Dose Course 2018_Flipping book

Alan E. Nahum

Energy Dependence

Cavity Theory

Intrinsic energy dependence

g sections the following two limiting cases have been analyzed:

Detector reading to the average dose to the material of the sensitive detecting element

D rs that are large compared to the electron ranges, and in which, there- E is approximately established (photon radiation only): section 2. det (Q) = F cal (Q)· M det (Q)

Ion chamber: F

=1 (W/e constant)

cal

TLD: TLD response per unit of dose varies between 5% and 15% for low energy photons rs that are small compared to the electron ranges and which there- as “sensers” of the electron fluence existing in the uniform medium Gray cavities): section 3.

Absorbed dose- energy dependence s involve measuring the dose from photon (or neutron) radiation that fall into neither of the above categories (see next section); for there is no exact expression for the ratio D med / D det . Burlin (1966) so-called “General Cavity Theory” to treat these cases approxi- osed a factor, which is a weighted mean f the s opping-power ratio nergy absorption coefficient ratio; this factor, slightly simplified

Relates the dose to the detector material to the dose to the medium

D

(Q) = f(Q) · D

(Q) =f

· D

(Q)

med

det

Q

det

Cavity Theory

det

det

æ è ç ç

d ö ø ÷ ÷ + - ( ) 1

æ è ç ç

ö ÷ ÷ ø

D

m

L

Burling

(3.29a)

det

en

D r

d

,

=

D

r

med

med

med

f(Q) is calculated by MC ighting actor which v ri s etween unity for small (or Bragg-Gray) ro for large cavities (or photon detectors). Burlin provided a formula ased on the exponential attenuation of the electron fluence entering ugh the wall (build-down), balanced by the exponential build-up of rated electron fluence: