Nuclear Spectroscopy: From Natural Radioactivity to Studies of Exotic Isotopes.

Prof. Paddy Regan Chair of Radionuclide Metrology,

Department of PhysicsUniversity 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

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

Radioactivity…..

The science of decay…

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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.

‘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.

14

61

Note, the number of 40K decays would then be equal to the number of 1461 keV gamma rays emitted, divided by the ‘branching ratio’ which is 0.1067 in this case.

Nuclei can also decay by emission..

ejection of a 4He nucleus….

Depends (again) on binding energies & masses

Before…

232Th, Z = 90 N = 142

After…

228Ra, Z = 88 N =140

4He, Z=2 N=2

Radioactive decays occur as a result ofconservation of mass/energy E=mc2

M(232Th) = 232.038055 u = mass / energy before alpha decay. M(4He) = 4.002603 u + M(228Ra) = 228.031070 u = mass after.

1 u = 1 atomic mass unit = 931.5 MeV/c2

mc2 = M(232Th) – [ M(228Ra) + M(4He)])c2

mc2=0.004382 uc2 = 4.08 MeV

4.08 MeV of ‘binding energy’ from 232Th is released in its decay to 228Ra by the emission of a 4He 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 228Ra nucleus (0.07 MeV).

Geiger-Nuttall rule links Q values to explain long lifetimes of 232Th, 238U 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.

238U 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.

238U 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 40K (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 bythe characteristic decay probability per secondfor that particular nuclear species ().

A = N

is related to the half-life of the radioactive species by = 0.693 / T1/2

One signature that a radioactive decay has taken place is the emission of gamma raysfrom 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.

226Ra

228Ac

40K

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:• 137Cs (T1/2 = 30 years)

• 131I (T1/2 = 8 days)

– Neutron-capture on fission products in reactors• 134Cs (T1/2 = 2 years)

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.