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 p.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 Copernicium Roentgenium 4

•ATOMS ~ 10-10 m •NUCLEI ~ 10-14 m •NUCLEONS-10-15 m •QUARKS ~? The Microscopic World… •ATOMS ~ 10-10 m •NUCLEI ~ 10-14 m •NUCLEONS-10-15 m •QUARKS ~?

Nuclear Isotopes 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 magnetic fields (B) set to exert equal and opposite forces on ions of a particular velocity → (v/B) = cont. The magnet then separates the ions according to mass since the bending radius is r = (A/Q) x (v/B) Q = charge of ion & A is the mass of the isotope Not all atoms of the same chemical element have the same mass (A) Frederick Soddy (1911) gave the name isotopes. (iso = same ; topos = place). 0.4% 2.3 11.6 11.5 57.0 17.3 Results for natural terrestrial krypton Krypton, Z=36 N = 42 44 46 47 48 50

Nuclear chart 8

Atomic Masses and Nuclear Binding Energies M(Z,A) = mass of neutral atom of element Z and isotope A The binding energy is the energy needed to take a nucleus of Z protons and N neutrons apart into A separate nucleons M(Z,A) m ( 11H ) + Nmn - Bnuclear energy Mass of Z protons + Z electrons + N neutrons (N=A-Z) = binding energy (nuclear + atomic) Mass of neutral atom  MeV  eV

increasing Z → increasing Z → A=125, odd-A even-Z, odd-N or odd-Z, even N A=128, even-A even-Z, even-N or odd-Z, odd- N increasing binding energy = smaller mass 125Sn, Z=50, N=75 125Xe, Z=54, N=71 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.

A=137 Mass Parabola Mass (atomic mass units) 137Xe83 137Ba81 137Cs82 Nucleus can be left in an excited configuration. Excess energy released by Gamma-ray emission. b - decay: 2 types: 1) Neutron-rich nuclei (fission frags) n → p + b- + n 2) Neutron-deficient nuclei (18F PET) p → n + b+ + n

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/n Beam Currents: 238U - 108 pps some 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 Ablation Formation of an exotic compound nucleus Reaction products travelling at Relativistic Energies Abrasion Beam at Relativistic Energy ~0.5-1 GeV/A Target Nucleus FIREBALL

Dream of every student to press a button and level scheme pop out

A few physics examples….

b+ decay/ec b- decay

How are the heavy elements made ? Is it via the Rapid Neutron Capture (R-) Process ? 205Au126 T1/2 = 10.4 s K-electrons L-electrons 202Pt Many of the nuclei which lie on the r-process predicted 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=82 N=126 Need to look at N=82 and 126 ‘exotic’ nuclei in detail….

Even-Even Nuclei Excited states spin/parities depend Excitation energy (keV) Ground state (Ex=0) config has Ip=0+ ; 2+ 0+ ~2 D = ‘pair gap’ 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’. First excited state in (most) even-N AND even-Z has Ip=2+

2+ 0+ Excitation energy (keV) Ground state Configuration. Spin/parity Ip=0+ ; Ex = 0 keV PHR, Physics World, Nov. 2011, p37

Is there evidence for a N=82 shell quenching ? r-process abundances exp. pronounced shell gap shell structure quenched mass number A Certainly all of you have seen this figure before. It shows the solar r-process abundances with the characteristic two peaks around mass 130 and 195 which are closely related to the N=82 respectively N=126 shell gaps. These gaps determine the properties of the waiting-point nuclei in the r-process path shown here as red squares and therefore the dynamics of the nucleosynthesis of elements above iron. Already in the nineties it has been shown that the assumption of a N=82 shell quenching leads to a considerable improvement in the global abundance fit in r-process calculations, namely the filling of the troughs around A=120 and 140 - shown here in blue compared to the non-quenched calculation in green. However ... Assumption of a N=82 shell quenching leads to a considerable improvement in the global abundance fit in r-process calculations !

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 g9/2 Since an 8+ seniority isomer based on two proton holes in the g9/2 orbit is expected to exist in 130Cd in complete analogy to the one known in 98Cd we decided to try to obtain independent experimental information about 130Cd using isomer spectroscopy within RISING. M. Górska et al., Phys. Rev. Lett. 79 (1997)

Evidence for nuclear shell structure… 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