Neutron Detectors

Neutron Detectors

Environmentally friendly neutron detectors use 10B and ZnS(Ag), both of which are harmless household chemicals.

The detectors do not use pressurized gases or corrosive chemicals. Nor do they rely on export-controlled materials such as 6Li.

Features

  • Powered via USB or other 5V supply.
  • No adjustments necessary.
  • No software needed to operate.
  • 3.3V and 5V, programmable-width pulse output.
  • Operate from -40°C to +60°C.
  • Optional read out via USB.
  • Can be equipped with LED for gain stabilization
  • Built-in Portal Monitor function that tracks background and alarms on a passing source
  • Built-in Sample − Background function to determine if a sample is more radioactive than background.

Efficiency vs Voltage

The neutron detection efficiency improves with applied operating voltage.

While we show the characteristic of a PMT-based neutron detector below, the behavior is quite similar for a detector that uses SiPM as the optical sensor.

Neutron count rate, in the presence of a 252Cf source, as a function of the applied high voltage.

Gamma-ray rejection

There is a trade off between the neutron detection efficiency and the gamma-ray rejection. We measure both, the response to a 252Cf source and to a 15mR/hr 60Co field as a function of applied high voltage. As can be seen in the graph, the application engineer can make the call to sacrifice a little efficiency and gain substantial gamma-rejection in return.

High voltage scan in 15mR/hr Co-60 field. (1): Total measured neutron rate; (2): Background due to stored neutron source; (3): Net count rate caused by the Co-60 sources.

PMT-based tubular detectors

The PMTN2K detectors are highly sensitive and convenient drop-in replacements for 3He-tubes. The detecting element is a 10B+ZnS(Ag) screen read out by a PMT. The neutron capture reaction is
10B + n → 4He + 7Li. Bridgeport Instruments' digital signal processing electronics and real-time pulse shape analysis efficiently separates PMT noise, cosmics and gamma-rays from true neutrons — with an outstanding gamma-ray rejection capability. All detectors will work well in a 10mR/hr 137Cs field, and even a 10mR/hr to 20mR/hr 60Co field. Smaller detectors can tolerate higher fields. Developers can adjust operation parameters, to balance neutron detection efficiency vs gamma-ray rejection.

The detectors emit a pulse with programmable width (>100ns) for every detected neutron. Neutron count rates can also be read via its USB interface.

The detectors use a hanging-rod design. This means that the PMT is gently pushed onto the detecting element (a 4.75cm diameter light guide rod) via a metal spring. However, the rod is mechanically fastened to the housing, and even when the detector is mounted with the electronics at the bottom end, the weight of the light guide rod does not add to pressure on the PMT. Hence the design is robust against moderate mechanical shock and vibration.

Performance

The R2D-NT detector efficiency strongly depends on the size and shape of the moderator. A wide moderator provides a higher efficiency. When using a narrow moderator, neutrons may simply fly past without having a chance to be moderated and then detected. Widening the moderator is a low-cost way of increasing the neutron detection efficiency of the system. We show neutron detection efficiencies in typical moderators. The sensitivities quoted are for operation in a 10mR/hr 137Cs field.

Portal Monitor Operation

The PMT-N2K detectors have an embedded ARM M0+ 32-bit processor programmed in C, with a built-in portal monitor capability.

It can evaluate count rates 20 times per second and issue an alarm when the count rate exceeds the background rate.

The unit continuously monitors the count rate and updates the measured background count rate to keep up with slow changes in the environment due to rain, wind and dust. For a portable system the background may slowly change with the terrain without causing an alarm.

The PMT-N2K performs a statistical analysis every time slice (50ms or longer) and alarms if the alarm probability exceeds a programmed threshold of typically 1:10k to 1:1000k (1k=1000)

The panel on the right shows the behavior in a high neutron background of about 100n/s.

A second source then passes nearby, momentarily increasing the count rate.

The PMT-N2K responds immediately with an alarm.

A more detailed description can be found in the user's manual .

PMT-N2K Alarm

Plot of Number of events, probability and alarm per 100ms time slice.

To create the above plot, we used the logger capability of the PMT-N2K. The two logger channels were set to 129 (input to the portal monitor data analysis) and 130 (-log10(probability that this is due to background)).

The alarm threshold was set at 1:1000 = 1e-4. This means that the unit alarms if the probability that the counts recorded in the last L=40(=4s) time slices can be attributed to background is less than 1.0e-4. In mathematical terms: P(n>N | mu)<1e-4. N is the number of counts observed in the last L time slices while mu is the number of expected background counts during the same time period.

Sample − Background Operation

The PMT-N2K detectors have an embedded ARM M0+ 32-bit processor programmed in C, with a built-in Sample vs Background capability.

Here the purpose is to measure the neutron radioactivity of samples.

First the user sets "active_bank=1" and counts the background until they achieve the desired accuracy as reported by the device.

Next the user sets "active_bank=0" and places a sample near the detector.

The PMT-N2K performs a statistical analysis every 1s and alarms if the alarm probability exceeds a programmed threshold of typically 1:10k to 1:1000k (1k=1000)

The panel on the right shows the behavior in a high neutron background of about 100n/s.

A second source then was placed nearby, which permanently increased the total neutron count rate.

The PMT-N2K measures the sample count rate and performs a statistical analysis every second.

Over time it becomes more and more certain that the added source does indeed increase the neutron count rate above the previously established background.

A more detailed description can be found in the user's manual .

PMT-N2K Alarm

Plot of probability and alarm updated every 1s.

To create the above plot, we used the logger capability of the PMT-N2K. The first logger channel was set to 131 (-log10(probability that this is due to background)). Consider -log10(probability) to be a measure of certainty that the sample has excess radioactivity. If it is more radioactive than background, our certainty will continue to rise with increasing measurement time.

The alarm threshold was set at 1:1000 = 1e-4. This means that the unit alarms if the probability that the counts recorded so far can be attributed to background is less than 1.0e-4. In mathematical terms: P(n>N | mu)<1e-4. N is the number of counts observed until now while mu is the number of expected background counts during the same time period.

Pricing

Part number 1 unit 100 units
PMT-N2K-48x2 $5800 $4200
PMT-N2K-24x2 $5300 $3850
PMT-N2K-12x2 $5300 $3850
Neutron detector part numbers and pricing in US Dollars. Call for other quantities. Add -LED for the optional LED-based gain stabilization

Downloads

www.bridgeportinstruments.com 11740 Jollyville Rd. Austin, TX 78759, phone: 512-533-9933.

© Bridgeport Instruments 2020