion exhibit extreme

conditions,approximating those

encounteredin star cores.

10

102

104

106

108

104 108 1012

density of charged particles (cm-3)

temperature(K)

1016 1020 1024 1028

liquids

solids

inertial fusion

core of sun

novae

magnetic fusion

auroraborealis

gaseousnebula

plasma screen

lightning

solarcorona

gases

10 mm

6 mm

polyimide

window

1m

CHGe capsule

holding

cryogenic DT

gas H2 + He gold

hohlraum

50m

capsule

physics

hohlraum

physicsinteraction

physics


3/13CLEFS CEA - No. 54 - AUTUMN 2006 49

Characteristics of ICF plasmasIn the target designed for ICF, on LMJ (see Figure,in Box), the laser beams, grouped into a number ofirradiation cones, on either side of the cylindricalhohlraum, enter through the two apertures, impac-ting the walls of the gold hohlraum, after passing

through the gas (the pressure of which slows downthe expansion of plasma from the hohlraum wall)and the polyimide(1)-covered aperture (retaining thegas).The main characteristics of ICF plasmas (see Table1) determine the requirements in terms of measu-rement dynamics, and resolution, for the investiga-tion of such plasmas.Dimensions range over four orders of magnitude,from the hohlraum (on the centimeter scale) to thehot spot, a few tens of micrometers (m) across,entailing micrometric spatial resolutions for the ima-ging devices. As for time scales, these range over three

orders of magnitude, from the duration of the laserpulse (~ 20 nanoseconds) to the DT burn time (afew tens of picoseconds [ps]). Power lasers, at thesame time, allow pressure levels of several hundredmegabars to be achieved, or even up to one gigabar(with LMJ).

Measurements in plasma physics

Controlling the various physical processes involvedcalls for mastery of the associated measurements,since validation of a simulation is dependent on mea-surement capabilities: measuring devices define theobservables, these being the meeting point of expe-

riment and simulation, and the performance of thesedevices determine measurementprecision, on whichdepends the precision of computation code valida-tion.

The measurement approachA good many quantities to be measured, of thosemost relevant to the description of the state of mat-ter, may not be directly accessed in plasmas. Such isthe case for energy, temperature, pressure, or den-sity At the same time, as a result of the unfeasibi-lity of effecting measurements inside, or in the vici-nity of plasma, owing to the extreme conditions

prevailing in it, measurement devices are set up at aremove, as instanced with all due proportion bymeasurements in astrophysics.Measurements that are accessible are those concer-ning size, by way of imaging systems, the evolutionover time of a process, by means of electrical or opti-cal analyzers, velocity, measured byDoppler effect,

or through a combination of the aforementionedmeans, finally the energyandnumber of particles relea-sed (including photons), these being measured bymeans of calibrated detectors, allowing the spectraldensity , or spectrum, to be retrieved.From these basic measurements, the most relevantquantities may be derived, in some cases with theaid of theoretical models: energy or temperaturefrom the spectrum, pressure from velocity readingsand the RankineHugoniot relations,(2) density

through the broadening of X-radiation emissionlines (Stark effect) or the profile of the neutronspectrumMost measurements rely on the manifold emis-sions from the plasma:visible light and ultravio-let (UV) (for the laser energy balance, or emissionfrom a shock front), X- and gamma radiation(from a few tens of electronvolts [eV] to severalMeV), particles (neutrons, electrons, ions, alphaparticles, protons). Examples of measurements inthe spatial, temporal and spectral domains are givenbelow.

LMJ experiment chamber being assembled at the CESTA site. On the outside of the spheremay be seen the square apertures, through which the 240 laser beams will enter (in groupsof four), and those, circular as a rule, destined to the many different measuring devices.

P.

Stroppa/CEA

Table 1.Characteristic quantities for ICF plasmas.

size stored energy duration temperature other parameters

hohlraum 1 cm 1 MJ (laser) 15 ns 3 MK(laser pulse)

capsule 1 mm 100 kJ 3 ns 3 MK implosion velocity(X-radiation) (implosion phase) ~ 300 km/s

DT prior to fusion 100 m 10 kJ 3 MK max. density ~ 300hot spot 30 m 1 kJ 30 MK max. density ~ 100

fusion 10 MJ yield 30 ps 1,000 MK(combustion time)

(1) Polyimide: a high-performance engineering polymer,formed from imide monomers, this being a functional groupmade up of two carbonyl groups and one primary amine.

(2) RankineHugoniot relations: relations (three in all)that relate values for momentum and energy, before and afterpassage of a shock wave set up in a compressed fluidby a piston moving at constant velocity, while not taking intoaccount the thermodynamic state of the medium considered.


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Measuring dimensions in the X regionRequirements for imaging systems in the X regionare quite plentiful, whether it be to investigate theplasmas own emission or to observe target evolu-tion by radiography. There are various types of ima-ging devices, functioning by way of projection, reflec-tion, or diffraction. The simple pinhole works byprojection: its lack of sensitivity may be remedied bymeans of coded apertures (assembling a number of

pinholes according to a variety of geometries), thesehowever requiring deconvolution algorithms, inorder to yield the object. Reflection is used in X-rayoptics involving grazing-incidence mirrors, whichmay be coupled with Bragg reflection (spectral reso-lution by means of multilayer interference mirrors,or curved crystals). Diffraction is employed by Fresnelzones (circular, variable-pitch diffraction slits: theanalogues, in the X region, of Fresnel lenses, as usedfor marine beacons, in the visible region), which maybe coupled to multilayer mirrors. The system usingreflection shown in Figure 2, for instance, achievesmicrometric resolutions with an operating flexibi-

lity that accommodates many different geometries.The basic configuration is that of the optical systemdevised by Kirkpatrick and Baez, comprising twoorthogonal spherical mirrors, operating in grazingincidence. Each mirror carries out focusing in onedirection, coupling of the two yielding a two-dimen-sional image. This device allows a 5-m resolutionto be achieved, over a restricted 100-m field of view,and 10-m resolution over a 1-mm field.Building on this basic structure, a multi-imager sys-tem was designed, using six spherical mirrors, two

of which are orthogonal to the others. By couplingthe mirrors two by two, eight images of the sameobjects are generated, and may be recorded at diffe-rent times, to analyze the objects evolution.By changing the nature of the mirror coating, andaltering the angle of incidence, different energy ran-ges may be worked with: two separate devices werethus developed (see Table 2).

Temporal measurementsThe extremely fast processes involved in hot plasmaphysics call, for their investigation, for temporal ana-lysis at a resolution higher than 100 ps. There is nodevice available, having the ability to record an imagedirectly at such a high time resolution. Solving this pro-blem entails the use of systems converting photonicsignals into electronic signals, which are more readilyprocessed to achieve the required resolutions.

Framing camerasFraming cameras (FCs; see Figure 3) have the capa-bility to take pictures every 100 ps, with an exposure

time lower than 70 ps. Several images of the objectto be analyzed are formed simultaneously, on theinput side of a microchannel plate, covered withphotocathodes. The photons generate electrons byphotoelectric effect, allowing low-intensity electro-nic images to be obtained. Inside the plate, each micro-channel, about 10m in diameter this defining thesystems spatial resolution plays the role of an adjus-table-release electron amplifier. The images obtainedat the back of the plate are more intense by far (by afactor up to 10,000), and electron fluxes exiting the

Figure 2.Multi-imager device for X-radiation. Use of six grazing-incidence spherical mirrors allows eight images to be produced,

by coupling the mirrors two by two (Kirkpatrick and Baez layout). Shown in the figure, the formation of the four images yieldedby reflections on the blue vertical mirror, and the four horizontal mirrors. The other four images are yielded by reflectionson the other vertical mirror. The eight images are captured at different points in time by means of an framing camera.

Table 2.Characteristics of the multi-imager systems developed for the Laser Integration Line (LIL).

multi-imager angle of incidence mirror spectral range magnificationcoating

soft X-rays 1.8 silicon 100 eV - 1keV 4hard X-rays 0.8 platinum 1 - 5 keV 8

silicon

spherical mirrors

image

of the plasma

plasmaspherical

mirror

framing

camera

CEA



channels impact a phosphor screen, thus formingagain a visible image, recordable by means of conven-tional image-capture systems, photographic films orCCDs.The temporal aspect is obtained by triggering themicrochannels, by means of a pulsed voltage propa-gating along gold strips, deposited on the front sideof the plate, playing the role of photocathodes. Thepropagation speed and full width at mid-height ofthe electric pulse, used to bias the plate, determineexposure time for each image. By using several goldstrips, triggered with varying delays, images taken atsuccessive points in time appear on the FCs backscreen, reproducing the images formed on the front

side.

Streak camerasThe streak camera (SC; see Figure 4) is an ultrarapidcinema camera, which has been in widespread use, fora long time, but for which developments in electronicshave enabled considerably enhanced performance. Aphotocathode, forming a slit, converts radiation intoelectrons by photoelectric effect. These are then acce-lerated, the electron beam being focused by electronicoptics positioned mid-tube, going on to yield in turn

Figure 3.Schematic of the functioningof a pulsed microchannelplate, in an framing camera(FC). X photons generatephotoelectrons at the inputend of the microchannels,

each of which acts asan electronic amplifierin the brief duration intervalof the high-voltage pulseapplied to the gold stripdeposited on the front sideof the plate.

Figure 4.Streak camera (SC).The X radiation falling on the

photocathode is convertedinto an electron beam which,after shaping by an electronicoptics, is deflected overtime by deflector plates,before impacting the outputscreen, which yields in turna visible photonic signal.

CEA

Framing camera, with its fourelectrode strips at the front,and high-voltage connectorsat the back.

CEA

Streak camera.

microchannel

plate

phosphor screen

visible

photons

microchannel input and ouput ends

X photonHV

gold deposit serving

as photocathode

e-

accelerator electrode

screen

deflector

plates

image of

photocathode

shutter electrode

quadrupolar

spatial focusing

lens LQtemporal

focusing

lenses F2

electron

beam

photocathode

incident photons

focusing

lenses F1


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the image of the photocathode on a phosphor screenat the back. Prior to impacting the screen, the beamtravels through a space between two deflector plates.Applying a pulsed voltage ramp to these plates allowsthe path of these electrons to be deflected as a func-tion of time, the lattermost electrons (yielded in turn

by the lattermost photons) undergoing the largestdeflection. A two-dimensional space-time image is thusformed, presenting the evolution over time (y-axis) ofthe intensity of radiation reaching the photocathodeslit (x-axis), with a resolution of about 10 ps. Finally,this image is recorded on film by way of a brightnessintensifier, or by means of a very-low-readout-noiseCCD array detector.

Towards picosecond resolutionsCurrent limitations, for such devices, are related topower electronics, and the difficulty encountered withthe propagation of steep-slope, high voltage pulses.

While some devices may achieve resolutions of theorder of the picosecond, or even less, this invariablyentails a loss in measurement dynamics: the analysisceases to be quantitative.While ICF investigations do not require such resolu-tions, research workers may find they need to use, forinvestigations of a more fundamental character, shortlaser pulses (< 1 ps), generating radiation sources thatare themselves sub-ps. Time resolution is then deter-mined by the duration of such sources, rather than bythe analytical device.

Measuring temperatureA large proportion of the experiments make use of an

indirect-drive configuration, whereby the laser inter-acts inside a hohlraum with a wall made of a high-Zmaterial (e.g. gold): the X-radiation energy generatedby the laser is recycled within the hohlraum walls(absorptionemission), and becomes thermalized.The X-radiation inside the hohlraum approximatesthat from a black body, and may then be characteri-zed by its radiation temperature, which has to be mea-sured.Determining the temperature of a very small object isno easy task, the more so since the aim is not merelyto measure the maximum temperature achieved insidethe capsule, but rather to ascertain its evolution over

time (with a 100-picosecond resolution).

A number of methods are used, such as measuring thetime the radiation flux takes to go through a gold foil(from which temperature is derived, by way of a theo-retical model), or measuring the time taken for theshock wave generated by the radiation flux to passthrough a material for which the equation of state is

know, this allowing the pressure generated within thesample to be worked out, and hence radiation tempe-rature.The most reliable measurement however, and one notrequiring use of theoretical models, remains that obtai-ned by observation of the spectrum emitted by thehohlraum wall, through an aperture. Any body, whenheated, undergoes a rise in luminosity, even as itsemis-sion spectrum shifts to shorter wavelengths. At a tem-perature of a few million degrees, the emissivity maxi-mum lies in the low-energy X region (100200 eV).Absolute measurement of this emissivity, by means ofa widebandspectrometer,yields hohlraum tempera-

ture.For that purpose, some 20 measurement channels,comprising filters, grazing-incidence mirrors andcoaxial-geometry vacuum diodes (see Figure 5) are setup at a distance of several meters. By changing thenature of the filter, and of the mirror in each channel,a different portion of the emission spectrum is selec-ted, in the 50 eV20 keV bracket, with spectral reso-lution standing at 1 < E/E < 6. The signal capturedby each diode is recorded on a high-speed oscillo-scope,(3) to achieve the required time resolution (100 ps).A specific software program readjusts the signals rela-tive to one another, reconstructs the radiation spec-trum from the hohlraum (typically lying in a range

from 1012W/sr/cm2 to a few 1015W/sr/cm2), and yieldsits temperature at every point in time, in the 50300 eVbracket, with an uncertainty of less than 5%.

One step further into the extreme:the LMJ experimental environment

The devices presented in the above are in use on variouslaser installations, in experiments serving as prepara-tions for those that will be carried out on LMJ, to

Figure 5.Principle of wideband spectrometer, for the measurement of hohlraum radiation temperature. The spectrometer comprisessome 20 measurement channels, each comprising in turn various components, serving to effect the selection, in spatial andspectral terms, of a segment of radiated spectrum. The spectral range of a given measurement channel depends on the natureof the filter, of the mirror, and of the photocathode of the coaxial diode. Through judicious modification of these components,a variety of spectral ranges may be defined, sampling the spectrum in the 50 eV20keV bracket. This device is fittedto the Omega installation, at the University of Rochester (New York State).

(3) Oscilloscope: an electronic apparatus allowing real-timevisualization of the instantaneous value, and evolution over

time, of signals varying as a function of time.

plasma

filter

knife

mirror

collimator

coaxial diode

tooscilloscope



achieve ignition. While these do involve extreme cha-racteristics, in terms of precision, measurement dyna-mics or spectral range, they do not, as yet, take in theconstraints involved in the LMJ experimental envi-ronment.Indeed, as and when an ICF target functions, the intense

neutron and gamma-radiation flux this will yield willgenerate severe perturbations, which may go so far asto result in destruction of part of the measuring devi-ces. Developing measuring equipment for such an envi-ronment sets a challenge that is all the more ambitioussince bringing matter to the required conditions isnever an ideal process, and the causes of ICF target

malfunction must be meticulously identified, by effec-ting the measurements appropriate for that purpose.Design of complete systems, from detection to signalrecording, more robust with regard to perturbation-inducing radiation, is one direction for research thathas already proved fruitful, but which yet provides a

vast area to be explored over the coming years, in anti-cipation of the first LMJ firings.

> Jean-Luc Miquel, Jean-Yves Boutinand Dominique Gontier

Military Applications DivisionCEA DAM le-de-France (DIF/DCRE)

The principle of thermonuclear fusionC

The fusion reaction that is most readily effectedis that between deuterium (D) and tritium (T),

two isotopes of hydrogen (see Figure). This reaction

requires that the DT mix be brought to a tempera-ture of 100 million degrees, and that it remain confi-ned for a time meeting the Lawson criterion: the pro-duct of density by confinement time must be greaterthan 1020 s/m3. To set up the conditions for fusion ofa light-element plasma, two confinement methods

have been developed: magnetic confinement, in toka-maks, corresponding to a stationary regime, wheredensity of the order of 1020 m 3 is maintained for seve-

ral seconds by means of a magnetic field; and iner-tial confinement by laser beams, or particle beams,

an explosive regime where density reaches 1031 m 3for some 10 11 s.

For further information: see Clefs CEA No. 49, pp. 4576.

deuterium neutron

tritium helium nucleus

Wideband spectrometer, to measure hohlraum radiation temperature, fitted to the Omega installation at the Universityof Rochester (New York State), in the United States.

RochesterUniversity


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Spectroscopy and spectrometryD

Spectrometric methods are subdivi-ded, as a whole, into two main cate-gories, radiation spectrometry itself

comprising absorption spectrometry,emission spectrometry, Raman scatte-ring spectrometry, and nuclear magne-

tic resonance spectrometry and massspectrometry.

Radiation spectroscopy and spectrome-try(1) cover a ensemble of analyticalmethods allowing the composition andstructure of matter to be ascertained,

based on investigation of the spectra yiel-ded by the interaction between atoms and

molecules, and various types of electro-magnetic radiation, emitted, absorbed,

or scattered by the former.Depending on their energy,photons inter-act selectively with the various electronshells, or levels, making up the electro-

nic structure of the atom, or molecule.The electrons involved are core electrons(close to the atoms nucleus), for X-rays,(2)

peripheral electrons (furthest from thenucleus, and involved in chemical bonds)for light absorbed, or emitted, in the nearultravioletand visible region. In the infra-

red radiation region, it is the leap fromone molecular vibration level to another

that is involved, the switch from one mole-

cular rotation level to another for micro-wave radiation, and atomic nucleusspinfor NMR.

Absorption spectrometryThose spectroscopy methods that rely onabsorption make use of the BeerLambertlaw, setting out the proportional relation

between the intensity of light absorbed,and the amount of absorbing matter:

A = log (I0/I) = l C,where A stands for the absorbance of themedium traversed, I0 for incident light

intensity, I for transmitted light intensity, is the characteristic molar extinctioncoefficient, for a given wavelength, for the

substance investigated expressed in

L mol 1cm 1 while l stands for the thick-ness passed through, expressed in cen-timeters, and C is the concentration, in

moles per liter.By measuring the mediums absorbance,for a given wavelength, the concentration

of a substance, in a sample, may thus bedetermined.In an absorption spectrum, as recorded

by means of a spectrometer, absorptionpeaks correspond to the wavelengths themedium is able to absorb. Just as the

spectrum from the Suns light is obtai-ned by making it pass through a prism,which breaks it up, spectrometers ana-

lyze the spectral distribution of the whole

range of electromagnetic radiations,separating them out according to wave-

length, by means of a reflection diffrac-tion grating. Spectra exhibit peaks, eachone corresponding to a specific wave-

length.Depending of the type of sample to be ana-lyzed, and the performance level being

sought, in the laboratory, absorption spec-trometry is used either on molecules in

liquid or gaseous phase, or on atomicvapor, obtained through thermal break-down of liquid or solid samples.

Molecular absorption spectroscopy, in the

UVvisible region, affords simplicity ofuse, however it is only applicable to sam-

ples of moderate complexity, since, owingto the width of molecular absorption bands,absorption spectra, as a rule, do not allow

specific discrimination of every consti-tuent, in a complex mixture.In infrared (IR) spectrometry, absorption

is the outcome of molecular vibration androtation processes. Infrared absorptionspectra thus allow the nature of chemi-

cal bonds to be determined, that make upa molecule, by ascertaining the bonds

elasticity constant (influencing vibrationfrequency, as for a spring), thus confir-ming structural hypotheses.

As the number of atoms increases, thespectrum rapidly exhibits growing com-plexity, and interpretation becomes highly

problematical, especially for organic com-pounds.

Atomic absorption spectrometry, in this

respect, brings higher performance, sinceabsorption by atoms yields very narrow

absorption lines. Very precise measure-

ments are thus feasible, even when thesample consists in a complex assembly

of chemical elements. Atomic absorp-tion is a reference technique for the ana-

lysis of trace elements in a wide varietyof samples, in particular for biologicalsamples.

Emission spectrometryAtoms or molecules brought to an exci-

ted state may deexcite by emitting radia-tion, known as emission radiation. Whenthe excitation is caused by selective

absorption, by the atoms or molecules tobe analyzed, of electromagnetic radiation,this represents a fluorescence emission

(or a phosphorescence emission, depen-ding on the electron excitation state invol-ved).

As with absorption, fluorescence may be

applied, in the UVvisible radiation region,to molecules, or atoms. X-ray fluores-

cence spectrometry, on the other hand,refers to the X radiation emitted by

atoms excited by absorption of X-radia-tion. Fluorescence techniques are morecomplex to implement than is the case

for absorption techniques, since theyentail that the particle subjected to ana-lysis be selectively excited by a mono-

chromatic radiation. On the other hand,since the radiation emitted is likewisespecific to the particle, fluorescence

spectrometry involves a double selecti-

vity, resulting in very low backgroundnoise, thus making it peculiarly well sui-

ted for the measurement of very lowconcentrations.Emission of radiation may also occur

when atoms are thermally excited, in anenvironment brought to high tempera-tures. Emission spectroscopy is based

on the fact that atoms, or molecules exci-ted to high energy levels deexcite to lowerlevels, by emitting radiation (emission,

or luminescence). This differs from fluo-rescence spectrometry in that excitation

is not applied selectively, rather it invol-ves indiscriminately all of the particlesmaking up the medium. Emission linesthus correspond to radiation directly

emitted by a body brought to a high tem-perature, and the emission spectrumallows the detection, and quantification,of all atoms present in the emissionsource.

Raman spectrometryInteractions between matter and elec-

tromagnetic radiation also give rise toscattering processes, such as elastic scat-

tering, and inelastic scattering. Scatteringmay occur when the interface between

(1) The term spectrometry, initially used onlyto refer to recording and measurementtechniques, has tended to become synonymouswith spectroscopy, as the eye was supplanted,for observation purposes, by other receptors andinstruments, while the visible region now onlyformed one special region, in analytical terms.

(2) It should be noted, at the same time, that X-ray crystallography is not deemed to be aspectroscopy method, in the strict sense of theterm.


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D

two media is encountered, or as a mediumis passed through. This process, in mostcases, is an elastic one, in other words

it takes place with no change in frequencyfor the radiation forming the beam invol-ved. Elastic scattering of solar radiation

by the atmosphere is, for instance, respon-sible for the blueness of the sky, obser-ved when the eye is not directed towards

the Sun (Tyndall effect). Indeed, scatteredintensity is all the greater, the shorter theradiation wavelength, which, in the case

of the solar spectrum, corresponds to thecolor blue.As regards spectrometry, the main use of

scattering concerns Raman spectrometry.

This involves the inelastic scattering ofincident radiation by the molecules makingup the sample. The difference betweenscattered radiation frequency, and inci-

dent radiation frequency allows the iden-tification of the chemical bonds involved.Raman spectrometry is a technique that

is widely used for structural analysis, tocomplement infrared spectrometry, andmass spectrometry.

Nuclear magnetic resonancespectrometryThe principle of nuclear magnetic reso-

nance (NMR) is based on the fact that anatom has a magnetic moment, just like a

spinning charge acting as a tiny magnet,governed by quantum mechanics, aligningin a magnetic field as the needle of a com-

pass in the Earths magnetic field. Theprinciple of NMR consists in inducing, anddetecting, the transition, for the nuclear

magnetic moment, from the lowest energylevel to the highest energy level, throughabsorption of electromagnetic radiation

of a wavelength lying in the radiofrequencyregion: when the energy of the photon

precisely matches the energy differencebetween the two levels, absorption occurs.Nuclei having numbers of protons, and

neutrons that are both even exhibit zerospin. Carbon 12 and oxygen 16 atoms,which are very widespread in nature, thus

have zero spin. On the other hand, hydro-gen only has one single proton, and itsnuclear magnetic moment equals 1/2: it

may thus take on two possible energy sta-tes, corresponding to the two orientationstates of its spin, relative to the magne-

tic field. Measuring the resonance fre-quency in the electromagnetic field allo-

wing transition from one of these energystates to the other enables the molecu-

les to be analyzed. This frequency is fixed,

however the various nuclei in a molecule

do not all resonate at the same frequency,since their magnetic environment is modi-

fied by their chemical (electronic) envi-ronment.Many NMR spectra exhibit more peaks

than there are protons in the nucleus,owing to the interactions between protonsand their neighbors. Two nuclei may inter-

act within the molecule, though they areseparated by several chemical bonds: thisis known as interatomic coupling. This

interaction endows the NMR spectrumwith a fine structure.

Mass spectrometryMass spectrometry is a highly sensitivedetection and identification technique, allo-wing determination of molecular structu-res, and thus of a samples composition.

This is not, strictly speaking, a form of spec-trometry, since it is not concerned withdiscrete energy levels. What is its princi-

ple? A compound introduced into the deviceis vaporized, and subsequently ionized byan electron bombardment source (at 70 eV).

The ion thus obtained, termed a molecu-lar ion, allows the compounds molar mass

to be determined. Breaking chemical bondswithin the compound may yield characte-

ristic fragment ions. These are then sor-ted according to their mass/charge ratioin an analyzer, through application of a

magnetic and/or electric field, then col-lected by a detector, which amplifies thesignal associated to the ions, which arrive

with varying delays. A data processing sys-tem converts the information from thedetector into a mass spectrum, readout of

which, by comparing it with reference spec-tra, allows the identity details of the mole-cule to be drawn up. Through use of a high-

resolution mass spectrometer, the exactmass of the compound may be determi-ned, together with isotope percentages for

each constituent atom.

Choice of ionization method is directlyrelated to the nature of the sample, andthe type of analysis. If mass spectrome-try has gradually adapted to meet the gro-

wing demands from chemists, and biolo-gists (separation of increasingly complex,highly polarized mixtures, determination

of ever higher molecular masses on sam-ples of ever more constricted sizes), thisis essentially due to advances in ioniza-

tion techniques, these including secondaryion mass spectrometry (SIMS), chemicalionization, thermospray ionization, and

fast atom bombardment (FAB) sources,

further comprising, from the 1980s,matrix-assisted laser desorption ioniza-

tion (MALDI), and electrospray ionization(ESI), together with advances in detectiontechniques, from time-of-flight (TOF) mea-

surement to ion traps (ITs), through qua-drupoles (MS or Q).In proteomics, for instance, only MALDI,

ESI and SELDI (surface-enhanced laserdesorption ionization) are employed.Ion mobility spectrometry (IMS) is a che-mical analysis technique in the gaseousphase, which consists in subjecting a gas

to an electric field. Ionized moleculesacquire a velocity that is characteristic forthe ion, since this depends on mass, and

charge. Arrival of the ions on one of theplates generating the field results in a cur-rent, which is recorded. The length of time

after which a peak occurs can be relatedto the nature of the ion causing it.Scientists often make use of a coupling of

devices each belonging to one of the twomain families of analytical techniques (seeBox E, What is chromatography?), e.g. of a

chromatograph with a mass spectrome-ter (or an electron-capture detector [ECD]),

particularly for the investigation of tracecomplex mixtures.

C.

Dupont/CEA

Spectromtre de masse d'ions secondairesutilis au CEA pour raliser des mesuresisotopiques rapides sur un chantillon

par exemple prlev sur une installationaux activits nuclaires suspectes.

(contd)


10/13

Fundamental interactions and elementaryparticles

B

The standard model of particle phy-

sics is the reference theoretical fra-mework describing all known elemen-

tary particles (see Table 1) and thefundamental interactions these parti-cles are involved in (see Table 2). The

basic constituents of matter, known asfermions, are partitioned into two maincategories, as determined by their part-

cipation in the fundamental interactions,or forces (the gravitational, electro-

magnetic, weak, and strong forces),which are mediated by vector bosons,the fundamental particles which carry

out the transmission of the forces ofnature(1) (see Table 2). Whether a par-ticle belongs to the category of fermions,

or to that of bosons depends on its spin(i.e. its intrinsic angular moment, orinternal rotation moment), depending

on whether it exhibits half-integer spin(fermions) or integer spin (bosons).At the same time, to every constituent

of matter is associated its antiparticle,a particle having the same mass, butthe opposite charge. The positron is thus

the positively charged antiparticle of theelectron, which exhibits a negative

charge.

Leptons and quarksFermions include, on the one hand, lep-

tons, which may travel freely and do notparticipate in the strong interaction,

which ensures the cohesion of atomicnuclei (it is consequently termed anuclearinteraction), and, on the other

hand, quarks, which participate in allinteractions but are not individually obs-erved, enmeshed and confined as they

are within hadrons, the particles sus-

ceptible to strong interaction, of whichthey are the constituents.(2)

In the lepton category, charged leptonsparticipate in the electromagnetic inter-

action (which ensures the cohesion of

atoms and molecules, and in the weakinteraction (which underlies decay pro-cesses, in particular radioactivity).

Neutral leptons, or neutrinos, for theirpart, participate in the weak interactiononly. Exhibiting very low mass, there is

one type of neutrino for each type ofcharged lepton.Independently from their involvement

in interactions, the basic constituentsof matter are classified into threegene-

rations, or families, of particles. From

one family to the next, quarks and lep-tons having the same charges only dif-

fer by their mass, each family being hea-vier than the preceding one.The electron, up quark (symbolized u)

and down quark (symbol d), whichbelong to the first generation, are the

lightest massive particles, and are sta-ble. These are the sole constituents ofnormal matter, so-called baryonic mat-

ter (a baryon is an assembly of quarks),which is made up of protons and neu-trons, this however only accounting for

4% of the Universes energy content!Particles in the other two families areheavier, and are unstable, except for

neutrinos, which on the other hand exhi-bit non-zero mass, but are stable.These latter particles may only be obs-

erved or detected in the final statesresulting from collisions effected inaccelerators, or in cosmic radiation,

and rapidly decay into stable first-gene-ration particles. This is why all the sta-ble matter in the Universe is made up

from constituents from the first family.According to quantum mechanics, for

an interaction to take place betweenparticles of normal matter, at least oneelementary particle, a boson, must beemitted, absorbed, or exchanged. The

photon is the intermediate (or vector)boson for the electromagnetic interac-

tion, the W+, W- and Z are the interme-diate bosons for the weak interaction,andgluons are those of the strong inter-

action, acting at quark level.As to the graviton, the putative vectorfor the gravitational interaction, it has

not so far been empirically discovered.

The gravitational force, which acts onall fermions in proportion to their mass,

is not included in the standard model,due in particular to the fact that quan-tum field theory, when applied to gra-

vitation, does not yield a viable scheme,as it stands. While gravitational effectsare negligible in particle physics mea-

surements, they become predominanton astronomical scales.

Interaction rangesQuarks and charged leptons exchangephotons. The photon having no electric

charge, these particles conserve theirelectric charge after the exchange. Since

the photons mass is zero, the electro-

magnetic interaction has an infiniterange. Having no electric charge, neu-

trinos are the only elementary fermionsthat are not subject to electromagneticinteraction.

In the electroweak theory (a unificationof the weak and electromagnetic inter-actions), the weak interaction has two

aspects: charged-current weak inter-action, for which the interaction vectors

are the W+ and W; and neutral-currentweak interaction, for which the media-tor is Z0. These two forms of weak inter-

action are active between all elemen-tary fermions (quarks, charged leptonsand neutrinos). The mass of these

bosons being very large (about 80 GeV/c2

for W, 91 GeV/c2 for Z0), the range of theweak interaction is tiny of the order of

10 18 m. Since W bosons have a non-zero electric charge, fermions exchan-ging such bosons undergo a change in

electric charge, as of nature (flavor).Conversely, since the Z0 boson has noelectric charge, fermions exchanging

one undergo no change in nature. Ineffect, neutral-current weak interaction

is somewhat akin to exchanging aphoton. As a general rule, if two fer-mions are able to exchange a photon,they can also exchange a Z0. On the other

hand, a neutrino has the ability toexchange a Z0 with another particle,

though not a photon.Only those quarks that have a colorcharge(1)exchange gluons, these in turn

being bearers of a color charge. Thus,

(1) The participation of basic constituents infundamental interactions is governed by theirinteraction charges (electric charge, colorcharge), or conserved quantum numbers.Color charge, a quantum number thatdetermines participation in stronginteractions, may take one of three values:red, green, or blue (these colors bearingno relation to visible colors). Every quarkbears one of these color charges, everyantiquark one of the three anticolor charges.Gluons are endowed with doublecoloranticolor charges (eight combinationsbeing possible).

(2) To take e.g. nucleons: the proton holdstwo up quarks and one down quark, theneutron two down quarks and one up quark.A meson is made up of just two quarks (onequark and one antiquark).


11/13

when a gluon exchange takes place bet-

ween quarks, the latter exchange theirrespective colors. Gluons have zero

mass, however, since they do bear acolor charge, they are able to interact

together, which greatly complicates

theoretical treatment of this interaction.The range of the strong interaction is

consequently very restricted of theorder of 10 15 m.

The quest for unification

The theoretical framework for the stan-dard model is quantum field theory,

which allows a quantitative descriptionto be made of the fundamental interac-

Tableau 1.Table showing the twelve elementary constituents for which the standard model describes the interactions involved. The three charged leptons(electron e-, muon, -, tau particle-) are subject to electromagnetic and weak interactions, neutrinos (e, , ) are only affected by weakinteraction, and the six quarks (up, charm, top or u, c, t bearing a charge of 2/3; and down, strange, bottom d, s, b bearing a charge of 1/3) are subject to all three interactions. Every elementary constituent has its antiparticle, having the same mass, and algebraic quantumnumbers (such as electric charge) of the opposite sign.

Fermions

Normalmatter is

made up ofparticles from

this group.

Most of theseparticles were

around justafter the Big

Bang.Presently onlyto be found incosmic rays,

and aroundaccelerators.

Vectorbosons

Fundamentalparticles

carrying outtransmission ofnatural forces.

responsible for electroweak symmetry breaking

electron (e)

responsible forelectricity andchemical reactionscharge:-1

mass: 0.511 MeV/c2

muon ()

a more massivecompanion to theelectron.

mass:105.658 MeV/c2

electron neutrino

(e)

has no electric charge,and interacts veryseldom with theambient medium.

muon neutrino

()

properties similar tothose of the electronneutrino.

tau neutrino ()

properties similar tothose of the electronneutrino.

up (u)

electric charge: +2/3

the proton holds two,the neutron one

mass: 1.5 4 MeV/c2

charm (c)

a heavier companionto down

mass:1.15 1.35 GeV/c2

top (t)

heaviest in the family(observed in 1995)

mass:171.4 2.1 GeV/c2

Firstfamily

Secondfamily

Thirdfamily

photon

elementary grain of light,vector for theelectromagnetic force

gluon

bearer of thestrong forcebetweenquarks

W, Z0

bearers of the weakforce, responsible for someforms of radioactive decay

Higgs boson?

nucleon

quarks

tau particle ()

heavier still.

masse:1,776.99 0.29 MeV/c2

down (d)

electric charge: -1/3

the proton holds one, theneutron two

mass: 4 8 MeV/c2

strange (s)

a heavier companionto up

mass:80 130 MeV/c2

beauty (b)

tau particle.

mass:4.1 4.4 GeV/c2

atom nucleus electronproton charge + 1mass: 938.272MeV/c2

neutron zero chargemass: 939.565MeV/c2

W+ W- Z0

leptonsable to move freely

quarksassembled into triplets, or quarkantiquark pairs,

to form the many subatomic particles

(contd)B


12/13

(contd)B

tions between elementary particles, while

respecting the principles of special rela-tivity, as those of quantum mechanics.

According to the latter theory, if one seeksto observe a microscopic structure athigh temporal and spatial resolution, this

entails transferring to it an amount ofenergymomentum, the greater, thehigher the resolution being sought.

However, according to the theory of rela-tivity, such an energymomentum trans-

fer is liable to undergo transformation,yielding particles not present in the initialstate: fermions may be generated, or

annihilated, in particleantiparticle pairs,while bosons may be so in any arbitrarynumber.

All processes involving one and the samefundamental interaction are interrela-ted. The quantum field theory approach,

in which properties of symmetry play afundamental part, seeks to describe allof the processes relating to each funda-

mental interaction, within overarchingtheoretical constructions.The strong and electromagnetic inter-

actions are formalized, respectively, inthe theories of quantum chromodyna-

mics, and quantum electrodynamics.The weak interaction, for its part, is notsubject to a separate description, beingdescribed jointly with the electroma-

gnetic interaction, in the unified forma-lism of electroweak theory. Theories of

thegrand unification of all fundamentalinteractions do exist, however theyremain as yet lacking any experimental

validation.All the predictions of the standard modelhave been corroborated by experiment,

except for just one, to wit, the existence

of the Higgs boson(s), which particle(particles?), it is hoped, will be discove-

red with LHC. The Higgs mechanism isthought to be responsible for the massexhibited by elementary particles, the

eponymous boson making it possible forzero-mass fermions interacting with itto be endowed with mass. This would

allow the unification, at high energies,of the weak and electromagnetic inter-actions within the electroweak theory,

while effectively accounting for the brea-king of this electroweak symmetry atlow energies, taking the form of two inter-

actions, which may be seen as distinctat that energy level (see The electroweak

interaction from one accelerator to the

next: the LHC roadmap and the yardstickof LEP measurements, p. 23).

Going beyond, or completing thestandard model?The standard model features a set ofparameters (such as the masses of ele-

mentary particles, or the intensities offundamental forces) which are ancho-red in experimental findings. It is, in any

event, a theory that is liable to be impro-ved, or further elaborated, or even sur-passed and left behind. It does not

account in any way for the classificationof the constituents of matter into threegenerations of particles, whereas it is

precisely the existence of these threegenerations which makes it possible toaccount forCP (chargeparity) invariance

violation (meaning that a physical pro-cess involving the weak interaction is notequivalent to its own mirror image), a

violation that is in all likelihood the sourceof the matterantimatter imbalance,running in favor of the former, in the pri-

mordial Universe. The model neitherallows quantum treatment of gravita-

tion, nor does it fully account for the fun-damental property of confinement, whichprevents quarks from propagating freelyoutside hadrons.

To go beyond, or to complete the stan-dard model, research workers are mainly

exploring two avenues: supersymmetry (widely known as

SUSY) would associate, to every particle

(whether a boson or a fermion) in thestandard model, a partner from the other

series, respectively a fermion or a boson.Supersymmetric partners would, at firstblush, be highly massive, the lightest of

them being a particle interacting veryweakly only. This would be an ideal can-didate to account for the hidden matter

(ordark matter) in the Universe, accoun-ting as it does for some 21% of the

Universes energy content, the remain-der (close to 75%) consisting in a darkenergy, the nature of which likewise

remains to be determined. These WIMPs(acronym for weakly interacting mas-sive particles) are actively being sought

(see EDELWEISS II, the quest for dark mat-ter particles); the substructure path assumes there

could be a new level of elementarity,underlying the particles in the standardmodel (or some of them). This would lead

to a veritable blossoming of new, com-posite particles, analogous to hadrons,but exhibiting masses two to three thou-

sand times heavier.It should be noted that, whereas super-

symmetry theories yield predictions thatagree with the precision measurementscarried out at LEP, the theories pro-pounding substructures (or their sim-

pler variants, at any rate) fail to do so.As for the more complex variants, theseare encountering difficulties at the theo-

retical level.

Tableau 2.Fundamental interactions, their vectors, and effects.

fundamental associated particles actionsinteraction (messengers)

gravitation graviton? having an infinite rangeresponsible for the mutual

attraction of any twomasses and for the law of

falling bodies

electromagnetic photon having an infinite rangeinteraction responsible for the

attraction between electronsand atomic nuclei, hencefor the cohesion of atoms

and molecules

weak interaction W+, W-, Z0 responsiblefor - and + radioactivity,

reactions involving particlesas neutrinos

strong interaction gluons ensures the cohesion of the(there are 8 gluons) atomic nucleus


13/13

E

Figure.An example of the combined use of mass spectrometry and chromatography: the separation of isomers

(sister molecules) of an explosive molecule (dinitrobenzene [DNB]), after solid-phase microextraction sampling,by gas chromatography, and their detection by mass spectrometry (SPMEGCMS).

What is chromatography?

Chromatography, together with the various forms

of spectroscopy and spectrometry (see Box D,Spectroscopy and spectrometry), represent the twomajor basic analytical techniques, the former ser-

ving for the separation, the latter for the identifi-cation of the constituents of a substance.Chromatography (from the Greek khrma, color,

andgraphein, to write), allows the separation ofthe constituents of a mixture in a homogeneousliquid or gaseous phase, as blotting paper might

spread out in concentric rings a liquid poured ontoit.A chromatograph comprises a sample injection

device, a column, a detector, and a recording andanalysis system. Its principle is based on the equi-librium of compound concentrations, between two

phases coming into contact: the stationary phase,

in the column, and the mobile phase, which movesacross it. Separation relies on the differential displa-cement of constituents inside the column, passingthrough in times that are proportional to their size,

or depending on their structure, or affinity for thestationary phase (polarity). As they reach the farend of the column, a detectormeasures, on a conti-

nuous basis, the quantities of each constituent.The most common form of chromatography is gaschromatography, carried out on gaseous samples,

or samples that may be vaporized without incur-ring breakdown. The mobile phase is a gas (helium,nitrogen, argon, or hydrogen), constantly sweeping

through the column, which is placed in a thermo-

stat oven. Detectors allow the selective analysisand identification of highly complex mixtures.

If the stationary phase is a nonvolatile, or not highlyvolatile liquid, exhibiting solvent properties for thecompounds to be separated, the process is termed

gasliquid chromatography, orpartition chroma-

tography. If the stationary phase is an adsorbent

solid (silica, alumina, zeolites, or polymers), thisis gassolid chromatography. Within this samefamily, of adsorption chromatography processes,

liquidsolid chromatography is characterized byits stationary phase, this being a polar solid.In high-performance liquid chromatography

(HPLC), the sample must be wholly soluble in themobile phase (elution solvent). The latter must bekept at high pressure (hence the alternative name

ofhigh-pressure liquid chromatography), to ensurea constant flow rate inside the column, and pre-clude any loss of head. HPLC involves solutemobile

phasestationary phase exchange mechanisms,based on partition or adsorption coefficients, depen-ding on the nature of the phases in contact. (1)

A chromatographic analysis yields a chromato-

gram, this being a graphical representation of theevolution of a parameter (intensity of the detectorsignal), related to instantaneous solute concen-tration, as function of time. This exhibits peaks,

rising above the baseline, which obtains in theabsence of any compounds (see Figure).

(1) There are two further types of liquid chromatography,ion chromatography, and exclusion chromatography.

N.B: This Box reproduces a number of excerpts froma presentation by Pascale Richardin, head of the DatationGroup at the Research and Restoration Centerof the French National Museums Administration(Muses de France), taken from the pages dealingwith analytical methods, as posted on the site :

ttp://www.culture.gouv.fr/culture/conservation/fr/biblioth/biblioth.htm

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