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Product Spotlight: Neutron Pulse Monitor (Part 1)

Quaesta’s Neutron Pulse Monitor (NPM-3000) has several unique features that separate it from a standard preamplifier and allow it to be used in a wider range of applications. The NPM-3000 interfaces with proportional style gas tube neutron detectors. It includes a high voltage supply (3000V), low noise charge sensitive amplifier (CSA), pulse shaper, gain stage (1-20) and discriminator. These are components of a standard neutron pulse counter preamplifier. However, this is where the similarity ends.

Our Neutron Pulse Monitor is a digital device and has a built-in multichannel analyzer, which is more typically found in NIM rack equipment and high end spectrometers. In the NPM-3000, the multichannel analyzer (MCA) produces a histogram of counts versus pulse energy for the associated gas tube detector. The MCA adds tremendous value to our device because it makes device configuration fast and easy and allows for long term monitoring of the detector tube and electronics.

Generally, the MCA allows the device to be configured in minutes without the need for a neutron source. Setting up a gas tube style neutron detector and Neutron Pulse Monitor (or standard preamplifier) involves selecting optimal high voltage and discriminator settings that maximize neutron counts and minimize counts from noise and gamma rays. In particular the lower discriminator should be set to capture as much neutron signal as possible while eschewing noise in the low end of the tube’s energy spectrum.

For a traditional preamplifier without an MCA, discriminator values are typically determined using the so-called ‘plateau method’. With this method, the lower discriminator level is fixed and the high voltage is adjusted in steps over the operating range of the gas tube. For each voltage setting, the count rate is measured. A graph of count rate versus high voltage produces a curve with a plateau region that indicates low count rate sensitivity to voltage changes, and this is considered an optimal discriminator setting. Without a radiation source, the operator may have to wait many minutes at each voltage setting to obtain enough counts. Often the operator will make additional changes to electronic gain and discriminator values, and repeat the ‘plateau’ exercise. Setting up a neutron detector in the field can sometimes take hours using this technique.

The pulse height spectrum produced by our Neutron Pulse Monitor allows the discriminator levels (upper and lower) to be identified by visual inspection. With some knowledge of the classical spectra for different tube types, an operator can optimize discriminator settings in minutes with no need for a neutron source (see the Neutron Detection section of our About page for more information on this topic). In a lab environment with a large number of detectors, setup can be drastically more efficient and more accurate with the use of the built in MCA. In remote monitoring applications, portal calibration, and even low-counting lab setups, our NPM-3000 is invaluable as no neutron source or high count rate environment is needed for a fast and precise setup.

Check back soon for Part 2 focusing on the diagnostic benefits of our Neutron Pulse Monitor with built in multichannel analyzer. In the mean time please contact us with any further questions about our neutron detection systems or our Neutron Pulse Monitor.

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