# Axial dose from an RPP source

Dear expert,

I have estimated dose at various locations from a RPP source whose only vertical 4 surfaces contain the source (uniform sampling on those 4 surfaces were carried out). I have estimated dose using the usrtrack output and using kerma conversion factors externally using a MATLAB program.

The dose at normal directions seems to be fine. I have placed detectors on two sides only (left and right face) and they are equal at equal distances (right and left detectors). But, the axial result is a little bit strange. Ideally it should be maximum at the centre , but it is little higher when y = 11 m and y = 15 m.

This also reflects in the fluence scoring. As I have checked the fluence scoring,

at location (x = -1 m, y = 10 m, z = 2 m), fluence = 2.3847031E-07 pr per cmq (22 sum)
at location (x = -1 m, y = 11 m, z = 2 m), fluence = 2.3971558E-07 pr per cmq (30 sum)
at location (x = -1 m, y = 15 m, z = 2 m), fluence = 2.5005934E-07 pr per cmq (31 sum)
Filter1.zip (49.7 KB)

Is there mistake in my understanding ?

Regards,
Riya

Dear Riya,

I donâ€™t see anything surprising in these results. The particle fluence increases as you are getting closer to the side â€śout_backâ€ť.

It is always good to add a USRBIN scoring with Cartesian coordinate system, to help visualize the result.

Cheers,
David

Thank you @horvathd , I will add usrbin with cartesian system. As you have mentioned this is normal, what I observed is, there is a peak in dose/fluence if I move towards the out_back, it is maximum at the location (-1, 15,2) (Det 31 output) . Due to symmetry, there should also be a peak at (-1, 5, 2) i.e. if I move towards the out_front ( Det 33 output). But here they are not nearly equal,

For example, fluence at (-1, 15, 2) is greater than fluence at (-1, 5, 2).

In other words, for left face, if I move towards right from the middle, there is a peak in fluence/ dose; where as if I move towards left from the middle, there is no peak and fluence/ dose is reducing.

Regards,
Riya

Dear Riya,

there are typos in your source routine. They should be:

``````*  Particle coordinates
XX = FLRNDM (XDUMMY)
``````

and

``````* SAMPLE FRONT SURFACE
ELSEIF ( XX .LT. 0.750D0)THEN
``````

It caused your source locations to be asymmetric.

Cheers,
David

1 Like

Thank you @horvathd , sorry for this mistake.

Regards,
Riya