Nuclear Spectroscopy: From Natural Radioactivity …...λ is related to the half-life of the...

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Nuclear Spectroscopy: From Natural Radioactivity to Studies

of the Most Exotic Isotopes. Paddy Regan

Department of Physics University of Surrey Guildford, GU2 7XH

p.regan@surrey.ac.uk &

Radioactivity Group National Physical Laboratory

Teddington, TW11 0LW Paddy.regan@npl.co.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?

•ATOMS ~ 10-10 m

•NUCLEI ~ 10-14 m •NUCLEONS-10-15 m

•QUARKS ~?

The Microscopic World…

Darmstadtium

Roentgenium Copernicium

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% 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

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?

Rapid-neutron capture path discussed, but only theoretical; nuclei through which the path ran were completely out of experimental reach…do they even exist?

How do you make ‘radioactive’ nuclei?

– Heavy-ion fusion evaporation

• makes ‘neutron-deficient’ nuclei at high angular momentum.

– High-energy projectile fragmentation: • makes neutron-deficient and neutron rich nuclei at medium spins.

– (Neutron-induced) fission of heavy nuclei (such as 235U)

• Makes neutron-rich nuclei at medium angular momentum.

= 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) ≈ Ζm ( 11H ) + Nmn - Bnuclear

The binding energy is the energy needed to take a nucleus of Z protons and N neutrons apart into A separate nucleons

Mass of Z protons + Z electrons + N neutrons (N=A-Z)

Mass of neutral atom

Nuclear chart

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=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 → increasing Z →

125Sn, Z=50, N=75

125Xe, Z=54, N=71

‘signature’

1461 keV

Some (odd-odd) nuclei can decay by competing types of beta decay (a)p → n + β− + ν ; (b) n → p + β+ + ν ; (c) p + e- → n+ v ).

Decay rate depends on energy released (Qβ value) and

CONSERVATION OF ANGULAR MOMENTUM. Big change in angular momentum and small Qβ →long half-life.

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.

(Heavy) nuclei can decay by α 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 of conservation of mass/energy E=∆mc2

M(204Hg) = 203.973493 u M(4He) = 4.002603 u M(200Pt) = 199.971440 u 1 u = 1 atomic mass unit = 931.5 MeV/c2

∆ mc2 = M(204Hg) – [ M(200Pt) + M(4He)])c2

∆mc2 = (203.973494 - 199.971440 - 4.002603 ) uc2

∆mc2 = -0.00055 uc2 = -0.511 MeV i.e. it requires an additional 511 keV of ‘energy’ to release an alpha particle from 204Hg (which is stable). Alpha decay of 204Hg is energetically forbidden.

Alpha decay masses show shell effect at N=126…

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

What is NORM? • Naturally Occurring Radioactive Materials

• Two main sub-groups…

– Cosmogenic (from cosmic ray interactions) • 14C (from 14N(n,p)14C), 7Be, 26Al

– Primordial (i.e. very old, here when the earth

formed) • Single nuclei (e.g., 40K, 138La,…) • Decay chains (232Th, 235U, 238U/226Ra)

Natural decay ‘chains’. Sequences of α and β decaying radioisotopes from Uranium (Z=92) or Thorium (Z=90) to Lead (Z=82). On earth since formation….isotopic ratios (e.g. for 235/238U) used to age the earth.

•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 α 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’.

Relevance to nuclear astrophysics?

• 210Po terminate the s-process (α decay T1/2=138 days)

• Existence of 235,238U, 232Th evidence for explosive r-process nucleosynthesis.

• Abundance ratios and half-lives used to estimate age of earth/solar system (~5,0000 million years).

• Presence of 244Pu (T1/2~80 million years) could be used to infer presence of nearby supernovae / r-process events?

Other nuclides in the ‘background’ • Man-made (‘anthropogenic’) radionuclides in

the environment.

– Nuclear weapons tests / Chernobyl / Fukushima • Fission fragment daughters such as 137Cs, 90Sr, 131I

• 241Am, decays to 237Np, which has a very long half-

life; can be used as an erosion tracer etc.

• 239Pu from neutron capture on 238U in fuel

• Neutron capture products (e.g., 134Cs)

‘Anthropogenic’ (= man made) Radiation…. Mostly 137Cs and 90Sr (fission fragments)

How do you measure the gammas?

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 (λ). A = λ N λ is related to the half-life of the radioactive species by λ = 0.693 / T1/2 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.

Fukushima?

• Big change in the ‘background’ gamma-ray spectra…expect other radionuclides present, specifically 134Cs (and briefly 131I) etc.

Evidence of 131I decay, formed either directly and/or from decays of A=131 precursors, is the 364 keV gamma-ray from an excited state in the 131Xe daughter nucleus. If production stops, activity should decay away with 8 day half-life.

Look for signature gamma ray of 131I decay (365 keV) in various samples.... such as Vancouver rainwater. Obvious effect of 8 day half-life of this particular activity as the 131I decays to form the (stable) 131Xe.

134Cs…a smoking gun…

134Cs (T1/2~2 years) can not be created by β- decay of heavier A=134 fission fragments since 134Xe is stable. Presence of 134Cs is evidence for nuclear reactor waste. 134Cs is made in reactors via (n,γ) capture on stable 133Cs. 134Cs is not present in nuclear weapons fallout.

100 mL samples in U8 container, net weight 81 gram/sample

Brown rice grown in Fukushima after the nuclear accident; measured in radioactivity department at NPL using low-background HPGe dets

Jun Saegusa, Fukushima Environmental Safety Center, Japan Atomic Energy Agency & Visiting Scientist at NPL

604 keV: 5702 counts, 661 keV (137Cs): 11643 counts, 795 keV: 3987 counts, 1461 keV (from 40K): 602 counts. 80,000 sec measurements on 05 Sep. 2013 Evaluations underway

From Jun Saegusa, Fukushima Environmental Safety Center, Japan Atomic Energy Agency & Visiting Scientist at NPL, Teddington.

Radioactivity in Fukushima Rice?

Japanese AEA inspected 10 million bags of rice at 160 Inspection centres last year. 71 bags showed radiocesium values which exceeded the reference level. (i.e. 99.9993 % were below this) Rice above reference level not shipped out.

Source: Jun Saegusa, Fukushima, Japan Atomic Energy Agency & Visiting Scientist at NPL

A few new physics examples….

‘isospin’ impurity in beta decay of 0+ decaying ground state of N=30 ; Z=32 nucleus 62Ge30 to N=31 ; Z=31 nucleus 62Ga31 is an important Input (correction) into the standard. 0+ → 0+ is ap ure Fermi superallowed beta Decay, 0+ →1+ decays are allowed Gamow-Teller decays which the transition rate has to be corrected for, Check unitary of the CKM matrix (in the conserved vector current hypothesis).

K-electrons

L-electrons

T1/2 = 10.4 s 205Au126

How are the heavy elements made ? Is it via the Rapid Neutron Capture (R-) Process ?

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=126 N=82

• Need to look at N=82 and 126 ‘exotic’ nuclei in detail….

First excited state in (most) even-N AND even-Z has Iπ=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 Iπ=0+ ;

~2 ∆ ∆ = ‘pair gap’

Even-Even Nuclei

Evidence for nuclear shell structure….. energy of 1st excited state in even-even nuclei….E(2+).

Excitation energy (keV)

Ground state Configuration. Spin/parity Iπ=0+ ; Ex = 0 keV

PHR, Physics World, Nov. 2011, p37

Protons and neutrons are Fermion particles (i.e., have intrinsic angular momentum projection of ½ ħ). The Pauli exclusion principle means that these are then arranged into ‘discrete, quantised’ energy levels. Basic (1950s) model of nuclear structure assumes a central, spherical ‘mean’-field with (empirical) correction terms for spin-orbit splitting etc.

V= SHO + l2.+ l.s.

1h11/2

1i13/2 Independent particle model (from 1950s) Protons & neutron (fermions) fill orbits by PEP. Each j=l+s level has 2j+1 projections of mj e.g., g9/2 orbit can have [(2 x 9/2)+1 ] = 10 protons with mj from -9/2, -7/2,…..,,+7/2, +9/2. Clustering of levels causes energy ‘gaps’ leading to MAGIC NUMBERS. Can ‘approximate the average field experienced by each nucleon by e.g., as: H = HO + al.l + bl.s Changes in the Hamiltonian alters the level ordering. The magic numbers can ‘change’ for exotic N/Z ratios

Evidence for a N=82,126 shell quenching ?

Assumption of a N=82, 126 shell quenching leads to a considerable improvement in the global abundance fit in r-process calculations !

mass number A

exp. pronounced shell gap shell structure quenched

Proton-hole – neutron particle interactions around 208Pb? Virtually no data on excited states in Z<82, N>126 nuclei….

R(4/2) = 3.33 for ideal rotor; 2.0 for ideal vibrator <2.0 for ‘seniority’

Evidence for three-body forces in nuclei ?

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.

Plenty more to look at….

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 heavy nuclei with

‘diffuse neutron skins’ ?

• FAIR will increase dramatically our reach of nuclear species for experimental study

Nuclear Spectroscopy: From Natural Radioactivity …...λ is related to the half-life of the...

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