Helium-3 Neutron Detectors: Operation and Associated Measurement Systems

Features

• Neutron detection by (n,p) nuclear reaction
• Operating mode: proportional
• Very low amplitude charge pulses: 0.05 to 2 pC
• High voltage power supply depending on detector and associated electronics
• Wide gain charge amplifier (0.05 pF, i.e. 20 V.pC^-1, low noise (0.1 nC) and high resolving time (1 to 10 µs)
• Current pulse amplifier with short resolving time (50 ns)

Description

The pulse amplitude delivered by neutron Helium-3 detectors operating in proportional mode are extremely low (in the order of 0.005 to 2 pico Coulomb). They must be associated to high gain and low background amplifiers.

Charge amplifiers are generally used. For special applications, current pulse amplifiers may be preferred.
The operating high voltage of Helium-3 neutron detectors highly depends on the associated electronics (charge amplifier, current amplifier, gain value, choice of the discrimination threshold).
A typical high voltage at +50 V above the beginning of the high voltage plateau is recommended.

The thermal peak resolution (FWHM) depends on electronics (optimization with a slow charge amplifier such as ACHEM7F for example), purity of the refilling gas (absence of electronegative ions) and increases with the filling pressure and the detector diameter.

Additional heavy gas (Argon, CF4) reduces the trace length of ionizing particles, reduces the wall effects, improves the thermal peak resolution and reduces the charge collection time (improvement of characteristics with a fast amplifier, ACHNP97 for example).