nuclear spectroscopy: from natural radioactivity to studies of the most exotic isotopes

Nuclear Spectroscopy: From Natural Radioactivity to

Studies of the Most Exotic Isotopes.

Prof. Paddy Regan Department of Physics

University of Surrey, Guildford, &

Radioactivity Group, National Physical Laboratory,

Teddingtonp.regan@surrey.ac.uk

Outline of talk

• Elements, Isotopes and Isotones

• Alpha, beta and gamma decay

• Primordial radionuclides…..why so long ?

• Internal structures, gamma rays and shells.

• How big is the nuclear chart ?

• What could this tell us about nucleosynthesis?

Darmstadtium

Roentgenium Copernicium

•ATOMS ~ 10-10 m

•NUCLEI ~ 10-14

m•NUCLEONS-10-15 m

•QUARKS ~?

The Microscopic World…

7

Mass Spectrograph (Francis Aston 1919)

Atoms of a given element are ionized.

The charged ions go into a velocity selector which has orthogonal electric (E) and magneticfields (B) set to exert equal and opposite forces on ions of a particular velocity → (v/B) = cont.

The magnet then separates the ions accordingto mass since the bending radius isr = (A/Q) x (v/B) Q = charge of ion & A is the mass of the isotope

Nuclear Isotopes

0.4% 2.3 11.6 11.5 57.0 17.3

Results for natural terrestrial krypton

Not all atoms of the same chemical element have the same mass (A)Frederick Soddy (1911) gave the name isotopes.(iso = same ; topos = place).

Krypton, Z=36

N = 42 44 46 47 48 50

Nuclear chartNuclear chart

9

= binding energy

MeV eV

(nuclear + atomic)

Atomic Masses and Nuclear Binding Energies

M(Z,A) = mass of neutral atom of element Z and isotope A

M(Z,A) m ( 11H ) + Nmn -

Bnuclear

The binding energy is theenergy needed to take a nucleus of Z protons and N neutrons apart into A separate nucleons

ener

gy

Mass of Z protons+ Z electrons + Nneutrons (N=A-Z)

Mass of neutral atom

10

ISOBARS have different combinations of protons (Z) and neutrons (N) but same total nucleon number, A → A = N + Z.

(Beta) decays occur along ISOBARIC CHAINS to reach the most energetically favoured Z,N combination. This is the ‘stable’ isobar.

This (usually) gives the stable element for this isobaric chain. A=125, stable isobar is 125Te (Z=52, N=73); Even-A usually have 2 long-lived.

incr

easi

ng b

indin

g e

nerg

y =

sm

alle

r m

ass

A=125, odd-A even-Z, odd-Nor odd-Z, even N

A=128, even-A even-Z, even-Nor odd-Z, odd- N

increasing Z → increasing Z →

125Sn,Z=50, N=75

125Xe,Z=54, N=71

decay: 2 types:

1) Neutron-rich nuclei (fission frags)n → p + - +

Neutron-deficient nuclei (18F PET)p → n + + +

137Cs82

137Ba81

137Xe83

A=137 Mass Parabola

Mass

(ato

mic

mass

unit

s)

Nucleus can be left in an excitedconfiguration. Excess energyreleased by Gamma-ray emission.

Some current nuclear physics questions

• 286 combinations of protons and neutrons are either stable or have decay half-lives of more than 500 million years.

– What are the limits of nuclear existence…i.e. how many different nuclear species can exist?

• N/Z ratio changes for stable nuclei from ~1:1 for light nuclei (e.g., 16O, 40Ca) to ~1.5 for 208Pb (126/82 ~ 1.5)

– How does nuclear structure change when the N/Z ratio differs from stable nuclear matter?

Accelerator facility at GSI-Darmstadt

The Accelerators:UNILAC (injector) E=11.4 MeV/n

SIS 18Tm corr. U 1 GeV/nBeam Currents:

238U - 108 ppssome medium mass nuclei- 109

pps (A~130)

FRS provides secondary radioactive ion beams:• fragmentation or fission of primary beams • high secondary beam energies: 100 – 700 MeV/u• fully stripped ions

An Efficient Way to Make Exotic Nuclei:Projectile Fragmentation Reaction Process

Abrasion

Beam at Relativistic Energy ~0.5-1 GeV/A

Target Nucleus

FIREBALL

Ablation

Formation of an exotic compound

nucleus

Reaction products travelling at Relativistic

Energies

A few physics examples….

+ decay/ec

- decay

K-electrons

L-electrons

T1/2 = 10.4 s205Au126

202Pt

How are the heavy elements made ?

Is it via the Rapid Neutron Capture (R-) Process ?

Many of the nuclei which lie on the r-processpredicted path have yet to be studied.

Do these radioactive nuclei act as we expect ?

SN1987a before and after !!

• A (big!) problem, can’t reproduce the observed elemental abundances.

• We can ‘fix’ the result by changing the shell structure (i.e. changing

the magic numbers)….but is this scientifically valid ? N=126N=82

• Need to look at N=82 and 126 ‘exotic’ nuclei in detail….

First excited state in (most)even-N AND even-Z has I=2+

Excited states spin/parities depend on the nucleon configurations.

i.e., which specific orbits the protons and neutrons occupy.

Result is a complex energy ‘level scheme’.

Excitation energy (keV)

Ground state (Ex=0) config has I=0+ ;

2+

0+

~2

‘pair gap’

Even-Even Nuclei

Excitation energy (keV)

Ground stateConfiguration.Spin/parity I=0+ ;Ex = 0 keV

2+

0+

PHR, Physics World, Nov. 2011, p37

Is there evidence for a N=82 shell quenching ?

Assumption of a N=82 shell quenching leads to a considerableimprovement in the global abundance fit in r-process calculations !

r-p

roce

ss a

bu

nd

ance

s

mass number A

exp.pronounced shell gapshell structure quenched

g9/2

Search for the 8+ (g9/2)-2 seniority isomer in 130Cd(structure should look lots like 98Cd…apart from size?)

two proton holes in the g9/2 orbit

M. Górska et al., Phys. Rev. Lett. 79 (1997)

Evidence for nuclear shell structure…..energy of 1st excited state in even-even nuclei….E(2+).

Facility for Anti-Proton and Ion Research (FAIR)

To be constructed at the current GSI site, near Darmstadt, Germany

Will bring currently ‘theoretical nuclear species’into experimental reach for the first time.

Summary• Radionuclides (e.g. 235U, 238U, 232Th, 40K) are everywhere.

• Radioactive decays arise from energy conservation and other (quantum) conservation laws.

• Characteristic gamma ray energies tell us structural info.

• The limits for proton-richness in nuclei has been reached.

• Neutron-rich nuclei are harder to make at the extremes, but we are starting to be able to reach r-process radionuclides.– Does the nuclear shell model remain valid for nuclei with ‘diffuse neutron

skins’ ?• FAIR will increase dramatically our reach of nuclear species for

experimental study