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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,
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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?
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Darmstadtium
Roentgenium Copernicium
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•ATOMS ~ 10-10 m
•NUCLEI ~ 10-14
m•NUCLEONS-10-15 m
•QUARKS ~?
The Microscopic World…
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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
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Nuclear chartNuclear chart
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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
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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
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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.
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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?
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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
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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
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A few physics examples….
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+ decay/ec
- decay
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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 ?
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SN1987a before and after !!
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• 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….
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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
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Excitation energy (keV)
Ground stateConfiguration.Spin/parity I=0+ ;Ex = 0 keV
2+
0+
PHR, Physics World, Nov. 2011, p37
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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
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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)
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Evidence for nuclear shell structure…..energy of 1st excited state in even-even nuclei….E(2+).
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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.
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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