Hi Tancredi,
If indeed the thresholds are 1.4 MeV equivalent electron energy, then
your estimate of 5 MeV protons is about right.
However, an electron threshold of 1.4 MeV seems to me to be somewhat
inconsistent with what I remember of the "beam on" ADC spectra. The
ADC peak corresponding to minimum ionizing electrons is up around
channel 1500 (I don't remember the exact number) and should correspond
to 5 MeV energy loss. A threshold of 1.4 MeV is almost 1/3 of that and
should clearly be visible in the ADC spectra as a span of about 400
channels or so above pedestal with no counts in them. I don't remember
seeing anything like that. One should see where the source end point
is on the ADCs compared with minimum ionizing electrons.
John
On Tue, 21 Oct 2003, Tancredi Botto wrote:
>
> I investigated the issue of the TOF threshold with Michael.
>
> We used a Sr/Yr90 beta emitter with an end point of 2.2 MeV, which has a
> range in plastic < 1", i.e. less than the detector thickness. The source
> was placed on the back side of a tof, where there is no lead sheet, and
> very close to a light guide and PMT. For all practical purposes the source
> deposited up to 2.1 MeV in the scintillator, even after accounting for
> energy loss.
>
> On the scaler for a single PMT the source is clearly visible up to a PMT
> threshold of 45 mV, see plot of rate in Hz vs threshold in mV (top panel)
> The source activity is 0.56 uC, corresponding to a rate of 10 kHz into the
> detector (1/2 solid angle or 2pi) Assuming the pulse height to be linear
> with energy deposit, then our "normal" threshold for the TOF photo-tubes
> (30 mV) corresponds to the energy deposit of a 1.4 MeV electron.
>
> If I take and invert Calarco's correction to small energy loss for
> protons/electrons this threshold corresponds to a proton energy loss of
> (dE+1.3)/0.6 = 4.5 MeV !! (for dE=1.4 MeV e- eloss) If I look it up in
> Knoll, pg 225, I get a proton threshold of 5.0 MeV
>
> This is the threshold for a single PMT, when light attenuation can be
> effectively assumed to be negligible (since we are so close to the tube).
> Note that the effect of the beta source was visible on a single pmt rate
> only, but not on the top/bottom coinc rate.
>
> Therefore it seems possible that the TOF proton threshold is very high
> (few MeV). More detailled info after we take source spectra, hopefully
> including Co56.
>
> I am asking for some help from the TOF folks:
> scintillator material, attenuation length and Birks formula parameters
> for e- and protons for our material. According to Knoll the main reference
> here is "R.L. Craun, D.L. Smith NIM 80, 239 (1970)" to which I only have
> access on campus.
>
>
>
-- John R. Calarco Dept. of Physics Univ. of New Hampshire Durham, NH 03824 phone: (603)862-2088 FAX: (603)862-2998 email: calarco@unh.edu
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