A. M. El-Khayatt, PHY464 Autumn 2014
Radiation Physics PHY471
A.M. El-Khayatt
Radiological physics is the science of ionizing radiation and its interaction with matter, with special interest in the energy thus absorbedJames E. Turner :Atoms, Radiation, and Radiation Protection
A. M. El-Khayatt, PHY464 Spring 2014
Introduction
• Radiological physics studies ionizing radiation and its interaction with matter.• Began with discovery of x-rays, radioactivity and radium in
1890s.• Special interest is in the energy absorbed in matter• Radiation dosimetry deals with quantitative determination of
the energy absorbed in matter.
A. M. El-Khayatt, PHY464 Spring 2014
Ionizing radiation
• By general definition ionizing radiation is characterized by its ability to excite and ionize atoms of matter
• Lowest atomic ionization energy is ~ eV, with very little penetration
• Energies relevant to radiological physics and radiation therapy are in keV – MeV range
A. M. El-Khayatt, PHY464 Spring 2014
Chapter 4:Radioactive Decay Activity
• Activity• The rate of decay → the number of atoms that decay per unit time
Bq 103.7ICi
,S 11Bq10
1
The traditional unit of activity is the curie (Ci), which was originally the activity ascribed to 1 g of 226Ra.
A. M. El-Khayatt, PHY464 Spring 2014
Exponential Decay
• The activity of a pure radionuclide decreases exponentially with time.
A. M. El-Khayatt, PHY464 Spring 2014
• Example: Calculate the activity of a 30-MBq source of after 2.5 d. What is the decay constant of this radionuclide?• Solution
Na2411
A. M. El-Khayatt, PHY464 Spring 2014
Specific Activity SA(Bqg-1)
The specific activity of a nuclide of half-life T (year) and atomic mass number A is therefore given by
Example: Calculate the specific activity of 226 Ra in Bq g-1.
This, by definition, is an activity of 1 Ci.
110
23226
.107.3
3600243651600226
1017.4)(
gBq
RaSA
Number of grams =A/SA
A. M. El-Khayatt, PHY464 Spring 2014
Example What is the specific activity of I4C(T=5730Y)?
Example: A sample contains 1 mCi of 191Os at time t = 0.
How many grams of 191Os are present at t = 0?
The mass of the sample, therefore, is
. 51.414
226
5730
1600)( 112 gCiCaSA
. 1049.4191
226
4.15
3651600)( 14191
gCi
day
dayOsSA
A. M. El-Khayatt, PHY464 Spring 2014
4.4 Serial Radioactive Decay
The activity of a sample in which one radionuclide produces one or more radioactive offspring in a chain.
Secular Equilibrium (T1
T2)At secular equilibrium ≥7 T2
1122
12
-20
-12
NN
e )e-(1 22
AA
AAA tt
A. M. El-Khayatt, PHY464 Spring 2014
A chain of n short-lived radionuclides can all be in secular equilibrium with a long-lived parent. Then the activity of each member of the chain is equal to that of the parent and the total activity is n + 1 times the activity of the original parent.
Transient Equilibrium (T1
T2)
112
22
12
AA
AA
At Transient equilibrium ≥7 T2
A. M. El-Khayatt, PHY464 Spring 2014
• Example: Starting with a 10.0-GBq (= 1010 Bq) sample of pure 90Sr at time t = 0, how long will it take for the total activity (90Sr + 90Y) to build up to 17.5 GBq?• Solution r 90
40)60(90
39)12.29(90
38 ZYSr hy
(T1 T2) → Secular Equilibrium
12 AA
,21 AAA
0 0at t )e-(1 20-
122 AAA t
128h t
)e-10(17.5
7.51017.5A
,202
60
0.693-
1
t
GBqAA
A. M. El-Khayatt, PHY464 Spring 2014
• Example• How many grams of 90Y are in secular equilibrium with 1 mg of 90Sr?
r 9040
)60(9039
)12.29(9038 ZYSr hy
The amount of 90Y will be that having the same activity as 1 mg of 90Sr. The specific activity, SA, of 90Sr (T1 = 29.12 y) is
Number of grams =A/SA
A. M. El-Khayatt, PHY464 Spring 2014
• Example page 111• A sample contains 1 mCi of 191Os at time t = 0. The isotope decays by
β– emission into metastable 191mIr, which then decays by γ emission into 191Ir. The decay and half-lives can be represented by writing
(a) How many grams of 191Os are present at t = 0?(b) How many millicuries of 191mIr are present at t = 25 d?(c) How many atoms of 191mIr decay between t = 100 s and t = 102 s?(d) How many atoms of 191mIr decay between t = 30 d and t = 40 d?
A. M. El-Khayatt, PHY464 Spring 2014
Natural Radioactivity• All of the heavy elements (Z > 83) found in nature are radioactive and decay
by alpha or beta emission.• The heaviest elements decay into successive radioactive daughters, forming
series of radionuclides that end when a stable species is produced.(n+2)The uranium series, for example, begins with 238U-92 andends with stable 206Pb-82 .(n+0)The thorium series, starting with 232 90Th and ending with 208Pb-82.(n+3) the actinium series, which begins with 235U-92 and ends with 207Pb-82.(n+1)neptunium series 237Np-93, has a half-life of 2.2 × 106 years, which is short on a geological time scale. Neptunium is not found in nature, but has been produced artificially, starting with 241Pu-93 and ending with 209Pb-82.
A. M. El-Khayatt, PHY464 Spring 2014
A. M. El-Khayatt, PHY464 Spring 2014
ExampleHow many alpha and beta particles are emitted by a nucleus of an atom of the uranium series, which starts as 238 92U and ends as stable 206 82Pb?SolutionNuclides of the four heavy-element radioactive series decay either by alpha or beta emission. A single disintegration, therefore, either (1) reduces the atomic number by 2 and the mass number by 4 or (2) increases the atomic number by 1 and leaves the mass number unchanged. Since the atomic mass numbers of 238 92U and 206 82Pb differ by 32, it follows that 8 alpha particles are emitted in the series. Since this alone would reduce the atomic number by 16, as compared with the actual reduction of 10, a total of 6 beta particles must also be emitted.
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