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Kai
and responders,
Most
neutron detectors EXCEPT proton recoil types (gas proportional with hydrogen or
scintillation plastic or liquid) rely on thermalisation before detection, e.g.
He-3 and BF3, simply because the thermal cross-sections of He-3 and B-10 are so
much larger (~1/v). Stick a G-M probe into an intense thermal neutron beam
and the aluminium immediately starts activating via neutron capture (n,gamma)
and beta-decaying (accompanied by a 1779keV gamma) with a 2.25min halflife to
Si-28. Activation and beta decay approach an equilibrium after about 3
halflives in the beam. The prompt gamma cascade from the capture
contributes to a response while the probe is in the beam, but the beta-decay
part continues for several minutes after the probe has been taken out of the
beam. The response of a G-M probe is indirect and the cross-sections are
quite small. The thermal neutron response can be boosted by covering the
probe with large (thermal) cross-section material, e.g. Cd, In, etc.for the
capture gamma-rays.
Your (alpha,n) type
sources deliver neutrons of several MeV which will register in proton recoil
detectors or after thermalisation in other types, like He-3 or BF3.
The latter types, if used as neutron monitors, usually have a removable wax or
plastic cover, which acts as a moderator for fast neutrons. If a booster
foil is used in conjunction with an optimised moderator cover, it might be
possible to coax some weak neutron response out of the G-M probe.
Proper neutron dosimetry is quite complex and a
specialised field.
Chris
Hofmeyr
-----Original Message-----
From: Kai Kaletsch [mailto:eic@shaw.ca] Sent: 29 January 2003 09:39 To: RadSafe Subject: GM response to neutrons?[Scanned]
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