# Keyword:DOSE

DOSE ndose {1} <dist> {0}

Dose rates are calculated for a semi-infinite slab of the material. This is the default if the keyword is not used or if ndose = 1, but if ndose = 2 then the calculations are done for a point source of 1 g of material at a distance of dist metres. dist is not used for the semi-infinite slab as the contact dose rate is always assumed. The minimum distance is 0.3 m; if a smaller value is specified then dist is set to 0.3 m and a message to this effect is printed.

An example of this keyword for a point source of 1 gram of the irradiated material at 1 metre is:

< -- Control phase -- >
...
FISPACT
* Title of the simulation
< -- Initial phase -- >
...
DOSE 2 1.0
...
< -- Inventory phase -- >
...


Note that the specific activity (Bq/kg), not the activity (Bq), is used to calculate dose rates, in both cases.

The surface gamma dose rate (Sv/hr), $D$ for a semi-infinite slab is calculated as below:

$D=C \frac{B}{2} \sum_{i=1}^{N_{\gamma}} \frac{\mu_{a}\left(E_{i}\right)}{\mu_{m}\left(E_{i}\right)} S_{\gamma}(E_{i})$

where:

$N_{\gamma}$ = number of energy groups in the $\gamma$ spectrum histogram

$E_{i}$ = mean energy of the $i^{th}$ energy group

$\mu_{a}$ = mass energy absorption coefficient ($\mu_{en}/\rho$) of air (m$^{2}$ kg$^{−1}$)

$\mu_{m}$ = mass energy attenuation coefficient ($\mu/\rho$) of the material (m$^{2}$ kg$^{−1}$)

$B$ = build up factor (= 2)

$S_{\gamma}$ = rate of $\gamma$ emission (MeV kg$^{−1}$ s$^{−1}$)

$C$ = 3.6 $\times$ 10$^{9}|e|$ converts (MeV kg$^{−1}$ s$^{−1}$) to (Sv/hr)

Whereas the dose rate (Sv/hr) from a point source in air, $D$, is calculated as below:

$D=C \sum_{i=1}^{N_{\gamma}} \frac{\mu_{a}}{4 \pi r^{2}} e^{-\mu\left(E_{i}\right) r} m_{s} S_{\gamma}(E_{i})$

where additional terms are defined as:

$m_{s}$ = mass of source (kg)

$r$= distance from source (m)

$\mu(E_{i})$ = energy attenuation coefficient of air (m$^{−1}$)

In both cases the emission rate, $S_{\gamma}$, is calculated using the specific activity, $A(t)$ (Bq/kg), as below:

$S_{\gamma}(E_{i}) = I_{i}A(t)$

where $I_{i}$ is the intensity of energy group $i$ (MeV).

Both equations are approximations suitable for FISPACT-II calculations, but it is noted that they may not be adequate for specific health physics problems.