Transcript REGAN-Emanuel-June2013-FINAL

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
[email protected]
Outline of talk
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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
The Microscopic World…
•ATOMS ~ 10-10 m
•NUCLEI ~ 10-14 m
•NUCLEONS-10-15 m
•QUARKS ~?
Nuclear Isotopes
Not all atoms of the same chemical
element have the same mass (A)
Frederick Soddy (1911) gave the
name isotopes.
(iso = same ; topos = place).
Results for natural terrestrial krypton
Krypton, Z=36
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.
N = 42
0.4%
44
2.3
The magnet then separates the ions according
to mass since the bending radius is
r = (A/Q) x (v/B) Q = charge of ion &7
46
47 48
50
A is the mass of the isotope
11.6 11.5 57.0
17.3
Nuclear chart
Atomic Masses and Nuclear Binding Energies
M(Z,A) = mass of neutral atom of element Z and isotope A
energy
M(Z,A) m ( 11H ) + Nmn -
The binding energy is the
energy needed to take a nucleus
Bnuclear of Z protons and N neutrons apart
into A separate nucleons
Mass of Z protons
+ Z electrons + N
neutrons (N=A-Z)
= binding energy
(nuclear + atomic)
Mass of neutral atom
 MeV
 eV
9
Radioactivity…..
The science of decay…
increasing binding energy = smaller mass
increasing Z →
increasing Z →
A=125, odd-A
even-Z, odd-N
or odd-Z, even N
125Sn,
Z=50,
N=75
A=128, even-A
even-Z, even-N
or odd-Z, odd- N
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.
11
125
A=125, stable isobar is Te (Z=52, N=73); Even-A usually have 2 long-lived.
Mass (atomic mass units)
A=137 Mass Parabola
137Xe
83
137Ba
81
137Cs
82
b- decay: 2 types:
1) Neutron-rich nuclei (fission frags)
n → p + b- + n
2) Neutron-deficient nuclei (18F PET)
p → n + b+ + n
Nucleus can
be left in an
excited
configuration.
Excess energy
released by
Gamma-ray
emission.
‘signature’
1461 keV
1461
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.
gamma
Some (odd-odd) nuclei can decay by competing types of beta decay
(a) p → n + b- + n ; (b) n → p + b+ + n ; (c) p + e- → n+ v ).
Decay rate depends on energy released (Qb value) and
CONSERVATION OF ANGULAR MOMENTUM.
Big change in angular momentum and small Qb →long half-life.
Nuclei can also decay by a emission..
ejection of a 4He nucleus….
Depends (again) on binding energies & masses
Before…
After…
a
232Th,
Z = 90
N = 142
228Ra,
Z = 88
N =140
4He,
Z=2
N=2
Radioactive decays occur as a result of
conservation of mass/energy E=Dmc2
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
Dmc2 = M(232Th) – [ M(228Ra) + M(4He)])c2
Dmc2=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 (a 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 Qa 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.
a
•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 a 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 by
the characteristic decay probability per second
for that particular nuclear species (l).
A =lN
l is related to the half-life of the radioactive
species by l = 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.
226Ra
Different radionuclides are identified by
their characteristic gamma-ray energies.
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:
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•
137Cs
(T1/2 = 30 years)
131I (T
1/2 = 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.