Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 69 (2015) 198 – 201
10 World Conference on Neutron Radiography 5-10 October 2014
Selective energy neutron radiographic imaging Origins and lessons for low cost systems J.P. Bartona* and J.D. Rogersb b
a Retired RadSci Consultancy Ltd, 23 Pant-y-Seren, Tonyrefail, South Wales CF39 8DX, U.K.
Abstract Major advances in selective energy techniques for neutron radiographic imaging have been demonstrated recently at very advanced, high flux, shared user facilities. The origins of selective energy methods for neutron radiography have been reviewed and options for low cost systems at lower flux, lower budget, single-user neutron source facilities are discussed. An original cold NR Imaging demonstration used a simple filter of polycrystalline beryllium and single crystal bismuth cooled by liquid nitrogen. An expensive refrigerated moderator source block is not essential. A less expensive option omits use of the single crystal bismuth. A low cost boost to cold neutron flux at a low power reactor uses a refrigerated source block of solid methane. For NR Imaging at selective epithermal energies, a single crystal neutron monochromator provides a low cost option. Alternatively a pulsed neutron source and time of flight technique is included in the original reports on selective energy methods. The original demonstrations using low cost systems indicate new advanced selective energy techniques pioneered at high flux sources may be developed at lower flux, single-user sources. © Published by Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license ©2015 2015The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of Paul Scherrer Institut. Selection and peer-review under responsibility of Paul Scherrer Institut Keywords: Neutron radiography; Neutron radiographic imaging; Selective energy neutron radiographic imaging.
* Corresponding author. Tel.: +1 8584888811 E-mail address:
[email protected]
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of Paul Scherrer Institut doi:10.1016/j.phpro.2015.07.028
J.P. Barton and J.D. Rogers / Physics Procedia 69 (2015) 198 – 201
1. Introduction The records of the International Atomic Energy Agency (IAEA) recently showed about 66 research reactors in about 40 countries that list neutron radiography (NR) or neutron imaging (NI) as one of their interests. Many of these neutron sources have multiple beams that are underused, presumably due in significant part to lack of sufficient funds for advanced equipment. Elsewhere, at higher flux, shared user facilities, such as HZB and TUM in Germany and PSI in Switzerland, impressive technique advances are being made including techniques using selective energy neutrons (Lehmann et al. 2010). The objective in this paper is to review the origins of selective energy neutron radiography using low-cost systems, and to consider whether there are low cost methods that could combine the advances at shared user systems with the lower cost techniques desired at lower flux single user facilities world-wide. Historically, most neutron radiography techniques have used the full spectrum of neutron energies from a thermal neutron reactor source. Thus we use the term “selective energy” to mean other than full thermal reactor spectrum. This includes fast neutron radiography methods using un-moderated accelerator and isotopic sources, and cold neutron radiography using filtered thermal neutron beams. The term “Neutron Radiographic Imaging” or NR Imaging (NRI) will be used in this paper to mean advanced neutron radiography, a special branch of “neutron radiography” as used by many authors in this conference series. 2. Origins of selective sub-thermal energy NR imaging High flux neutron facilities are expensive to build and operate. They may justify high cost refrigerated moderators and high cost guide tubes for beam extraction. But at lower flux, lower budget, facilities such high cost neutron energy selection features are not necessary. Much lower cost alternatives can provide neutron beams of equal quality in terms of energy selection, and the lower fluxes can be compensated by use of longer exposure periods. The original demonstration of cold neutron radiography used a low cost neutron energy selection by filters but without the need for a refrigerated moderator or guide tube (Barton, 1965,-1). The source was a simple volume of ambient temperature light water placed in a radial beam tube of the reactor Herald at AWRE Aldermaston (Barton, 1965-2). The full spectrum of radiation from the source volume was filtered by 200 mm of polycrystalline beryllium followed by 100 mm of single crystal bismuth measured in the direction of the beam. The bismuth single crystal was aligned such that it would provide high transmission of cold neutrons but high attenuation for gamma radiation. The combined filter was cooled by liquid nitrogen. The incident face of the beryllium filter was positioned about 2.5 m from the water source. The beam incident on the filter was collimated by a tube of diameter 65 mm and length 700 mm. This simple low-cost filter system provided a nominal energy cut-off for neutrons above 0.005 eV. (Equivalent to wavelengths below about 4 angstrom units). At a reactor power of 5 MW, the cold neutron flux incident at the object for radiographic tests was 3x105 neutrons/cm2 /s. This was ample for neutron imaging with converter-film methods, and would presumably be ample for imaging with advanced digital methods. 3. Low cost filters for sub thermal NR Imaging. While polycrystalline beryllium at liquid nitrogen temperature is not quite as efficient for gamma discrimination as single crystal bismuth, it can be acceptably efficient in certain circumstances. Thus the cost and other disadvantages of a single crystal bismuth component to the filter can be avoided by use of a simple long beryllium filter. Studies have been published on some options for low-cost cold neutron radiography capabilities at the Oregon State University reactor, USA (Bossi and Barton, 1977) and the University of California, Davis, MNRC reactor, USA (Barton, 1999-1, and 2). The latter reactor has been operating for neutron radiography for over 20 years without a cold filtered beam. There are four tangential beams available including one filtered to improve the thermal neutron spectrum. A low-cost cold neutron beam option may be an option to consider. Similar low cost
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J.P. Barton and J.D. Rogers / Physics Procedia 69 (2015) 198 – 201
designs making use of tangential rather than radial beam tubes should be applicable to many of the other low power, low budget facilities in operation world-wide. 4. Low-cost cold moderators for NR Imaging One example in the history of cold neutron radiography applications justified dedication of a low power reactor (TRIGA 1.5 MW) to operate for a long period (years) exclusively for repetitive cold NR inspections. In this case, the budget justified design and installation of a moderate cost cooled moderator based using methane solidified at 20oK. (Rogers, 1981). A patent that was filed on this inexpensive design has presumably expired due to the passage of time (Larsen, 1979). 5. Origins of monochromatic resonance energy NR Imaging A radial beam from the reactor Herald (5MW) at Aldermaston was used for an original demonstration of selective energy neutron radiography using a crystal monochromator (Barton 1965-3 and 4). The beam cross sectional area was 7.6 cm x 3.8 cm. A correspondingly large single crystal of aluminum (12 cm x 6 cm cross section) was rotatable in the incident neutron beam to provide radiographic beams of resonance energies such as the indium resonance at 1.4 eV. Using the crystal planes 111 and a Bragg angle of 2o 54’ the measured flux at 1.4 eV was measured to be over 10 4 n/cm 2/s. 6. Origins of resonance NR Imaging using a pulsed source. Resonance Neutron Radiography using a time of flight method was originally demonstrated using the spallation source IPNS (Intense Pulsed Neutron Source) at the Argonne National Laboratory (Strauss et al. 1981). Because of the pulsed nature of this spallation source, the peak available epithermal flux could be much higher than could be obtained from a typical steady state reactor source combined with a neutron chopper. A time sensitive digital detection system was based on lithium glass scintillator and an array of photomultiplier tubes to give large dimension readouts sorted by time of flight. The demonstration resonance neutron radiographs covered the neutron energy range 0.2 to 20 eV. The repetition rate was 30 pulses per second. The energy resolution was 0.05%. The effective L/D ratio was 1000. 7. Summary Selective energy neutron radiography started with relatively simple, relatively inexpensive demonstrations using each of three approaches: filters, crystal monochromators and pulsed source time of flight methods. Progress, primarily at the advanced, high flux, shared-user facilities has opened new areas of interest. Complementary to this, rapid advances are being made in digital detection primarily in the medical radiology field (Lanca and Silva, 2013). Some of the large number of lower flux, lower budget neutron sources may have beams available for single-users. The lower flux may, in part, be compensated by longer exposure times and custom system design. The performance reported for low cost systems used in original demonstrations of selective energy neutron radiography suggests ways that low cost modern systems may be considered at some single-user facilities. References Barton, J.P., 1965-1, US patent 3496358 A Radiographic Examinations- Sub thermal Neutrons. May 1965. Barton, J.P., 1965-2, Radiographic examination using cold neutrons, Brit. J. of Appl. Phys. 16, 1833-1840. Barton, J.P., 1965-3, Neutron radiography using a crystal monochromator, J. of Scientific Instr. 43: 549. Barton, J.P., 1965-4, Radiography with Resonance Energy Neutrons, Phys. Med. Biol. 10, No 2, 209. Barton, J.P., 1999-1, Long Be Filter Compared with Be-Bi Filter for Dedicated Cold Neutron Radiography Beam, WCNR-6, Fujine, S. et al, Eds. 121-128 Barton, J.P., 1999-2, Filters for thermal neutron radiography, WCNR-6, 185-194, Fujine, S. et al. ( Eds).
J.P. Barton and J.D. Rogers / Physics Procedia 69 (2015) 198 – 201 Bossi, R.H. and Barton, J.P., 1977, Performance of an inexpensive cold neutron radiography facility, Am. Soc. for NDT, National Topical Meeting, Aug 2-5 1977. Lanca, L., Silva, A., 2013, Digital Imaging Systems for Plain Radiography, Book, Springer publication. Larsen, John E., 1979, US patent 4134016, Cold neutron radiographic apparatus and method, Jan 1979. Lehmann, E. , Vontobel, P.,Frei, G.,Kuehne, G. Kaestner, A., 2010, How to organize a neutron imaging user lab? at PSI, CH, WCNR-9, De Beer et al. (Eds). 1-5. Rogers, J. D., 1981, Cold neutron spectra of the General atomic TRIGA F Neutron Radiography facility, WCNR-1, Barton J.et al (Eds.). 977981. Strauss, M.G. et al., 1981, Resonance neutron radiography using a pulsed neutron source, WCNR-1, Barton et al. (Eds). 519- 530.
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