Physics and Engineering of Radiation Detection

11.2. Quantities Related to Dosimetry 613

Combining all of the above, we can reach at the following relation for the exposure
from a radiation source.

X ∝

λdNt

r^2

=Γ

λdNt

r^2

, (11.2.14)

wheretis the exposure time and Γ is generally known as thegamma constant.
Gamma constant is specific to the type of radioisotope and its values for most
isotopes are available in literature (see Table 11.2.3). The usual quoted units of Γ
areRcm^2 /M BqhandRcm^2 /mCih.

Table 11.2.3: Gamma constants and predominant decay modes other thanγ-decays
of some radioisotopes (4). All values are given inRcm^2 mCi−^1 h−^1.

Isotope (mode) Γ Isotope (mode) Γ Isotope (mode) Γ

227

89 Ac(β) 2.2

198

79 Au(β) 2.3

228

88 Ra(β) 5.1

124

51 Sb(β) 9.8

181

72 Hf(β) 3.1

106

44 Ru(β) 1.7

72

33 As(γ, e

+) 10.1 124

53 I(EC) 7.2

46

21 Sc(β)) 10.9

140

56 Ba(β) 12.4

130

53 I(β) 12.2

75

34 Se(EC) 2.0

7

4 Be(EC) 0.3

132

53 I(β) 11.8

110

47 Ag(EC, β) 14.3

47

20 Ca(β) 5.7

192

77 Ir(β) 4.8

22

11 Na(EC) 12.0

11

6 C(EC) 5.9

59

26 Fe(β) 6.4

24

11 Na(β) 18.4

134

55 Cs(EC, β) 8.7

140

57 La(β) 11.3

85

38 Sr(EC) 3.0

137

55 Cs(β) 3.3

28

12 Mg(β) 15.7

182

73 Ta(β) 6.8

38

17 Cl(β) 8.8

52

25 Mn(EC) 18.6

121

52 Te(EC) 3.3

57

27 Co(EC) 0.9

65

28 Ni(β) 3.1

187

74 W (β) 3.0

60

27 Co(β) 13.2

95

41 Nb(β) 4.2

88

39 Y(EC) 14.1

154

63 Eu(EC, β) 6.2

42

19 K(β) 1.4

65

30 Zn(EC) 2.7

72

31 Ga(β) 11.6

226

88 Ra(α) 8.25

95

40 Zr(β) 4.1