Neutron-Induced Gamma-Ray Physics at LANSCE [wkshop procs]

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The Nowotny-Juza compound LiZnAs has been demonstrated as a neutron detector; [54] however, the material is difficult and expensive to synthesize, and only small semiconductor crystals have been reported. Finally, traditional semiconductor materials with neutron reactive dopants have been investigated, namely, Si Li detectors.

Neutrons interact with the lithium dopant in the material and produce energetic reaction products. Prompt gamma-ray emitting semiconductors, such as CdTe, [55] [56] and HgI 2 [57] [58] have been successfully used as neutron detectors. However, these semiconductor materials are designed for use as gamma-ray spectrometers and, hence, are intrinsically sensitive to the gamma-ray background. With adequate energy resolution, pulse height discrimination can be used to separate the prompt gamma-ray emissions from neutron interactions.

However, the effective neutron detection efficiency is compromised because of the relatively small Compton ratio. In other words, the majority of events add to the Compton continuum rather than to the full energy peak, thus, making discrimination between neutrons and background gamma rays difficult. Consequently, most thermal neutrons are absorbed near the detector surface so that nearly half of the prompt gamma rays are emitted in directions away from the detector bulk and, thus, produce poor gamma-ray reabsorption or interaction efficiency. Activation samples may be placed in a neutron field to characterize the energy spectrum and intensity of the neutrons.

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Fast neutrons are often detected by first moderating slowing them to thermal energies. However, during that process the information on the original energy of the neutron, its direction of travel, and the time of emission is lost. Typical fast neutron detectors are liquid scintillators, [61] 4-He based noble gas detectors [62] and plastic detectors.

Fast neutron detectors differentiate themselves from one another by their 1. The capability to distinguish between neutrons and gammas is excellent in noble gas based 4-He detectors due to their low electron density and excellent pulse shape discrimination property. Detection of fast neutrons poses a range of special problems. A directional fast-neutron detector has been developed using multiple proton recoils in separated planes of plastic scintillator material. The paths of the recoil nuclei created by neutron collision are recorded; determination of the energy and momentum of two recoil nuclei allow calculation of the direction of travel and energy of the neutron that underwent elastic scattering with them.

Neutron detection is used for varying purposes. Each application has different requirements for the detection system.

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Experiments that make use of this science include scattering experiments in which neutrons directed and then scattered from a sample are to be detected. Neutron detection in an experimental environment 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. 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.

If 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.

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. 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. 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.

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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. 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. Thus, the ADC samples out approximately one in every 30 events from the oscilloscope for analysis.

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Since the typical event rate is around 10 6 neutrons every second, [68] this sampling will still accumulate thousands of events every second. 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. 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. 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. 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. 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.

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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. 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.

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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.

Detection rates can be kept low in many ways. Kanda J.