Neutron Detector

This project started with the need for some means of verifying the presence of fast neutrons generated during fusor operation. As of April, 2007, the detector unit itself is finished and the HV/preamp electronics are working.

Description

The fast neutron detector, in testing

Detector unit

The detector itself is an ancient (manufactured some time in the late 1960s) boron trifluoride neutron detector tube. The tube itself is designed to detect thermal neutrons, but it is also marginally sensitive to gamma rays. A bias voltage of about 1700 Volts is applied between its central electrode (positive) and the outer wall (ground), and ionization events in the fill gas result in tiny and brief (under a microsecond long) current pulses, which are then amplified and processed. Neutrons generate ionization events via capture by, and subsequent alpha decay of, the boron nuclei.

Because the particles of interest in this case are fast (2.45 MeV) neutrons, the tube is enclosed in a moderator made from circular plates of high-density polyethylene (HDPE) plastic, which serves to absorb energy from incoming fast neutrons and slow them down to detectable energy levels.

HV bias supply, preamp

The tube is driven by an Eberline model IB-1, which generates the bias voltage and provides substantial signal amplification, with pulses a couple of Volts high for neutron detection events, and gamma noise floor at about 100mV.

Counting, pulse height analysis, etc.

Currently, a digital storage oscilloscope is used to capture and measure output pulses from the amplifier. In the future, a dedicated electronic system will perform pulse height discrimination and peak capture, and detection events will be recorded by a computer for analysis.

Parts sources and cost breakdown

Nuclear Chicago DN-3 neutron probe under disassembly

The detector tube was removed from a non-functional Nuclear Chicago model DN-3 neutron probe, purchased for $98.89 from a surplus dealer on Ebay. A component of the original probe housing was re-used to secure the tube to the moderator and shield the electrode lead.

The HDPE plastic was