Neutron Detection

Survey Meters and Dosimeters

 

Pug-7N fast-slow neutron detector

 Model Pug 7N Fast-Slow Neutron Monitor The Neutron PUG-7N detects fast or slow neutrons by means of a thermal neutron scintillation detector and a moderator.

Technical Associates PUG-7 with fast neutron detector

 Model Pug 7 with Fast Neutron Detector Fast Neutron Scintillation Probe insensitive to gamma radiation in fields below 100 R/hr. The n-p reaction is used to measure energy deposited by neutrons.
Technical Associates PUG-7 with slow neutron detector
 Model Pug 7 with Slow Neutron Detector Thermal neutrons are detected by means of the boron n-alpha reaction. Probe delivers approximately 60 cpm per neutron/cm2/second and requires a 900 volt supply.
NeutronRAE Neutron - Gamma Dosimeter
NeutronRAE II Gamma-Neutron Dosimeter The NeutronRAE II is personal radiation detector that provides rapid detection of both gamma and neutron radiaton sources in robust environments.

 


 

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notes:

For neutron detection, we provide survey meters and personal dosimeters. An effective neutron detection system comprises of the effective detection of neutrons entering a well-positioned detector. There are two key aspects to effective neutron detection: hardware and software. Neutron detection hardware refers to the type of neutron detector and to the instrumentation used in the neutron detection system. The most common probe used in neutron detection is the neutron scintillation probe. The other common probes are H-3 and BF-3 gas probes. The hardware configuration defines key experimental parameters, such as source-detector distance, solid angle and detector shielding. Neutron detection software consists of analysis tools for graphical analysis to measure the quantity of neutrons and the energies of neutrons striking the detector.

Neutron detection exists at nuclear reactors and where californium-252 is used as a neutron source. Neutron detection and neutron shielding is a necessity at any facility that generates nuclear fission. Neutron detection is used for varying purposes. Each application has different requirements for the neutron detection system.

Reactor instrumentation: Neutron flux is an important measure of power in nuclear power and research reactors. Boiling water reactors may have dozens of neutron detectors, one per fuel assembly. Most neutron detectors used in nuclear reactors are optimized to detect thermal neutrons.
Particle physics: Neutron detection is proposed as a method of enhancing neutrino detectors.
Materials science: Elastic and inelastic neutron scattering enables to characterize the morphology of materials from scales ranging from Angtroems to about one micron.
Radiation safety: Neutron radiation is a product of nuclear fission reactors. Neutron detectors are used for radiation safety to determine the energies of neutron radiation to help determine the potential degree of damage to human tissue.
Cosmic ray detection: Secondary neutrons are one component of particle showers produced in Earth's atmosphere by cosmic rays. Dedicated ground-level neutron detectors are used to monitor variations in cosmic ray flux.

Obstacles to Neutron Detection

Neutron radiation is not easily detected. Common challenges in neutron detection include background noise, high detection rates, neutron neutrality, and low neutron energies.

Background noise
The main components of background noise in neutron detection are high-energy photons, which aren’t easily eliminated by physical barriers. The other sources of noise, such as alpha and beta particles, can be eliminated by various shielding materials, such as lead, plastic, thermo-coal, and other mediums. Photons cause major interference in neutron detection, because it is uncertain if neutrons or photons are being detected by the neutron detector. Both neutron and gamma radiation register similar energies after scattering into the detector from the target or ambient light and are difficult to distinguish. Some of our detectors include gamma rejection to alleviated this problem. Coincidence detection can be used to discriminate real neutron events from photons and other radiation.

High Neutron Detection Rates
Because the neutron detector lies in a region of high beam activity, it is bombarded with neutrons and background noise. This confuses collected data with an extreme overlap in measurement. Separate events are not easily distinguished from each other. Part of the challenge consists of designing a detector that can block background photons.

Neutral charge of Neutrons
Neutrons are neutral and thus do not respond to electric fields. This makes it difficult to direct their path toward a detector to initiate neutron detection. Furthermore, neutrons do not ionize atoms except by direct collision; gaseous ionization detectors are ineffective.

Varying Energy
Detectors relying on neutron absorption are generally more sensitive to low-energy thermal neutrons, and are much less sensitive to high-energy neutrons. Conversely scintillation detectors have trouble registering the impacts of low-energy neutrons.

Gas detectors

Gaseous ionization detectors can be adapted to detect neutrons. While neutrons do not typically generate ionization, the addition of a nuclide with high neutron cross-section allows the detector to respond to neutrons. Nuclides commonly used for this purpose are boron-10, uranium-235 and helium-3. Further refinements are usually necessary to isolate the neutron signal from the effects of other types of radiation. As elemental boron is not gaseous, neutron detectors containing boron use boron trifluoride (BF3) enriched to 96% boron-10.

With common neutron detection systems, the incoming particles, comprising neutrons and photons, strike the neutron detector. Most often, the neutron probe is a scintillation detector consisting of scintillating material, a waveguide, and a photomultiplier tube (PMT). The neutron scintillation detector will be connected to a data acquisition (DAQ) system to register detection details.

The detection signal from the neutron detector is connected to a scaler unit, gated delay unit, trigger unit and the oscilloscope. The scaler unit is used to count the number of incoming particles or events. The scaler increments its tally of particles as it detects a surge in the detector signal from the zero-point. Low dead time prevents failure to detect fast neutron particles. The trigger unit coordinates the electronics of the system and provides a logical high to these units when the neutron detection system is ready to record an event run.

The oscilloscope registers a current pulse with every event. The pulse is the ionization current in the detector caused by this event plotted against time. The total energy of the incident particle can be found by integrating this current pulse with respect to time to yield the total charge deposited at the end of the PMT. This integration is carried out in the analog-digital converter (ADC). The total deposited charge is a direct measure of the energy of the ionizing particle (neutron or photon) entering the neutron detector. This signal integration technique is an established method for measuring ionization in the detector in nuclear physics. The ADC has a higher dead time than the oscilloscope, which has limited memory and needs to transfer events quickly to the ADC. Thus, the ADC samples out approximately one in every 30 events from the oscilloscope for analysis. Since the typical event rate is around 106 neutrons every second, this sampling will still accumulate thousands of events every second.

Separating neutrons from photons

The ADC sends its data to a DAQ unit that sorts the data in presentable form for analysis. The key to further analysis lies in the difference between the shape of the photon ionization-current pulse and that of the neutron. The photon pulse is longer at the ends (or "tails") whereas the neutron pulse is well-centered. This fact can be used to identify incoming neutrons and to count the total rate of incoming neutrons. The steps leading to this separation are gated pulse extraction and plotting-the-difference.

Gated pulse extraction

Ionization current signals are all pulses with a local peak in between. Using a logical and gate in continuous time, the tail portion of every current pulse signal is extracted. This gated discrimination method is used on a regular basis on liquid scintillators. The gated delay unit is precisely to this end, and makes a delayed copy of the original signal in such a way that its tail section is seen alongside its main section on the oscilloscope screen. After extracting the tail, the usual current integration is carried out on both the tail section and the complete signal. This yields two ionization values for each event, which are stored in the event table in the DAQ system.

Plotting the difference

In this step lies the crucial point of the analysis: the extracted ionization values are plotted. Specifically, the graph plots energy deposition in the tail against energy deposition in the entire signal for a range of neutron energies. Typically, for a given energy, there are many events with the same tail-energy value. In this case, plotted points are simply made denser with more overlapping dots on the two-dimensional plot, and can thus be used to eyeball the number of events corresponding to each energy-deposition. A considerable random fraction (1/30) of all events is plotted on the graph.

If the tail size extracted is a fixed proportion of the total pulse, then there will be two lines on the plot, having different slopes. The line with the greater slope will correspond to photon events and the line with the lesser slope to neutron events. This is precisely because the photon energy deposition current, plotted against time, leaves a longer "tail" than does the neutron deposition plot, giving the photon tail more proportion of the total energy than neutron tails.

The effectiveness of any neutron detection analysis can be seen by its ability to accurately count and separate the number of neutrons and photons striking the detector. Furthermore, the effectiveness of the second and third steps reveals whether event rates in the experiment are manageable. If clear plots can be obtained in the above steps, allowing for easy neutron-photon separation, the detection can be termed effective and the rates manageable. On the other hand, smudging and indistinguishability of data points will not allow for easy separation of events.

Rate control

Detection rates can be kept low in a variety of methods. Sampling of events can be utilzied to choose only a few events for analysis. If the rates are so high that one event cannot be distinguished from another, physical experimental parameters (shielding, detector-target distance, solid-angle, etc.) can be manipulated to give the lowest rates possible and yield more distinguishable events.