[Halld-cal] FCAL radiation damage (fwd from Lubomir)

[Matthew Shepherd]

Hi all,

Thanks to Lubomir for the paper!

I think the relevant comparison is with the figure on slide 8 of Brad's talk that Lubomir references.  The RCS figure shows transmission through 4 cm of glass as a function of distance from the face.  As one goes further from the face the dose drops (the calorimeter shields itself) and therefore the transmission increases.  It is not clear what the 3-6 kRad dose corresponds to -- assuming this is 3-6 kRad at the front face then one assumes a transmission of about 85% through 4 cm of glass at 430 nm for 3-6 kRad.

Going to Brad's plot (slide 8) we see transmission through 1.4 cm of glass for several different wavelengths as a function of dose.  Unfortunately there is no line for 430 nm and 3-6 kRad is very near the y-axis.  But it is not crazy to think that if one had a line for 430 nm (which Brad should now add to his plot) one would get a transmission through 1.4 cm that is consistent with 85% through 4 cm.

The RCS paper is a nice cross check, but it would be really helpful to understand how their dose is measured.  By definition dose is energy deposited per unit mass.  The problem is that dose in the block is not uniform.  Any easy mistake is to estimate dose by simulating the total energy deposition in the block and then dividing by the mass of the block.  The problem is that this grossly underestimates the actual dose near the front face of the block where the low energy electromagnetic stuff gets absorbed.  The dose in the first 1 cm of glass may be orders of magnitude different from the dose in the last 1 cm of glass.

The plots on slides 10 and 11 that Lubomir refers to are plotting average transmission (averaged over the correct spectrum) through from some point (z) on the x-axis to the end of the block.  There is no way to compare these to the figure in the RCS paper.  (These figures are practical for implementing radiation damage in our HDGeant simulation.)

-Matt


On Dec 1, 2010, at 8:17 AM, Elton Smith wrote:

> 
> 
> ---------- Forwarded message ----------
> Date: Tue, 30 Nov 2010 17:56:43 -0500 (EST)
> From: Lubomir Pentchev <pentchev at jlab.org>
> To: Elton Smith <elton at jlab.org>
> Subject: FCAL radiation damage
> 
> 
> Hi Elton,
> 
> As we talked on the last calorimetry meeting, I was looking in the
> literature to find examples of radiation damage that can be compared to
> the Brad's results for FCAL. The RCS experiment at JLab used the same
> type of lead glass (TF-1), although produced in Yerevan. They estimated a
> total dose of 3-6kRad for the whole experiment. At the end they measured
> the glass transparency. Look at:
> 
> http://www1.jlab.org/Ul/Publications/documents/ACF6D6.pdf
> 
> Fig.22 at page 13. So, they had ~85% transparency at the front of the
> glass. Comparing this to the Brad's numbers for ~10kRad/year (at 10^8
> gammas/sec), at:
> 
> http://argus.phys.uregina.ca/gluex/DocDB/0016/001641/002/RHG-Fall2010.pdf
> 
> pages 10 and 11, one concludes that the Bard's calculations underestimate
> the radiation effects. Or maybe these plots are for 10^7 gammas/sec?
> 
> In any case, it will be useful to compare the Brad's calculations to
> the results of the above paper.
> 
> Lubomir