[Gate-users] Impossible (or Improbable ) timestamps for Randoms Coincidences

[KB]

Thanks for the message, Paul.

I noticed that the timing for the lower end of the coincidences has a sharp
cut off as one would expect. the problem is with the upper end. So if one
were interested in count rates, I wonder what is the right method is to
treat the randoms (with timing greater than the coincidence window):

a) ignore them by imposing a sharp cut off in the root analysis file.
b) Use all the parameters (detection position, energy etc) but assign a
random time to make it fall within the coincidence window.

I list these obvious steps (a and b ) because it seems the no of events with
the 'wrong' time is significant (about 8-9% for the NEMA scatter phantom
with 2.2 mCi).

Total Randoms              = 362852
Randoms with TOF > 0  = 31539       (the wrong timing ones).

So is the 'correct' no of randoms (362852-31539) or 362852.

Thanks.
Karthik.

On Mon, Jun 30, 2008 at 2:15 PM, Paul Vaska <vaska at bnl.gov> wrote:

> We have seen this as well and it seems like a bug.  It looks like the
> coincidence processor is using a different time than the time reported in
> the output data.  For example, if the coincidence code uses the exact event
> time and the output code reports the event time after time resolution
> blurring, then you can have events like this.  But we've not proven that
> this is the specific cause.
>
> Paul
>
> Paul Vaska, Ph.D.
> Medical Department, Building 490
> Brookhaven National Laboratory
> Upton, NY 11973
> vaska at bnl.gov (631)344-6228 fax:(631)344-4146
>
>
>
>
> On Jun 30, 2008, at 1:45 PM, KB wrote:
>
>  Hello all.
>>
>>    Plotting (time1-time2) for all the randoms coincidences, I notice that
>> the lower value is a sharp cutoff (just as I expect) but the higher value is
>> over and beyond the coincidence window. With my coincidence window set to 6
>> ns, I see (time1-time2) values as high as 89 ns.
>>
>> I have plotted (time1-time2) and uploaded the plot here: http://
>> img337.imageshack.us/img337/4598/gatetalkhm9.gif
>>
>> The relevant lines from my digitiser are:
>>
>> /gate/digitizer/Coincidences/setWindow 6.0 ns
>> /gate/digitizer/Coincidences/minSectorDifference 7
>> /gate/digitizer/Coincidences/MultiplesPolicy takeAllGoods
>> /gate/digitizer/name delay
>> /gate/digitizer/insert coincidenceSorter
>> /gate/digitizer/delay/setWindow 6.0 ns
>> /gate/digitizer/delay/setOffset 500.0 ns
>> /gate/digitizer/delay/MultiplesPolicy  takeAllGoods
>>
>>
>> I am interested in knowing why I get randoms coincidences beyond the
>> coincidence window. Thanks for any help.
>>
>> Karthik.
>> <GATE_Talk.jpg>
>> _______________________________________________
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>> Gate-users at lists.healthgrid.org
>> http://lists.healthgrid.org/mailman/listinfo/gate-users
>>
>
>
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