RAD-04

Bragg–Gray cavity dose

D_med / D_gas = (S/ρ)_med / (S/ρ)_gas. Gas cavity in a medium.

Reading speed

DosimetryBragg–Gray

Governing equation

D_{med}=D_{gas}\dfrac{(S/\rho)_{med}}{(S/\rho)_{gas}}

$D_{gas} \in \left[0.01, 20\right]$

where

D_{gas}: Gas dose (Gy)
(S/\rho)_{med}: Medium stopping (MeV cm²/g)
(S/\rho)_{gas}: Gas stopping (MeV cm²/g)
D_{med}: Medium dose (Gy)

The figure, part by part

Each numbered marker wears the color of its card. Hover a card to light the marker. Tap a marker to jump to that card. The slider walks a red point along the curve; Animate drawing traces it like a pen.

00:0 / 00:08

①

Governing law — red

D_{med}=D_{gas}\dfrac{(S/\rho)_{med}}{(S/\rho)_{gas}}

$D_{gas} \in \left[0.01, 20\right]$

$D_{gas} = 0.01$ $D_{med} = 0.011$

Along the curve

②

Domain of the variable — blue

D_{gas}_{\min} = 0.01,\quad D_{gas}_{\max} = 20

$D_{gas} \in \left[0.01, 20\right]$

$D_{gas} = 0.01$ $D_{gas} = 0.01$

Along the curve

③

Response curve — green

D_{med}=D_{gas}\dfrac{(S/\rho)_{med}}{(S/\rho)_{gas}}

$D_{gas} \in \left[0.01, 20\right]$

$D_{gas} = 0.01$ $D_{med} = 0.011$

Along the curve

④

Rate of change — amber

\frac{\mathrm{d}D_{med}}{\mathrm{d}D_{gas}}

$D_{gas} \in \left[0.01, 20\right]$

$D_{gas} = 0.136$ $D_{med}^{\prime} = 1.111$

Along the curve

Lecture brief

Historical brief

Beer attenuation, Compton (1923), Klein–Nishina, Bragg–Gray cavity and KERMA are the transport of photons and charged particles in matter. The sheets compute fluence, kerma and stopping. This sheet (RAD-04 — Bragg–Gray cavity dose) is the form associated with Bragg–Gray. Working symbols: $D_{gas}$ , $(S/\rho)_{med}$ , $(S/\rho)_{gas}$ $\rightarrow$ $D_{med}$ . Requires charged-particle equilibrium across a small cavity. Spencer–Attix refines the cutoff.

Purpose

Purpose: compute

D_{med}

from

D_{gas}

(S/\rho)_{med}

(S/\rho)_{gas}

in Radiation physics via

D_{med}=D_{gas}\dfrac{(S/\rho)_{med}}{(S/\rho)_{gas}}

D_med / D_gas = (S/ρ)_med / (S/ρ)_gas. Gas cavity in a medium. Use it when a real radiation physics question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

D_{gas} = 1.000\,\mathrm{Gy}

(S/\rho)_{med} = 2.000\,\mathrm{MeV cm^{2}/g}

(S/\rho)_{gas} = 1.800\,\mathrm{MeV cm^{2}/g}

, the governing relation

D_{med}=D_{gas}\dfrac{(S/\rho)_{med}}{(S/\rho)_{gas}}

yields

D_{med} = 1.111\,\mathrm{Gy}

. A phantom, a tiny air cavity, two doses. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Gas dose D_{gas}1 GyMedium stopping (S/\rho)_{med}2 MeV cm²/gGas stopping (S/\rho)_{gas}1.8 MeV cm²/g

Outputs

Medium dose D_{med}1.111 Gy

Reading speed

Watch on YouTube

Free library

Full library

Free PDF / open book

YouTube channels

00:0 / 00:08

Narration of this film

A phantom, a tiny air cavity, two doses.

Requires charged-particle equilibrium across a small cavity. Spencer–Attix refines the cutoff.

Reading speed

Watch on YouTube