•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!)

Qa(210Po) = 5.41 MeVEa = 5.30 MeV E(206Pb) = 0.11 MeVT1/2 = 138 days.

‘218Po =Radium A’

‘218At =Radium B’

C

D

E

210Po=Radium ‘F’ Radon

=‘Emanation’

‘Radium’

C’

C’’

The Natural Decay Chain for 238U

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 137Cs, 90Sr, 131I, & 111Ag (recent NPL work)

• 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

Uranium-235nucleus

Uranium-235nucleus plus a neutron = Uranium-236

Uranium-236splits, releasing energy.

2 primary, fission fragment nuclei.A~135 and A~90 plus fast neutrons.

Neutron-induced Nuclear fission.

e.g., 235U + n →236U* →137Xe + 97Sr + 2 neutrons + approx 200 MeV of energy.

137Xe and 97Sr are both radioactive…subsequently also decay, e.g.,137Xe , T1/2= 4 mins →137Cs, T1/2=30 years →137Ba (stable)

http://www.intechopen.com/articles/show/title/decay-heat-and-nuclear-data

~ 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….

• 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)

Nuclear Physics 101 (lecture 2)

See http://www.nndc.bnl.gov/ensdf/

Line of ‘constant A = N+Z Isobars, e.g., 131Te, 131I, 131Xe

NEUTRON NUMBER, N →

PR

OT

ON

NU

MB

ER

, Z

→

Nucleus T1/2 (b-) Daughter Eg (keV)131I 8 days 131Xe 365132I 2 hours 132Xe 668, 773 133I 21 hours 133Xe 530134I 53 mins 134Xe 847, 884135I 7 hours 135Xe 1260, 1132136I 83 seconds 136Xe 1313, 1321137I 25 seconds 137Xe 1218, 601

Nucleus T1/2 (b-) Daughter Eg (keV)133Cs STABLE134Cs 2 years 134Ba 605, 795135Cs 2 Myears 135Ba -136Cs 13 days 136Ba 819, 1048137Cs 30 years 137Ba 662138Cs 33 minutes 138Ba 1436, 463139Cs 1.4 hours 139Ba 1283, 627140Cs 1 minute 140Ba 602 141Cs 25 seconds 141Ba 190, 304142Cs 2 seconds 142Ba 360, 1327143Cs 2 seconds 143Ba 195, 233

Nucleus T1/2 (b-) Daughter Eg (keV)

137BaSTABLE

137Cs 30 years 137Ba 662137Xe 4 minutes 137Cs 456

13IXe STABLE131I 8 days 131Xe 365

131Te 25 seconds 131I 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…

X

134Cs (T1/2~2 years) can not be created by b- 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,g) capture on stable 133Cs.

134Cs is not present in nuclear weapons fallout.

(courtesy, Dr.Pieter Doornenbal, RIKEN, Japan)

Single half-life decays

0

100

200

300

400

500

600

700

800

900

1000

0 20 40 60 80 100

Time after t=0 (weeks)

Rel

ativ

e A

ctiv

ity

131I (t1/2 = 8 days)

95Nb (t1/2 = 64 days)

134Cs (t1/2 = 2 years)

137Cs (t1/2 = 30 years)

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

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

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

Radioactivity in Fukushima Rice?

Japanese AEA inspected10 million bags of rice at 160 Inspection centres last year.

71 bags showed radiocesiumvalues which exceeded the reference level.(i.e. 99.9993 % were below this).

Rice above reference level notshipped out.

Source: 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. 2013Evaluations underway

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

Summary

• Radioactive material surrounds us from NORMS (U, Th decay series; 40K) and nuclear weapons fallout (137Cs, 90Sr). It’s part of our daily lives.

• 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.