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

[Lubomir Pentchev]

Hi Matt and Brad,

I talked to the RCS people, to understand how the 3-6 kRad were 
measured in their experiment. They had many blocks with PMTs directly 
connected to ADC, without any capacitors. They were monitoring 
continuously the shift of the pedestals, which is proportional to the 
energy deposition. This statement is valid for the e.m. particles above 
~few MeV, since it is the Cherenkov light that makes the signal. To 
calculate the Rads, they estimated the depth of the energy deposition 
based on the transparency measurements.

Looking at the Bard's plot at slide 8, it  is difficult for me to say 
whether there's an agreement with the RCS result. Not only because there's 
no line for 430nm and the thickness is 1.4cm instead of 4cm, but mainly 
because 3-6 kRad is at the very beginning of the range. Maybe Brad can 
make the plot that can be directly compared to the RCS result, just using 
that parametrization of the transmission function.

Lubomir

On Wed, 1 Dec 2010, Matthew Shepherd wrote:

>
> 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
>
>
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