Nuclear Spectroscopy: From Natural Radioactivity to Studies of Exotic Isotopes. Prof. Paddy Regan Chair of Radionuclide Metrology, Department of Physics University of Surrey, Guildford, & Radioactivity Group, National Physical Laboratory, Teddington

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 ~ m NUCLEI ~ m NUCLEONS 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 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 Nuclear Isotopes 0.4% 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 =

Nuclear 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 ( 1 1 H ) + Nm n - B nuclear The binding energy is the energy needed to take a nucleus of Z protons and N neutrons apart into A separate nucleons energy Mass of Z protons + Z electrons + N neutrons (N=A-Z) Mass of neutral atom

Radioactivity….. The science of decay…

11 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 125 Te (Z=52, N=73); Even-A usually have 2 long-lived. increasing binding energy = smaller mass 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 Z → 125 Sn, Z=50, N= Xe, Z=54, N=71

 decay: 2 types: 1) Neutron-rich nuclei (fission frags) n → p +  - +  Neutron-deficient nuclei ( 18 F PET) p → n +  Cs Ba Xe 83 A=137 Mass Parabola Mass (atomic mass units) Nucleus can be left in an excited configuration. Excess energy released by Gamma-ray emission.

‘signature’ 1461 keV gamma Some (odd-odd) nuclei can decay by competing types of beta decay (a)p → n +    + ; (b) n → p +   + ; (c) p + e - → n+ v ). Decay rate depends on energy released (Q  value) and CONSERVATION OF ANGULAR MOMENTUM. Big change in angular momentum and small Q  → long half-life Note, the number of 40 K decays would then be equal to the number of 1461 keV gamma rays emitted, divided by the ‘branching ratio’ which is in this case.

Nuclei can also decay by  emission.. ejection of a 4 He nucleus…. Depends (again) on binding energies & masses Before… 232 Th, Z = 90 N = 142  After… 228 Ra, Z = 88 N =140 4 He, Z=2 N=2

Radioactive decays occur as a result of conservation of mass/energy E=  mc 2 M( 232 Th) = u = mass / energy before alpha decay. M( 4 He) = u + M( 228 Ra) = u = mass after. 1 u = 1 atomic mass unit = MeV/c 2  mc 2 = M( 232 Th) – [ M( 228 Ra) + M( 4 He)])c 2  mc 2 = uc 2 = 4.08 MeV 4.08 MeV of ‘binding energy’ from 232 Th is released in its decay to 228 Ra by the emission of a 4 He nucleus (  particle). Due to conservation of linear momentum, this energy is split between the energy of the emitted alpha particle (4.01 MeV) and the recoil energy of the residual 228 Ra nucleus (0.07 MeV).

Geiger-Nuttall rule links Q  values to explain long lifetimes of 232 Th, 238 U compared to other ‘heavy’ nuclei. ‘Classic’ evidence for quantum mechanical ‘tunnelling’ effect through a barrier.

Alpha decay can also leave daughter in excited states which can then decay by (characteristic) gamma emission. 

Radiation occurs in nature…the earth is ‘bathed’ in radiation from a variety of sources. Humans have evolved with these levels of radiation in the environment. Naturally Occurring Radioactive Materials These include Uranium-238, which has radioactive half-life of 4.47 billion years. 238 U decays via a series of alpha and beta decays (some of which also emit gamma rays). These create radionuclides including: Radium-226 Radon-222 Polonium-210

Radiation occurs in nature…the earth is ‘bathed’ in radiation from a variety of sources. Humans have evolved with these levels of radiation in the environment. Naturally Occurring Radioactive Materials These include Uranium-238, which has radioactive half-life of 4.47 billion years. 238 U decays via a series of alpha and beta decays (some of which also emit gamma rays). These create radionuclides including: Radium-226 Radon-222 Polonium-210 (all of which are  emitters). Other NORM includes 40 K (in bones!)

Bateman equations, for ‘secular equilibrium’, The activity (decays per second) of cascade nuclide equals the activity of the ‘parent’.

How do you measure the gammas? i.e., How do you see inside the nucleus?

Little ones…single hyper-pure germanium detector, CNRP labs, U. of Surrey

Bigger ones…the RISING array at GSI-Darmstadt, Germany, 105 Germanium detectors (see later)…

How do you know how much radioactive material is present? Activity ( A ) = number of decays per second The activity ( A ) is also equal to the number of (radioactive) nuclei present ( N ), multiplied by the characteristic decay probability per second for that particular nuclear species ( ). A = N is related to the half-life of the radioactive species by = / T 1/2 One signature that a radioactive decay has taken place is the emission of gamma rays from excited states in the daughter nuclei. If we can measure these, we can obtain an accurate measure of the activities of the different radionuclides present in a sample.

Not all the gamma rays observed have to originate from the same radionuclide. Different radionuclides are identified by their characteristic gamma-ray energies. 226 Ra 228 Ac 40 K

Making a Radiological Map of Qatar Arabic Gulf state, Oil Rich (oil industry all around) To host World Cup (2022)

662 keV

Characteristic gamma signatures can be used to measure emissions of radionuclides from ‘man-made sources’ such as Fukushima, Chernobyl, nuclear weapons tests…etc. –Nuclear Fission fragments: 137 Cs (T 1/2 = 30 years) 131 I (T 1/2 = 8 days) –Neutron-capture on fission products in reactors 134 Cs (T 1/2 = 2 years)

Summary Radionuclides (e.g. 235 U, 238 U, 232 Th, 40 K) are everywhere. Radioactive decays arise from energy conservation and other (quantum) conservation laws. Characteristic gamma ray energies tell us structural info.