# 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), [math]D[/math] for a semi-infinite slab is calculated as below:

[math] 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}) [/math]

where:

[math]N_{\gamma}[/math] = number of energy groups in the [math]\gamma[/math] spectrum histogram

[math]E_{i}[/math] = mean energy of the [math]i^{th}[/math] energy group

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

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

[math]B[/math] = build up factor (= 2)

[math]S_{\gamma}[/math] = rate of [math]\gamma[/math] emission (MeV kg[math]^{−1}[/math] s[math]^{−1}[/math])

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

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

[math] 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}) [/math]

where additional terms are defined as:

[math]m_{s}[/math] = mass of source (kg)

[math]r[/math]= distance from source (m)

[math]\mu(E_{i})[/math] = energy attenuation coefficient of air (m[math]^{−1}[/math])

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

[math] S_{\gamma}(E_{i}) = I_{i}A(t) [/math]

where [math]I_{i}[/math] is the intensity of energy group [math]i[/math] (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.