Neutron Collimator Research Articles

Performance of the upgraded multichannel neutron collimator

The original ZnS based NE451 detectors become saturated at the highest levels of D-D operation. A new less sensitive detector configuration, also based on ZnS crystals, was designed and tested at PPPL in order to extend the collimator capabilities to D-T plasma operation. The new detector design is composed of a thin layer of ZnS powder uniformly deposited between two layers of Plexiglas. The detector capabilities are further extended by upgraded signal processing electronics for a total enhancement of ∼1000× compared to the original detector system. An array of these detectors has been installed on TFTR in tandem with the original detector system. The current mode is presently being implemented on the original NE451 detectors. This will extend their capabilities and allow for greater overlap in data taken in the crossover region between the two detector systems. The performance of the collimator system will be illustrated by neutron data obtained during D-T operation.

Absolute calibration of TFTR helium proportional counters (abstract)a)

The TFTR helium proportional counters are located in the central five (5) channels of the TFTR multichannel neutron collimator. These detectors were absolutely calibrated using a 14 MeV neutron generator positioned at the horizontal midplane of the TFTR vacuum vessel. The neutron generator position was scanned in centimeter steps to determine the collimator aperture width to 14 MeV neutrons and the absolute sensitivity of each channel. Neutron profiles were measured for TFTR plasmas with time resolution between 5 and 50 ms depending upon count rates. The He detectors were used to measure the burnup of 1 MeV tritons in deuterium plasmas, the transport of tritium in trace tritium experiments, and the residual tritium levels in plasmas following 50:50 DT experiments.

EVALUATION OF THREE DIMENSIONAL UNSHARPNESS ON NR IMAGES

Unsharpness of neutron radiography (NR) image has been discussed usually two dimensionally. The unsharpness depends on size and shape of a neutron diaphragm, and on an object position and its shape. Although a circular diaphragm is often installed in the neutron collimator, rectangular diaphragms are also selected in some NR facilities. Anisotropic unsharpness is expected for these less symmetrical diaphragms. In this paper, the unsharpness is analyzed with respect to these parameters. The spatial unsharpness in depth direction should be required in case of thickness determinations and is analyzed in addition to the two dimensional unsharpness

Book review

Ten years after the publication of Fast Neutrons in the Treatment of Cancers by Mary Catterall and David K. Bewley in 1979, David Bewley has published an update on fast neutron therapy which concentrates on physical and radiobiological aspects. Since the first applications of fast neutrons at Hammersmith Hospital in the 1970s significant improvments have been achieved. Improvements have also been seen in the neutron generators and collimation systems as well as in dosimetry and radiobiology. Therefore, it was a good time to present an updated overview and synthesis of the situation of fast neutron therapy, or at least, the developments in its physical and radiobiological aspects.

Measurement of DT neutron emission from TFTR with helium-4 proportional recoil counters

The DT neutron emission from Triton Burnup has been measured with four helium-4 proportional counters in TFTR’s Multichannel Neutron Collimator. The operating characteristics of an ‘‘alpha’’ recoil proportional counter are presented. The theoretical response function of these detectors, which is complicated by the strong anisotropy of the (n,α) reaction, is compared with the measured response of these detectors from TFTR. A computer simulation predicts wall and end effects. The frequency responses of the detector and electronics have also been tested. A model predicts the saturation effects seen during high-power plasmas. The calibration techniques for this detector array will be discussed. The detectors have been used to measure the radial profile of the 1 MeV tritons produced by the d(d,p)t fusion reaction. The burnup produces 14 MeV neutrons in the d(t,n)α fusion reaction.

Validation of spatial profile measurements of neutron emission in TFTR plasmas (invited)

The TFTR multichannel neutron collimator provides spatially localized information on nuclear reaction rates in fusion plasmas. Absolutely calibrated NE 451 ZnS scintillators detect chord-integrated neutron emission along ten vertical sightlines, which are defined by a massive shielding structure in the facility basement. If such observations are to be of value, virgin neutrons along each sightline must be distinguishable from a background of gamma rays and scattered neutrons, and the measurements must be sufficiently precise, over a wide range of plasma conditions, to allow meaningful comparison with results of transport analysis computer codes. As a useful check, the volume-integrated neutron emission must agree with independent measurements of the global neutron source strength. Although NE 451 detectors are insensitive to gamma rays and to neutrons with energy less than ∼2 MeV, the scintillation pulse height distribution depends on both the incident neutron energy spectrum and the physical parameters of the individual detector. In order to establish the validity of our measurements, we have carried out a detailed characterization of our detectors, including computer modeling of scintillation pulse height spectra and of temporal pulse pileup effects in the counting circuitry. Satisfactory operation of the system has been demonstrated for all deuterium plasmas encountered in TFTR. The detector modeling, together with further laboratory measurements, will enable us to prepare for deuterium–tritium experiments.

Control of the neutron detector count rate by optical imaging

The signal processing electronics used for the NE451 detectors on the TFTR multichannel neutron collimator are presently showing saturation effects at high counting rates equivalent to neutron yields of ∼1016 n/s. While nonlinearity due to pulse pileup can be corrected for in most present TFTR experiments, additional steps are required for neutron source strengths above ∼3×1016 n/s. These pulse pileup effects could be reduced by inserting sleeves in the collimator shielding to reduce the neutron flux in the vicinity of the detectors or by reducing the volume of detector exposed to the flux. We describe a novel method of avoiding saturation by optically controlling the number neutron events processed by the detector electronics. Because of the optical opacity of the ZnS-plastic detectors such as NE451, photons from a proton-recoil scintillation arise from a spatially localized area of the detector. By imaging a selected portion of the detector onto a photomultiplier, we reduce the effective volume of the detector in a controllable, reversible way. A prototype system, consisting of a focusing lens, a field lens, and a variable aperture, has been constructed. Results of laboratory feasibility tests are presented.

Design studies related to an in vivo neutron activation analysis facility for measuring total body nitrogen

Design studies relating to an in vivo prompt capture neutron activation analysis facility measuring total body nitrogen are presented. The basis of a design is a beryllium-graphite neutron collimator and reflector configuration for ( alpha ,n) type radionuclide neutron sources (238PuBe or 241AmBe), so as to reflect leaking, or out-scattered, neutrons towards the subject. This improves the ratio of thermal neutron flux to dose and the spatial distribution of thermal flux achieved with these sources, whilst retaining their advantage of long half-lives as compared to 252Cf based systems. The common problem of high count-rate at the detector, and therefore high nitrogen region of interest background due to pile-up, is decreased by using a set of smaller (5.1 cm diameter*10.2 cm long) NaI(Tl) detectors instead of large ones. The facility described presents a relative error of nitrogen measurement of 3.6% and a nitrogen to background ratio of 2.3 for 0.45 mSv skin dose (assuming ten 5.1 cm*10.2 cm) NaI(Tl) detectors.

TFTR multichannel neutron collimator

The TFTR multichannel neutron collimator provides time-resolved neutron emission profiles from ten vertical chords spaced in major radius across the vacuum vessel. An in situ calibration of the system has been made using a 120 mCi 252Cf source scanned inside the vacuum vessel. A mapping of the effective neutron acceptance aperture was performed and showed reasonable agreement with the optical predictions testifying to the effectiveness of the interstitial shielding material. Crosstalk between channels was evaluated and found to be negligible. Periodic renormalization of the system has shown a standard deviation among the sensitivities of only 2.0% over the first seven months of operation.

Real-Time Neutron Radiography System Performance—Measurements and Methods

The University of Virginia's real-time neutron radiography facility was characterized by measurement of the total neutron flux, gold/cadmium ratio, neutron/gamma ratio, and the effective collimar length-to-aperture diameter (L/D) ratio. The real-time neutron imaging system and collimator were further characterized by measuring the modulation transfer function (MTF) of the system. The collimator effectiveness was measured by using the MTF to determine the ''unparallelism'' of the neutron beam. The computer-based image processing system allowed rapid calculation of the MTF and the collimator effectiveness. The results of these measurements, using no collimator and a simple tube collimator, demonstrated the method's ability to determine the effective L/D ratio. The MTF measurement scheme provided a fast, reliable, and reproducible means of monitoring any changes in the real-time system, including both the neutron beam and the electronic components. This paper describes the methods and measurements implemented to characterize the changes in the radiographic system performance. These include standard measurements of the neutron, flux, neutron-to-gamma ratio, the effective collimator length-to-aperture diameter (L/D) ratio, and effective neutron temperature.

Head phantom experiment and calculation for boron neutron capture therapy

Head phantom experiments with various neutron beams and calculations were carried out in order to provide useful information for boron neutron capture therapy (BNCT). Thermal neutron beams for thermal neutron capture therapy were used for phantom experiments with various neutron collimator aperture sizes. The filtered beam neutrons of 24 and 144 keV generated with iron and silicon filters were also used to investigate the possible application of BNCT in the treatment of deep-seated cancers. Thermal neutron fluence and induced capture gamma dose distributions within the phantom were calculated with a transport code DOT 3.5 and compared with the experimental results. The results showed that the calculation used was consistent with the experimental results and provided useful information on BNCT. The filtered beam neutron may be very useful for the treatment of deep or widespread cancer, if there were a high power research reactor constructed for this purpose.

The UCLA neutron therapy facility

The UCLA Neutron Therapy Facility (NTF) has recently been completed and is treating cancer patients with fast neutrons. The negative-ion, variable-frequency cyclotron for this facility is capable of accelerating 12–48 MeV protons and 7–22 MeV deuterons. Two variable energy ports and four fixed-energy ports are available on the cyclotron. A 46 MeV proton beam is used for the neutron therapy that is conducted in two separate treatment caves. A fixed, horizontal and an isocentric treatment assembly are available. The fixed energy beam ports are reserved for isotope production. This report describes the facility, the cyclotron and both neutron treatment assemblies, including the neutron collimator and the beryllium target. Neutron therapy and dosimetry are discussed in a companion paper.

Spectral measurement of angular neutron flux on the restricted surface of slab assemblies by the time-of-flight method

An experimental method of neutron angular spectrum measurement in a slab geometry with D-T neutrons has been developed and its characteristics were examined. This method separates collided flux from uncollided flux. A neutron collimator restricts the measuring area on the surface of a slab assembly and the detector-collimator response function was determined. The time-of-flight method with two discrimination levels was adopted using an NE213 scintillation detector with pulse shape discrimination to obtain an accurate efficiency. Such measurements provide useful information in fusion blanket research. The results of the spectrum measurement were compared with calculated neutron spectra. The estimated overall error of the spectrum was within 5%.

Collimating neutrons

Your annotation on page 111 of the March 1985 issue of Physics Bulletin gives the impression that a variable aperture neutron collimator for medical use is a new development. In fact they have existed for several years.

Variable aperture neutron collimator — II

A second model of a completely adjustable, variable aperture neutron collimator has been developed. This redesigned model can be scaled to a full-size device appropriate for use in a medical facility. The collimator aperture can be easily adjusted to provide a square or rectangular cross-sectional exposure while maintaining a uniformly diverging line-of-sight from the neutron source.

Multiple scattering from an infinite plane slab

A straightforward iterative procedure is used to calculate multiple scattering from an infinite slab over a wide range of slab thicknesses. For thin slabs, the calculations agree well with earlier work. In particular, we find that the ratio of total scattered to primary scattered neutron currents for a thin slab is represented by the function 1(1−δ), where δ is the ratio of secondary to primary currents. For thick slabs, the results are compared with Monte Carlo simulations of polyethylene and iron. The agreement is generally good, but the method exaggerates the ability of polyethylene to attenuate 10 MeV neutrons. A model of two parallel slabs is used to discuss the reflectivity of the walls of a neutron collimator.

Variable aperture neutron collimator

We describe a completely adjustable, variable aperture neutron collimator that provides uniform sheilding outside the aperture. This aperture can be adjusted smoothly from a square-to-rectangular cross section while maintaining either a uniform or diverging channel.

Use of plastic track detectors for studying the characteristics of fast neutron collimators

Use of CA80-15 and LR-115 plastic track detectors has been made in the study of the characteristics of fast neutron collimators. The results have yielded important information regarding: (a) the beam profiles; (b) neutron intensity variation within the beam; (c) degree of collimation at various stages; and (d) neutron flux variation along the length of the collimator. Useful information concerning the exact location of the neutron beam, and the total irradiated area of the target lying in the reaction chamber has also been obtained.

Remodeling and Dosimetry on the Neutron Irradiation Facility of the Musashi Institute of Technology Reactor for Boron Neutron Capture Therapy

The remodeling of the neutron irradiation facility of the Musashi Institute of Technology Reactor (TRIGA Mark II, 100 kW) was carried out for the purpose of boron neutron capture therapy. The gamma contamination was reduced by the bismuth scatterer technique, and the thermal-neutron intensity was enlarged by virtue of the cavity effect. A 6LiF sheet was used instead of a 10B sheet for neutron collimation to minimize production of the secondary gamma rays. The characteristics of the optimized field are as follows: ɸth ≈ 1.3 × 1013 m-2· s-1 (1.3 × 109 n/cm2·s), gamma rays ≈ 1.8 × 10-6 O kg-1· s-1 (25 R/h), γ/n ≈ 0.5% in dose equivalent.When a phantom head was placed at the irradiation aperture, the neutron fluence rate (flux) and gamma-ray exposure rate increased to ∼2 × 1013 m−2· s-1 (2 × 109 n/cm2 · s) and 1.1 × 10-5 C· kg-1 · s-1 (150 R/h), respectively, by the reflection of neutrons and capture gamma rays due to the phantom itself.The facility was licensed by the Japanese government to be used for the medical irradiation purposes on July 20, 1976.

Design Calculations for a 14-MeV Neutron Collimator

A number of SS-316 right circular cylindrical shells of varying lengths have been analyzed using two-dimensional discrete ordinates transport methods together with first and last flight particle estimators to aid in the design of neutron collimators for the Tokamak Fusion Test Reactor. In the TFTR the 14-MeV neutron source has a very large spatial extent and the collimators must be designed to allow spectral measurements that refer to only a small spatial region of this extended source. The analysis identifies the 14-MeV neutrons from scattering in the SS-316 immediately adjacent to the collimator opening as the dominant contributor to detector background. Collimator lengths greater than 0.60 m were found sufficient to attenuate uncollided background neutrons for reasonable source-detector distances. The lower energy (less than 13.8 MeV) neutron background and gamma background were not found to be significant.