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, Teddington [email protected]
Jan 12, 2016
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,
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