radiation physics phy471 a.m. el-khayatt a. m. el-khayatt, phy464 autumn 2014 radiological physics...

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.


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


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.


Exponential Decay

• The activity of a pure radionuclide decreases exponentially with time.


• Example: Calculate the activity of a 30-MBq source of after 2.5 d. What is the decay constant of this radionuclide?• Solution

Na2411


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


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


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 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


• 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


• 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


• 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?


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.


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.

radiation physics phy471 a.m. el-khayatt a. m. el-khayatt, phy464 autumn 2014 radiological physics...

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