Photon Instrumentation

First Mexican Particle Accelerator School

Guanajuato

Oct 6, 2011

Outline

•The Electromagnetic Spectrum

•Photon Detection

•Interaction of Photons with Matter

•Photoelectric Effect

•Compton Scattering

•Pair production

•Instrumentation

•Solid State Devices

•Gas Filled Devices

•Scintillation Counters

•Charge Coupled Devices

Outline

•Beam Line Measurements

•Beam Position

•Beam Profile

•Beam Intensity

•Summary

The Electromagnetic Spectrum

http://www.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html

Another way of looking at the spectrum

The Electromagnetic Spectrum

Wavelength () nm

Frequency () THz

Energy (eV)

infrared ~750 – 106 ~400 – 0.3 ~ 1.7– 1.2*10-3

Visible ~400 –750 ~750 – 400 ~3.1 – 1.7

Ultraviolet ~10 – 400 ~3*104 – 750 ~124 – 3.1

X-rays ~0.01 – 10 ~3*107 – 3*104 ~124*103–124

ϒ-rays < 0.01 >3*107 >124*103

Velocity of light (c) = Wavelength () x Frequency() Photon Energy E = h

h is Planck’s constant (≈4.136*10−15 eV·s)

Light Sources

Light sources provide photon beams over a wide spectrum

The wide variety of instrumentation does not yield to even a cursory description

We will limit ourselves to a small set of techniques and devices

Photon Detection

Detection requires that

• the photons interact with the material in the detector

• the interaction generates some identifiable signal

Example: Light detection by your eye

The eye can do more than just detection, it can distinguish colors and also intensity

Limitations of eye as a detector (among other things):

Range, which is the visible spectrum

Photon Detection

(Hamamatsu Photomultiplier Basics and Applications)

Photon Detection

A detector’s ability is related to the photon wavelength

To state it another way, a detector’s response is dependent on the photon energy

It is not sufficient for us to detect photons

We want to measure with some desired precision

•beam properties because experiments depend on knowing the characteristics of the photon beam

•The interactions of the beam with targets

Interactions of Photons with Matter

We will consider the following mechanisms by which photons lose energy

• Photoelectric effect

• Compton Scattering

• Pair production

Photoelectric Effect

Photoelectric effect is the dominant process at low photon energies with high Z materials

Probability of photoelectric interaction Zn/(h)3, n is between 3 and 4

(picture credit: http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html)

Kinetic Energy of the electron = h – W, where W is the binding energy of the electron

Compton Scattering

Energy transfer from photon to electron increases with photon energy. Probability of Compton scattering is approximately proportional to Z (picture credit http://hyperphysics.phyastr.gsu.edu/hbase/quantum/compeq.html)

Pair Production

Pair production probability increase when the incident energy is greater than 2*electron mass and approximately as Z2

Nucleus

e+

e- Photon

Photon Interactions with matter

In the energy regime below 1 MeV, the dominant processes are photoelectric effect and Compton scattering

Instrumentation

The information on energy dependent photon interactions with matter guides the choice of detectors

Solid state devices can work at very low photon energies (<10 eV)

Gas filled detectors are suitable when photon energies are around 30 eV

Scintillation detectors cover a large range from around 10 eV to very high energies

Solid State Devices

• The energy deposited by photons solid state devices creates electron/hole pairs

• The electrons move from valence bond to conduction band

• The migration of electrons creates holes in the valence bond

• The number of electron/hole pairs is proportional to the energy deposited

• Application of an electric field generates a pulse

Solid State Devices

Hamamatsu

Solid State Devices

The contribution of statistics to the energy resolution is given by

ΔE/E = 2.35 (Fε/E)1/2

F is called the Fano Factor, E is the photon energy in eV and ε is the energy needed to create an electron/hole pair

Signal to Electronics

Current

Gas Filled Devices

While in the semiconductor devices, electron/hole pairs are created by radiation, in the gas counters electron/ion pairs are created. The anode is kept at a positive potential and the walls are at ground. As the electrons drift towards the cathode, an avalanche can form and the signal is collected from the anode

Ground

Gas Filled Devices

Pulse height is given by

A*N*e/C,

where N is the number of electrons, e is the electron charge, C is the capacitance of the device and A is the amplification factor

Depending on the applied voltage, gas filled counters can work as ionization counters, proportional or Geiger-Mueller counters

Similar to solid state detectors, energy resolution is given by (note that F will have a different value)

ΔE/E = 2.35 (Fε/E)1/2

Scintillation Counters

Scintillation counters are widely used in Nuclear and Particle physics

A basic scintillation counter consists of a scintillator optically coupled to a photomultiplier tube

Large selections of the scintillating material and photomultiplier tubes are available for applications

Some uses of scintillation counters can be used are

• Particle Counting

• Measuring Particle Energy

• Triggering

• Time of flight

Scintillation Counters

Common scintillating materials are inorganic crystals, plastics liquids. Many specialty materials such as lead tungstate are available for specific applications

Scintillation Mechanism

Photomultiplier Tubes Electron Multiplication Alkali Photocathode

(Most of the material on scintillation counters is taken from Hamamatsu Photomultiplier Handbook)

Scintillation Counters

Ik is the cathode current and B is the bandwidth of the measurement system

Fano Factor

Fano factor is an adjustment factor introduced to relate the variance of the observed distribution to the mean of the distribution

If <N> is the average number of electron/ion or electron/hole pairs due to ionization, the fluctuation in the ionization is given by

σ2<N> = F <N>

Where F is the Fano factor

Fano Factor

Energy Resolution: Some Detectors

http://xdb.lbl.gov/Section4/Sec_4-5.pdf

(Imaging) Charge Coupled Devices

CCDs are based on Metal Oxide Semiconductor (MOS) capacitors

Charge stored on one area of the CCD can be transferred to another area

The area where the charge is stored is called a potential well

Referring to the figure when a voltage is applied to the gate electrode P2, (with P1 and P3 at zero volts), a potential well is created

Charge Coupled Devices

By adjusting the voltages in a time sequence on the gate electrodes, the charge can be sequentially transferred, somewhat like a shift register

Groups of electrodes form a pixel

In the figure the three electrodes form a pixel

Thus, charge is created by photoelectric effect, the charge is transferred sequentially by applying differential voltage at some frequency and the charge is converted to a voltage

CCDs have very high quantum efficiency (~80%)

Require cooling to reduce noise

Beam Line Measurements

We consider four types of measurements in the beam line

• Beam Position

• Beam Profile

• Beam Intensity

Beam Position

The following table gives examples of processes used for measuring beam position

(S. Hustache-Ottini,, Proceedings of CERN Accelerator School)

Beam Position

Blade type Beam position monitors

(H. Aoyagi, T. Kudo, H. Kitamura, Nuclear Instruments and Methods)

(S. Hustache-Ottini,, Proceedings of CERN Accelerator School)

Tungsten Blades

Beam Position

The x and y positions are given by

x [(IuR + IdR) - (IuL + IdL)]/ [(IuR + IdR) + (IuL + IdL)]

y [(IuR + IuL) - (IdR + IdL)]/ [(IuR + IuL) + (IdR + IdL)]

Where I is the current and u,d,R & L represent up, down, Right and Left (IuR represents current in the upper right blade)

Current is converted to Voltage using an I to V converter and the voltage is digitized by an ADC. The proportionality constants which have to be determined through calibration

Beam Position

A harp (wire scanner) works on the same principle of generating current in the wire due to photon interaction with the electron in the material of the wire

Four quadrant photodiodes provide another way to measure beam position

(Hamamatsu)

Beam Profile

Beam profile may be measured by imaging the beam, i.e. converting the beam into a visible image using a fluorescent screen

The light from the screen can be viewed by a CCD camera using suitable optical elements

The image can then be processed using a commercial or custom hardware/software systems

Screen

CCD camera and Optical elements

Beam Profile

Of critical importance in deciding the choice of a fluorescent screen is the beam power density. The thermal characteristics of the screen should allow power dissipation in the form of heat without damaging the screen

Beam Intensity

(S. Hustache-Ottini,, Proceedings of CERN Accelerator School)

Beam Intensity

A thin material (suited to the energy of the photon beam) is inserted in the beam path

The scattered or fluorescent photons are detected by, for example, a scintillation counter

The beam intensity is proportional to the number of detected photons

Counter

Scatterer

Beam Intensity

Responsivity is a measure of the detector’s sensitivity to radiant energy

It is the ratio of number of electrons generated per incident photon to photon energy. Its units are amps/watt

R (amps/watt) = Y/h = exp(-μt)/W

Y is the quantum yield

h is the photon energy

μ is the thickness of the surface oxide layer of the photodiode

t is the thickness of the oxide layer

Beam Intensity

W is the electron/hole pair creation energy

This could be rewritten as

i = e*N*A*h/W

i is the current in the photodiode

e is the electron charge

N is the number of photon/s

A is the fraction of x-rays absorbed by the diode

Beam intensity can be obtained by measuring i

)X. Zhang, H. Fujimoto and A. Waseda IOP Conference series, Materials and Engineering, Vol. 24, 2011)

(E. M .Gullickson, R. Korde, L. R. Canfield and R. E. Vest Journal of Electron Spectroscopy and Related Phenomenon. Vol. 80, 313-316,1996)

Beam Intensity

Responsivity and transmissivity of a 5 μm thick photodiode made by IRD http://www.ird-inc.com/axuvtransmission/axuvtrans.html

Summary

The field of beam diagnostic instrumentation is huge

Physicists and engineers have a vast array of materials, detectors and data acquisition systems from which to choose when designing a detector system

The references given at the end are a good starting point for in-depth knowledge and understanding of detection systems

References

• http://ocw.mit.edu/courses/nuclear-engineering/22-01-introduction-to-ionizing-radiation-fall-2006/lecture-notes/energy_dep_photo.pdf

• http://www.science.mcmaster.ca/medphys/images/files/courses/4R06/note4.pdf

• http://www.deqtech.com/Resources/PDF/scintillation-detectors.pdf

• http://sales.hamamatsu.com/assets/applications/ETD/pmt_handbook_complete.pdf

• http://pdg.lbl.gov/2011/reviews/rpp2011-rev-particle-detectors-accel.pdf

• http://old.iupac.org/publications/analytical_compendium/Cha10sec324.pdf

• http://sales.hamamatsu.com/assets/applications/SSD/fft_ccd_kmpd9002e06.pdf

• http:// www.pep.uni-bremen.de/services/.../raja_pres_tech_07_detectors.ppt

• http:// www.nuc.berkeley.edu/courses/.../XRayDetection_NE107_Fall10.pdf

• Review of Particle Properties, Particle Data Group, Journal of Physics G, Vol. 37,No. 74, July 2010

• http://xdb.lbl.gov/Section4/Sec_4-5.pdf

References

• S. Hustache-Ottini,, Proceedings of CERN Accelerator School on Beam Diagnostics, Dourdan, France, May 28- 6June 2008

• H. Aoyagi, T. Kudo, H. Kitamura, Nuclear Instruments and Methods in Physics Research, Section A. Vol 467-468. 21 July 2001, 252-255

• http://www.slac.stanford.edu/pubs/icfa/spring96/paper6/paper6.pdf

• http://www-physics.lbl.gov/~spieler/physics_198_notes_1999/PDF/IX-1-Signal.pdf

• http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5402210

• http://www.nist.gov/pml/data/xraycoef/index.cfm

• http://agamemnon.cord.org/cm/leot/Module4/module4.htm

• E. M .Gullickson, R. Korde, L. R. Canfield and R. E. Vest Journal of Electron Spectroscopy and Related Phenomenon. Vol. 80, 313-316,1996

• X. Zhang, H. Fujimoto and A. Waseda IOP Conference series, Materials and Engineering, Vol. 24, 2011

• http://www.ird-inc.com/axuvtransmission/axuvtrans.html