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.
-- P.S.At 2.2 MeV the specific energy losses from plastic, poly-ethylene, poly-vinyl, kapton polymide film are 1.8, 1.9, 1.8, 1.7 MeV cm2/gr and they are already rather flat with energy. Densities for the same materials are 1.0, 0.94, 1.2, 1.4 gr/cm3 I do not really know what our tape is made of, except that it is "plastic". With the above data I get energy losses/cm of the order of 1.8 Mev/cm to 2.2 MeV/cm. If I consider 10 mils (0.25 mm) of plastic material wrapping the scintillator, this attenuates the beta spectrum by ca. 0.05 MeV +- 10 %.
________________________________________________________________________________ Tancredi Botto, phone: +1-617-253-9204 mobile: +1-978-490-4124 research scientist MIT/Bates, 21 Manning Av Middleton MA, 01949 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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