1) Why study nuclei2) Basic facts about Nuclei 3) Nuclear structure and nuclear reactions4) Basic facts about collisions and reactions5) Where we do our experiments 6) How we do the experiments7) What one can learn from debris

Part I: An introduction to basic nuclear science

Romualdo T. de Souza

Nuclear Chemistry at Indiana: http://nuchem.iucf.indiana.edu

Romualdo de Souza

1) Why study nuclei?

• Nuclei are at the heart of every atom; What is their structure, properties? What is the nature of the force that hold them together?

•Necessary to understand the formation of the elements – nucleosynthesis

• Important in understanding the properties of astrophysical objects such as neutron stars ( a giant nucleus with a radius of ~ 0.6 km) nuclear equation-of-state.

• Important in understanding the thermodynamic properties of small, finite systems (ties to the study of atomic clusters).

• Important in understanding nuclear fission and nuclear fusion (energy source/ weapons)

Neutrons

Pro

tons

Stable Nuclei

Known Nuclei

Terra Incognita

Fundamentals of supernova explosions are not understood!

Synthesis of the heavy elements is not understood

Limits of nuclear stability (superheavy elements, N/Z exotic) poorly known

Only elements Z=1-4 produced in the Big Bang

1) Why study nuclei?

2) Basic facts about nuclei:

• Nuclei behave like microscopic drops of liquid (fairly incompressible yet deformable).

•Nuclei are small (R= 1-10 x 10-15 m);104-105 times smaller than an atom; requires measuring instruments of a comparable size to measure them e.g. other nuclei

• Nuclei are positively charged so one has to overcome the mutual repulsion between two nuclei (Coulomb repulsion) i.e. Particle accelerators are required.

Chemistry Nuclear Chemistry

Distance 10-10 m 10-15 m

Density 1-10 g/cm3 2 x 1014 g/cm3

Time 10-14 s 10-22 s

2) Basic facts about nuclei: Binding energy

nNHZTBEXAZ

10

1 TBE= Total Binding Energy

Analogous to the heat of vaporization

Binding energy curve for nucleiTBE/A = “average

bond strength”

How can one understand this binding energy curve?

2) Basic facts about nuclei: The liquid drop model

Charge denisty,

Radial distance (r)

3/10 ArR

fmr 4.12.10

TBE = C1A C2A2/3 C3Z

2/A1/3 C4(N-Z)2/A2 + C6/A1/2

<BE> = C1 C2A1/3 C3Z

2/A4/3 C4(N-Z)2/A3 + C6/A3/2

volume surface Coulomb symmetry pairing

<BE> = TBE/A

2) Basic facts about nuclei: The liquid drop model3/1

0 ArR fmr 4.12.10

The first three terms in the liquid drop model (Volume, surface, and Coulomb) already explain the shape and magnitude of the Binding energy curve for nuclei.

2) Basic facts about nuclei: The shell modelNuclei are not “formless blobs”. They have an internal structure in which protons and neutrons occupy orbitals much as in the atom (though with differences).

Proton number Z

Neutron number N

(Mm

easu

red –

Mli

quid

dro

p)c2

Red arrows indicate nuclei of additional stability. They occur at the MAGIC NUMBERS: 2,8,20,50,82, and 126

3) Nuclear structure and nuclear reactions

Nuclear structure involves studying the internal levels in a nucleus. Since the transition between levels involves the emission of gamma rays, nuclear structure involves gamma ray spectroscopy

110 Ge detectors on a 10 inch radius sphere

The next generation: Segmented Gamma ray detectors (GRETINA)

3) Nuclear structure and nuclear reactions

QEDCB AZ

AZ

AZ

AZ ...4

433

22

11

total number of protons is conserved

total number of neutrons is conserved

Q (energy release) can be either positive (exothermic) or negative (endothermic)

to get the nuclei to react one must get into the range of the short range nuclear force (projectile and target nuclei must touch)

The reaction products are quite likely excited (their protons and neutrons are not in the ground state) and they will de-excite by emission of gamma rays, neutrons, protons, alpha particles and other clusters.

t=0 ms 30 ms 60 ms 90 ms 120 ms 150 ms

Fusion-like event

Impact parameter selection: direct inspection

Strongly Damped/Deeply inelastic event

Deep inelastic + neck emissions event

Classical drops: Collisions of mercury drops

camera

Deposit of a fraction of initial kinetic energy into heat and stretching the drops. How strong is the inter-atomic interaction? Role of surface tension.

We want to study the same type of processes but with nuclear drops to learn about the forces holding nuclei together!

4) Basic facts about collisions and reactions?

Supercomputer simulations of 114Cd + 92Mo at E/A = 50 MeV; b=7.37 fm

Antisymmetrized Molecular Dynamics

IU Cyclotron FacilityThe Indiana University Cyclotron Facility (IUCF) is a multidisciplinary laboratory performing research and development in the areas of accelerator physics, nuclear physics, materials science, life science and biomedical applications of accelerators.

Accelerator PhysicsDefining the physics of producing and handling beams of sub-atomic particles

Nuclear Physics and ChemistryProbing matter and forces at the sub-atomic scale

Neutron PhysicsUsing neutrons to explore the molecular structure of proteins, crystals, surfaces, and much more

Materials ResearchImaging, modeling and manipulating macromolecules

Biomedical and Life SciencesHarnessing the power of radiation for research in biology and medicine

http://www.iucf.indiana.edu

5) Where we do our experiments (the accelerator side)

5) Where we do our experiments (the accelerator side )

• Up to C at 96MeV/A or U at 24MeV/A

• CSS1, CSS2 K=380

• SISSI - fragmentation beams

• SPIRAL - re-acceleration of radioactive beams with CIME

Ion sources

4 dipole magnets act to bend the moving charged particle in a circular orbit a voltage applied at radiofrequency as the particle moves between the dipoles causes the particle to accelerate, therefore spiraling outward When the particle reaches the maximum radius of the cyclotron it is at the maximum energy and is extracted by a small electrostatic deflection

Principle of acceleration of a cyclotron

A sense of scale : A K=200 cyclotron (IUCF)

Remember that GANIL has two K=380 cyclotrons coupled sequentially Michigan State has two coupled superconducting cyclotrons (K=500 and K=1200)

6) How we do our experiments (the detector side)

Interaction of radiation with matter!

Charged particles: protons, deuterons, tritons, alpha particles, intermediate mass fragments (IMF: 3≤Z≤20), fission fragments

Neutral particles:• gamma rays • neutrons

Gas detectors (incident particles cause ionization)

Solid state detectors: Si, Ge (incident particles cause

electron-hole pairs)

Scintillators: liquid, plastic (incident particles cause

scintillation)

6) How we do our experiments (the detector side)

E detector

Incident particle with

(Z,A,E)dx

E detector

dE Z2A

dx E

Interaction of radiation with matter!

Different “bands” represent different isotopes.

6) How we do our experiments (the detector side)

Segmented Si detectors

Backed by CsI(Tl) with photodiode readout …

Are stacked to make a telescope…

And electronics…

4x CsI(Tl) 4cm

16 strips v. (front)

Target

Beam

Si-E 65 m

16 strips v (front)

Si-E 1.5 mm

pixel

16 strips h. (back)

6) How we do our experiments (the detector side)

Many telescopes are combined together to give as complete a measurement as possible.

Collision of a nucleus with a light-ion (Z<3) or a heavy-ion (Z>2) converts kinetic energy of relative motion into intrinsic excitation i.e. heats the

nucleus.

From the debris – the fragmentation pattern we need to determine what happened

• identity of all the particles

• number of clusters (Z>2)

• number of light particles Z=1,2

• energy of all the particles

• angles of all the particles

7) What one can learn from debris

• 162 individual telescopes covering 74% of 4

• Gas Ionization chamber/500 µm Si(IP)/CsI(Tl(PD)

• Each telescope measures Z,A, E, and

• Identification of Z for 0.6≤E/A≤96 MeV

• Identification of A for E/A ≥ 8 MeV for Z≤4

ISiS: Indiana Silicion Sphere

We measure all information collision-by-collision (event-by-event).

4 measurements

Physical Chemistry, R. Chang, 2000

H2 gas

v (m/s)

P(v)

Charity, et.al., PRC (2001)

• Maxwell Boltzmann distribution• Coulomb Barrier for α-particles

T

EEEP f exp

Helium Isotopes

Kinetic equilibrium: motion of all

particles reflects a common temperature

Kinetic energy spectra fit Maxwell-Boltzman distribution TSlope

Thermometers

Angular distribution: comparing emission time to rotation time

Circular ridge PLF* emission

“Isotropic” component

Projectile velocityOther emission

(mid-rapidity, ...)

When the rotation time is short compared to the emission time, a uniform emission pattern is observed.

Emission from a hot nucleus

Chemical equilibrium: different partitions are

populated according to their statistical weights.

Emitting system

10B

6Li

F. Zhu et al., PRC52, 784 (1995)

Relative energy spectrum of daughters reflects internal quantum levels of parent

Pm = (2Jm+1)e-(E*-Em/T)

Pm/Pn = (2Jm+1)/(2Jn+1)e-(En-Em)/T

Extract temperature T

Another Thermometer: Excited state populations

Phase transitions for small, finite, open systems

Transition from one phase to an other at constant T

Constant PInfinite matterClosed system

“Caloric curve” for nuclear matter

Liquid phase

Gas phase

Liquid-gas coexistenceBOILING ?

J. Pochodzalla et al., PRL 75, 1040

(1995)Chemistry Nuclear

Chemistry

Distance 10-10 m 10-15 m

Density 1-10 g/cm3 2 x 1014 g/cm3

Time 10-14 s 10-22 s