Parasitic neutron beam monitoring: proof of concept on gamma monitoring of neutron chopper phases

Neutron beam monitors are an essential diagnostic component of neutron scattering facilities. They are used to measure neutron flux, calibrating experiments performed on the instruments, allowing measurement of facility performance, understanding of beam-line components (such as choppers), calibration of detectors and tracking of beam stability. Ideally beam monitors do not perturb the beam. Previous work shows commercial beam monitors attenuate the beam by few percent in the worst case due to the 1-2 mm thick Aluminium entrance and exit windows and the material inside. Parasitic methods of neutron beam diagnostics, where there is no beam monitor directly in the beam, would be preferable. This paper presents the concept of a parasitic method of monitoring the beam which can be used for neutron chopper phasing. This is achieved by placing a gamma detector close to a rotating chopper and measures a signal proportional to the flux absorbed by the chopper. Neutrons interact with the Boron absorber on the chopper disc lead to gamma emission at 480 keV. Detection of these gamma rays is used to determine the chopper phasing and timing. Potentially information on the flux of the beamline can be extracted. Results from a proof of concept implementation show that diagnosis of neutron chopper phases is feasible.

Introduction . -Neutrons from spallation sources such as at SNS [1], ISIS [2] and J-PARC [3] and ESS [4] are created by a primary proton beam accelerated with a specific repetition rate toward a heavy target which can be tungsten, lead or mercury. The produced neutrons are created with high energy (up to ca. GeV); the neutron's energy is reduced by a moderator made of light elements such as hydrogen. The neutrons are then transported from the moderator to the sample position by neutron guides. As the neutron's energy defines its velocity (about 2200m/s at thermal energies of 25meV which is equivalent to a wavelength of about 1.8 Angstrom), the time-of-flight of the neutron can be used to select the neutron energy. This selection could be achieved by several sets of mechanical "choppers" [5]. Choppers are mechanical rotating discs, with slits which define the timing of the neutrons which are allowed to proceed down the beamline. Typically several chopper pairs are used per instrument proposed for the European Spallation Source (ESS) facility [6]. The chopper system and the length of the instrument control the neutron wavelength and flux. The chopper discs are coated with a neutron absorbent material such as Gd or B in order to stop the unwanted neutrons.
The length of a neutron instrument varies from few metres up to 160 metres. A set of beam monitors is needed per instrument in order to diagnose its main components such as choppers and guides section and to determine flux on sample.
The requirements for the monitors vary greatly with respect to their location and purpose. Several types of monitors are needed to fulfil all these requirements. In recently built instruments, and in particular for instruments planned for the ESS facility, the number of beam line components and their complexity is increasing. This is especially the case for neutron choppers, where the number of choppers planned in the baseline instrument suite are comparable to the number of choppers currently installed globally. This in turn implies an increased need in beam- Typically neutron beam monitors are simple neutron detectors with sufficiently low efficiency (10 −6 -10 −1 ) so that a low percentage of the incoming beam is absorbed or scattered as shown in fig. 1. They are used to ensure that the neutron flux, beam distribution, and pulse timing correspond to those expected from the design of the instrument. In addition, they are used to determine the neutron flux at the sample in order to correctly interpret the scattering data. Different types of beam monitors from a variety of suppliers have been characterised in previous work showing a high attenuation factor for most of the monitors mainly due to the entrance and exit window [7] as shown in fig. 1.
The desirderatum is a parasitic method of beam monitoring that avoids attenuation of the beam. One such quasi-parasitic method has recently been investigated [8]. The parasitic concept developed here takes advantage of a chopper disc coated with Boron carbide. Neutrons blocked by this disc interact with Boron leading to gamma emission at 480 keV based on the interaction shown below: The emitted gamma rays can be measured using a detector which could be a scintillator made of e.g. NaI or LaBr. This can be placed close to the chopper disc and therefore requires no additional material in the direct neutron beam; thus no perturbation of the beam occurs. The concept is shown in fig. 2.  Concept of Measurement. -For a traditional neutron beam monitor, the measured flux is: where M is the measured detected flux, and I is the neutron intensity incident on the beam monitor. S and A are the neutrons scattered from the beam monitor and those absorbed in the non-sensitive material of the beam monitor respectively. These neutrons are not transmitted and are lost from the beam. It should be noted that, experimentally, the difference between scattered and measured neutrons is one of detector geometry, as has been discussed elsewhere [9].
is the efficiency of the beam monitor. λ is the neutron wavelength.
The flux transmitted by the monitor down the beamline is thus: The measured flux is thus dependent upon the neutron wavelength. There is an additional correction from the fraction absorbed in dead material and scattered out of the beamline, which can be sizable [7]. The corrections for wavelength are calculable and good design can reduce the attenuation of the incident beam to the percent level. However these corrections represent complications to the measurement schema; often these are neglected.
In contrast, the concept of measuring the gamma rays emitted relies on measuring neutrons absorbed in the Boron of the neutron chopper disc. This is actually the incident flux which is not transmitted by the chopper, or F . The neutron chopper is a rotating mechanical device and therefore has a time varying function of neutron transmission. This means that its transmission function can be expressed as: p-2 Parasitic neutron beam monitoring where I is the neutron intensity incident on the beam monitor, A is the neutrons absorbed in the chopper and T is the neutrons transmitted down the neutron beamline. Scattering from the beamline is neglected here.
As T is the transmitted flux through the chopper, this corresponds to F , i.e. the flux of the beamline. A corresponds to the flux not transmitted, i.e. F .
The gamma detector measures the gammas emitted with an efficiency which can be expressed as: where M is the detected flux and A is the neutrons absorbed in the chopper. n is the fraction of neutrons incident on the Boron Carbide coating of the chopper which are absorbed. K BR is the branching ratio (94%) for the gamma emmission fraction, given in equation 2. Ω is the fraction of solid angle subtended by the sensitive area of the gamma detector to the area where the neutron beam impinges on the chopper disc. Lastly, γ is the efficiency of the gamma detector to 480 keV gamma rays.
As neutron choppers aspire to a high "blackness" to neutrons when closed, the n factor will, by design, be close to 1 and almost constant across the neutron wavelength of interest. The other factors are not variable with neutron wavelength. This means that the measurement efficiency is not variable with wavelength.
In summary, this concept should give information on absorbed flux on the beamline as a function of time. In particular, it should give prompt time information on changes in flux, i.e. the opening and closing edges of chopper slits.
Experimental Setup . -A LaBr gamma detector from Saint Gobain [10] with 3 inch scintillator crystal was used to detect the emitted gamma from a Boron interaction. The LaBr detector was calibrated using different gamma sources.
To get the signal the detector was connected to a photo multiplier tube (PMT) base supported with a preamplifier then the signal was shaped and amplified using the 885 dual spectroscopy amplifier from ORTEC [11]. This analogue signal was then digitised using a multi-channel analyser from FastComTec [12]. The high voltage was provided using a CAEN high voltage power supply.
Measurements were performed using an AmBe neutron source, to verify sensitivity to the 480 keV gamma emissions from Boron in a high γ background environment. The source is placed in polyethylene moderator to moderate the emitted fast neutrons [13]. Good sensitivity over background was observed with the detector.
After measuring using a neutron source this measurement was performed at a beamline using the same setup using the time-stamped option on the multi-channel analyser electronics. The V20 beamline [14] at the BERII research reactor at Helmholtz-Zentrum Berlin(HZB) provides a complete wavelength frame multiplication chopper system and is designed to replicate the ESS pulse time structure [15].
The source chopper was set to mimic the ESS pulse (2.86 ms pulse length with a repetition rate of 14Hz) and the wavelength band choppers were set to prevent frame overlap within the repetition rate. All other choppers were left open. The integrated neutron flux is determined as 3 × 10 6 n.cm −2 .s −1 . The beam size can be collimated in both horizontal and vertical direction using several sets of slits in the beam line.
For the purpose of this measurement a LaBr gamma detector without any shielding was placed close to a mini chopper [16] which was placed in the direction of the neutron beam. The outer diameter of the chopper disc is 175 mm and made of 3 cm thick aluminum and coated with Boron Carbide on both sides with a total thickness of 3.5 mm in order to stop the unwanted neutrons. The chopper rotates at 14 Hz and it has two openings to allow the neutrons with a specific energy range to go through. Thus the neutrons have to pass first the aluminum window which is 25 by 25 mm wide and 0.5 mm thick before they either pass through the disc opening or get absorbed by the Boron coating. A schematic of the experimental setup is shown in fig. 3. A photograph of the setup is shown in fig. 4.     fig. 8. Note that the data from the LaBr detector has a much sharper edge than the conventional beam monitor, due to the promptness of the signal. This feature proves the feasibility of this method to be used for phasing of the chopper without the addition of any material in the beamline.
Conclusions. -A LaBr gamma detector was placed close to a mini chopper on a pulsed beamline with the chopper running. A clear time-variant gamma peak at 480 keV was detected. This gamma emission is due to the interaction of the incoming neutrons with Boron coating on the chopper system . A remarkable dip in the gamma at 480 keV curve was observed indicating the time of opening and closing of the chopper. This is a proof of concept of the parasitic method for monitoring the beam nearby a chopper system. The results applied here are equally applicable to choppers using Gadolinium absorbers, though a different, higher, energy window would need to be used. This concept, developed into an engineered system, could be a good diagnostic tool without adding any material in the beam, i.e. a parasitic beam monitor. It is particularly appropriate for determining and verifying chopper phases.  fig.7 the time-of-flight spectra between 25 and 50 ms. Shown is the monitor after the chopper, the data from the LaBr detector, and the result of deficit of events from the fitted data, due to the chopper opening. The line shows the fitted trend of the LaBr data.