Physics and Radiobiology of Nuclear Medicine

It is defined as the thickness of the absorber that reduces the intensity of a
photon beam by one-half. Thus, an HVL of an absorber around a source of
g-radiations with an exposure rate of 150 mR/hr will reduce the exposure
rate to 75 mR/hr. The HVL depends on the energy of the radiation and the
atomic number of the absorber. It is greater for high-energy photons and
smaller for high-Zmaterials.
For monoenergetic photons, the HVL of an absorber is related to its
linear attenuation coefficient as follows:

(6.7)

Because mhas the unit of cm−^1 , the HVL has the unit of cm. The HVLs of
lead for different radionuclides are given in Table 6.2.
Another important quantity, tenth-value layer (TVL), is the thickness of
an absorber that reduces the initial beam by a factor of 10. It is given by

(6.8)

=3.32 HVL (6.9)

Problem 6.2
If the HVL of lead for the 140-keV photons of 99mTc is 0.03 cm of lead, cal-
culate the linear attenuation coefficient of lead for the 140-keV photons

=

2 .30

m

TVL=−

ln .() 01

m

HVL=

0 693.

m

Interaction of g-Radiations with Matter 67

Table6.2. Half-value layer values (HVLs) of lead for

commonly used radionuclides.

Radionuclides HVL, Lead (cm)* HVL, Water (cm)†

(^137) Cs 0.65 —
99mTc 0.03 4.6
(^201) Tl 0.02 —
(^99) Mo 0.70 —
(^67) Ga 0.10 —
(^123) I 0.04 —
(^111) In 0.10 —
(^125) I 0.003 1.7
(^57) Co 0.02 —
(^131) I 0.30 6.3
(^18) F 0.39 11.2

• Adapted from Goodwin PN. Radiation safety for patients
  and personnel. In: Freeman LM, ed.Freeman and Johnson’s
  Clinical Radionuclide Imaging. 3rd ed. Philadelphia: WB
  Saunders; 1984: 320.
  †HVL in water is considered equivalent to HVL in tissue.