Neutron detection

Neutron detection is the effective detection of neutrons entering a well-positioned detector. There are two key aspects to effective neutron detection: hardware and software. Detection hardware refers to the kind of neutron detector used (the most common today is the scintillation detector) and to the electronics used in the detection setup. Further, the hardware setup also defines key experimental parameters, such as source-detector distance, solid angle and detector shielding. Detection software consists of analysis tools that perform tasks such as graphical analysis to measure the number and energies of neutrons striking the detector.

Experiments that make use of this science are typically scattering experiments whose scattered particles of interest are neutrons. Perhaps the most noteworthy among these experiments is the trademark experiment of the European Muon Collaboration, first performed at CERN and now termed the "EMC experiment." The same experiment is performed today with more sophisticated equipment to obtain more definite results related to the original EMC effect.

Neutron detection is also used at nuclear reactors and where californium-252 is used as a neutron source. Neutrons are a fundamental part of any experiment or technique involving nuclear fission, and thus detection of neutrons is an important part of the radiation protection strategy of such establishments.

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Applications

Neutron detection is used for varying purposes. Each application has different requirements for the 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 has been proposed as a method of enhancing neutrino detectors.^[1]

• Radiation safety: Neutron radiation is a hazard in nuclear reactors. Neutron detectors used for radiation safety must take into account the way damage caused by neutrons varies with energy.

• Cosmic ray detection: Secondary neutrons are one component of particle showers produced in Earth's atmosphere by cosmic rays. Dedicated ground-level neutron detectors, namely neutron monitors, are employed to monitor variations in cosmic ray flux.

Challenges in neutron detection

Neutron detection is not an easy science. The major challenges faced by modern-day 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, etc. Thus, photons cause major interference in neutron detection, since it is uncertain if neutrons or photons are being detected by the neutron detector. Both register similar energies after scattering into the detector from the target or ambient light, and are thus hard to distinguish. Coincidence detection can also be used to discriminate real neutron events from photons and other radiation.

High detection rates

Since the detector lies in a region of high beam activity, it is hit continuously by neutrons and background noise at overwhelmingly high rates. This obfuscates collected data, since there is extreme overlap in measurement, and separate events are not easily distinguished from each other. Thus, part of the challenge lies in keeping detection rates as low as possible and in designing a detector that can keep up with the high rates to yield coherent data.

Neutrality of neutrons

Neutrons are neutral and thus do not respond to electric fields. This makes it hard to direct their course towards a detector to facilitate detection. Neutrons also do not ionize atoms except by direct collision, so gaseous ionization detectors are ineffective.

Varying behavior with energy

Detectors relying on neutron absorption are generally more sensitive to low-energy thermal neutrons, and are orders of magnitude less sensitive to high-energy neutrons. Scintillation detectors, on the other hand, 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 cause 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.

Experimental setup and method

Figure 1 shows the typical main components of the setup of a neutron detection unit. In principle, the diagram shows the setup as it would be in any modern particle physics lab, but the specifics describe the setup in Jefferson Lab (Newport News, Virginia).

In this setup, the incoming particles, comprising neutrons and photons, strike the neutron detector; this is typically a scintillation detector consisting of scintillating material, a waveguide, and a photomultiplier tube (PMT), and will be connected to a data acquisition (DAQ) system to register detection details.

The detection signal from the neutron detector is connected to the scaler unit, gated delay unit, trigger unit and the oscilloscope. The scaler unit is merely used to count the number of incoming particles or events. It does so by incrementing its tally of particles every time it detects a surge in the detector signal from the zero-point. There is very little dead time in this unit, implying that no matter how fast particles are coming in, it is very unlikely for this unit to fail to count an event (e.g. incoming particle). The low dead time is due to sophisticated electronics in this unit, which take little time to recover from the relatively easy task of registering a logical high every time an event occurs. The trigger unit coordinates all the electronics of the system and gives a logical high to these units when the whole setup is ready to record an event run.

The oscilloscope registers a current pulse with every event. The pulse is merely 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.^[2] 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,^[3] 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.^[3] This fact can be used to identify incoming neutrons and to count the total rate of incoming neutrons. The steps leading to this separation (those that are usually performed at leading national laboratories, Jefferson Lab specifically among them) 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 (having a stream of "1" and "0" pulses as one input and the current signal as the other), the tail portion of every current pulse signal is extracted. This gated discrimination method is used on a regular basis on liquid scintillators.^[4] 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 detection analysis can be seen by its ability to accurately count and separate the number of neutrons and photons striking the detector. Also, 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 many ways. Sampling of events can be used 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 thus distinguishable events.

Finer detection points

It is important here to observe precisely those variables that matter, since there may be false indicators along the way. For example, ionization currents might get periodic high surges, which do not imply high rates but just high energy depositions for stray events. These surges will be tabulated and viewed with cynicism if unjustifiable, especially since there is so much background noise in the setup.

One might ask how experimenters can be sure that every current pulse in the oscilloscope corresponds to exactly one event. This is true because the pulse lasts about 50 ns, allowing for a maximum of 2×10⁷ events every second. This number is much higher than the actual typical rate, which is usually an order of magnitude less, as mentioned above.^[3] This means that is it highly unlikely for there to be two particles generating one current pulse. The current pulses last 50 ns each, and start to register the next event after a gap from the previous event.

Although sometimes facilitated by higher incoming neutron energies, neutron detection is generally a difficult task, for all the reasons stated earlier. Thus, better scintillator design is also in the foreground and has been the topic of pursuit ever since the invention of scintillation detectors. Scintillation detectors were invented in 1903 by Crookes but were not very efficient until the PMT (photomultiplier tube) was developed by Curran and Baker in 1944.^[2] The PMT gives a reliable and efficient method of detection since it multiplies the detection signal tenfold. Even so, scintillation design has room for improvement as do other options for neutron detection besides scintillation.

See also

• Bonner sphere – tool for determining neutron energies
• Large Area Neutron Detector
• Neutron Probe
• European Muon Collaboration

References

1. ^ John F. Beacom and Mark R. Vagins (2004-10-20). "Antineutrino Spectroscopy with Large Water Čerenkov Detectors". Phys. Rev. Lett. 93. doi:10.1103/PhysRevLett.93.171101.
2. ^ ^{a b} Leo, W.R. (1994). Techniques for Nuclear and Particle Physics Experiments. New York: Springer. 
3. ^ ^{a b c} Cerny, J.C., Dolemal, Z., Ivanov, M.P., Kuzmin, E.P., Svejda, J., & Wilhelm, I. (2003-03-11). "Study of neutron response and n–γ discrimination by charge comparison method for small liquid scintillation detector" (PDF). Nuclear Instruments and Methods in Physics Research: Section A. 527: 512–518. doi:10.1016/j.nima.2004.03.179. Retrieved on 2007-08-07.
4. ^ Jastaniah, S.D. & Sellin, P.J. (2003-08-19). "Digital techniques for n–γ pulse shape discrimination capture-gated neutron spectroscopy using liquid". Nuclear Instruments and Methods in Physics Research: Section A 517: 202–10. Retrieved on 2007-08-07.

• Cates, G.D., Day, D., & Liyanage, N. (2004). Neutron Tagged Bound Proton Structure to Probe the Origin of the EMC Effect (PostScript). Jefferson Lab Papers. Retrieved on 2005-06-09.
• Pozzi, S.A., Mullens, J.A., & Mihalczo, J.T. (2003-12-22). "Analysis of neutron and photon detection position for the calibration of plastic (BC-420) and liquid (BC-501) scintillators". Nuclear Instruments and Methods in Physics Research: Section A 524 (1–3): 92–101. doi:10.1016/j.nima.2003.12.036. Retrieved on 2005-06-21.
• Cecil, R.A., Anderson, B.D., & Madey, R. (1979-05-15). "Improved Predictions of Neutron Detection Efficiency for Hydrocarbon Scintillators from 1 MeV to about 300 MeV". Nuclear Instruments and Methods in Physics Research 161 (3): 439–447. doi:10.1016/0029-554X(79)90417-8. Retrieved on 2005-10-15.
• DOE Fundamentals Handbook on Instrumentation and Control, Volume 2