•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!)
The Natural Decay Chain for 238U
Qa(210Po) = 5.41 MeV
Ea = 5.30 MeV
E(206Pb) = 0.11 MeV
T1/2 = 138 days.
‘Radium’
‘218At
=Radium B’
210Po
Radon
=‘Emanation’
=Radium ‘F’
C’
E
D
C
C’’
‘218Po
=Radium A’
Alpha decay can also leave daughter in excited states which
can then decay by (characteristic) gamma emission.
Other nuclides in the ‘background’
• Man-made radionuclides in the environment.
– Nuclear weapons tests / Chernobyl / Fukushima
• Fission fragment daughters such as
& 111Ag (recent NPL work)
•
137Cs, 90Sr, 131I,
236Np, 237Np, 239Pu, 240Pu, 241Am, 242Am
etc. (from
neutron capture reactions and decay on 235,8U in fuel)
or residues from weapons tests.
Fukushima…
UK press response….
Neutron-induced
Nuclear fission.
2 primary, fission
fragment nuclei.
A~135 and A~90
plus fast neutrons.
neutron
Uranium-235
nucleus
Uranium-235
nucleus plus a
neutron =
Uranium-236
Uranium-236
splits, releasing
energy.
e.g., 235U + n →236U* →137Xe + 97Sr + 2 neutrons + approx 200 MeV of energy.
and 97Sr are both radioactive…subsequently also decay, e.g.,
137Xe , T = 4 mins →137Cs, T =30 years →137Ba (stable)
1/2
1/2
137Xe
~ 200 MeV of energy released per fission event in a reactor.
~ 10-20 MeV of energy in the form of ‘decay heat’.
Reactor fuel still generates some heat after the fission stops….
http://www.intechopen.com/articles/show/title/decay-heat-and-nuclear-data
Nuclear Physics 101 (lecture 2)
• Beta (b-) decay changes a neutron into a proton
• This keeps A constant but increases Z by 1 ( Z → Z+1 ).
• For each A value, there is a preferred, most stable (N,Z) combination.
– 131Xe (Z=54, N=77, A=131) stable.
– 131I (Z=53, N=78, A=131) unstable and b- decays 131Xe.
• Radioactive nuclei can emit characteristic gamma rays which can be
used to identify each nuclear species.
• The number of radioactive nuclei decreases over time.
The time for radioactivity to reduce by x2 = the half-life (T1/2).
• Half-lives are radionuclide specific, range from fractions of seconds
to millions of years (‘physics’ dependent).
• Number of radioactive nuclei present / decaying decreases with time.
• For a single radionuclide this is given by the law of radioactive decay.
N(t) = N0 exp (- 0.693 * time / T1/2)
PROTON NUMBER, Z →
NEUTRON NUMBER, N →
Line of ‘constant A = N+Z
Isobars, e.g., 131Te, 131I, 131Xe
See http://www.nndc.bnl.gov/ensdf/
Nucleus
T1/2 (b-)
Daughter
131I
131Xe
8 days
132I
132Xe
2 hours
133I
133Xe
21 hours
134I
134Xe
53 mins
135I
135Xe
7 hours
136I
136Xe
83 seconds
137I
137Xe
25 seconds
Eg (keV)
365
668, 773
530
847, 884
1260, 1132
1313, 1321
1218, 601
Nucleus
T1/2 (b-)
Daughter
133Cs
STABLE
134Cs
134Ba
2 years
135Cs
135Ba
2 Myears
136Cs
136Ba
13 days
137Cs
137Ba
30 years
138Cs
138Ba
33 minutes
139Cs
139Ba
1.4 hours
140Cs
140Ba
1 minute
141Cs
141Ba
25 seconds
142Cs
142Ba
2 seconds
143Cs
143Ba
2 seconds
Eg (keV)
605, 795
819, 1048
662
1436, 463
1283, 627
602
190, 304
360, 1327
195, 233
Nucleus
137Ba
137Cs
137Xe
13IXe
131I
131Te
T1/2 (b-)
Daughter
Eg (keV)
137Ba
662
456
STABLE
30 years
4 minutes
STABLE
8 days
25 seconds
137Cs
131Xe
131I
365
190, 304
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 of a
reactor fuel leak…
(T1/2~2 years) can not be created by b- decay of
heavier A=134 fission fragments since 134Xe is stable.
134Cs
Presence of 134Cs is evidence for nuclear reactor waste.
134Cs
is made in reactors via (n,g) capture on stable 133Cs.
134Cs
is not present in nuclear weapons fallout.
X
(courtesy, Dr.Pieter Doornenbal, RIKEN, Japan)
Single half-life decays
1000
900
137Cs
(t1/2 = 30 years)
800
700
Relative
Activity
600
134Cs
500
(t1/2 = 2 years)
400
95Nb
300
(t1/2 = 64 days)
200
131I
(t1/2 = 8 days)
100
0
0
20
40
60
Time after t=0 (weeks)
80
100
Brown rice grown in Fukushima after the nuclear accident; measured
in radioactivity department at NPL using low-background HPGe dets
100 mL samples in U8 container, net weight 81 gram/sample
Jun Saegusa, Fukushima Environmental Safety Center, Japan Atomic Energy Agency
& Visiting Scientist at NPL
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 Environmental Safety Center, Japan Atomic Energy Agency
& Visiting Scientist at NPL
From Jun Saegusa, Fukushima Environmental Safety Center, Japan Atomic Energy Agency
& Visiting Scientist at NPL, Teddington.
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
Summary
• Radioactive material surrounds us from NORMS (U, Th decay series;
and nuclear weapons fallout (137Cs, 90Sr). It’s part of our daily lives.
40K)
• By measuring characteristic energy a and g decays we can identify what
and how much radioactive materials is present.
• Understanding the quantum nature of the nucleus allows us to interpret
these decays via nuclear energy level schemes.
• We can utilize novel detection technologies developed for fundamental
nuclear structure physics research and apply them to ‘routine’ industrial
metrological / medical measurements of radioactivity.