Neutron News Energy resolved neutron radiography ...

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Energy resolved neutron radiography at LANSCE pulsed neutron facility a

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A.S. Tremsin , S.C. Vogel , M. Mocko , M.A.M. Bourke , V. Yuan , R.O. Nelson , D.W. Brown & W.B. Feller

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University of California at Berkeley , 7 Gauss Way, Berkeley , CA , 94720 , USA

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Los Alamos National Laboratory , Los Alamos , NM , 87545 , USA

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NOVA Scientific, Inc. , 10 Picker Rd., Sturbridge , MA , 01566 , USA Published online: 11 Oct 2013.

To cite this article: A.S. Tremsin , S.C. Vogel , M. Mocko , M.A.M. Bourke , V. Yuan , R.O. Nelson , D.W. Brown & W.B. Feller (2013) Energy resolved neutron radiography at LANSCE pulsed neutron facility, Neutron News, 24:4, 28-32, DOI: 10.1080/10448632.2013.831612 To link to this article: http://dx.doi.org/10.1080/10448632.2013.831612

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Scientific Reviews Energy resolved neutron radiography at LANSCE pulsed neutron facility

Downloaded by [the LANL Research Library] at 10:27 06 March 2015

A.S. TREMSIN1, S.C. VOGEL2, M. MOCKO2, M.A.M. BOURKE2, V. YUAN2, R.O. NELSON2, D.W. BROWN2, AND W.B. FELLER3 1University of California at Berkeley, 7 Gauss Way, Berkeley, CA 94720, USA 2Los Alamos National Laboratory, Los Alamos, NM 87545, USA 3NOVA Scientific, Inc., 10 Picker Rd., Sturbridge, MA 01566, USA Introduction Neutron radiography is one of the non-destructive testing techniques which can provide characterization complimentary to X-rays and in many cases unique information on the samples. The relatively high attenuation of hydrogen-containing phases and good penetration capability for most materials provide the possibility to reveal information not available with other non-destructive testing methods. The majority of neutron thermal radiography experiments are performed at continuous neutron sources, where fluxes are relatively high (106–109 n/cm2/s) and beam collimation and divergence are very good, enabling high spatial resolution [1–3]. The internal features of samples in these experiments are revealed due to the difference in the attenuation for various materials, integrated over a certain range of neutron energies present in the neutron beam. Typically thermal and cold neutrons are used in radiographic experiments as the attenuation contrast usually increases towards lower neutron energies. However, variation of attenuation with neutron energy is not monotonic for most materials, which can be utilized for quantitative studies of various sample characteristics if the measurement is energy dispersive. Therefore an extension of conventional neutron radiography into energy resolved radiography provides a number of very attractive novel opportunities [4–8]. In these experiments the sample transmission is measured as a function of neutron energy and thus in each pixel of the image not only the integrated intensity is measured, but also the sample transmission spectra. To measure spectra in each pixel of the image either the detection system has to be able to measure the energy of each neutron or the incoming neutron beam has to be monochromatic and then scanned over various energies. Energy sensitive neutron imaging detectors with good efficiency and spatial resolution are not available yet,

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and the scanning of monochromatic energies is not very flexible in terms of energy resolution and it substantially increases the duration of experiments if high energy resolution is to be achieved. The very attractive alternative for the energy selective imaging is the implementation of time of flight technique at a pulsed neutron source, where the energy of each neutron can be obtained from its time of flight, providing detectors with good spatial and temporal resolution are used in the experiment. Moreover, pulsed neutron beams extend the range of energies to the epithermal neutrons, where most materials have very sharp variation of transmission, referred to as absorption resonances. The short-pulse nature of the spallation neutron sources at Los Alamos Neutron Science Center (LANSCE) as well as only few others in the world (ISIS, UK; J-PARC, Japan; and partially SNS, USA where the pulse duration is longer leading to a lower energy resolution in the epithermal range) provides the unique opportunity to conduct high resolution energy resolved neutron imaging in a wide range of energies from epithermal to cold neutrons, spanning the energy range from tens of keV down to meV energies. The 300 nanosecond proton pulse width is a particular strength of the LANSCE accelerator since it enables measurements of resonances at energies exceeding 100 keV, providing the background level at the detector position is low enough and pulse broadening be the moderator is not too large at these high energies. This short pulse duration, in principle, should enable unique resolution of ~300eV at 10 keV energies. The combination of neutron energy resolved imaging with other experimental techniques available at LANSCE, namely conventional neutron diffraction and proton radiography as well as the capability to handle special materials, enables unique characterization capabilities at this facility.

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Scientific Reviews Recent development of high resolution neutron counting detectors capable of sub-100 μm and sub-100 ns (epithermal) and sub-1 μs (thermal) neutron imaging proved to provide some unique opportunities for energy resolved imaging at a pulsed neutron beamline [6, 7]. Some experiments conducted at Flight Path 5 at LANSCE described in this paper demonstrate some unique opportunities of energy resolved imaging at a short-pulsed neutron source for materials and energy research, engineering and many other fields. This article is not intended to provide a comprehensive review of all the opportunities, but rather demonstrates some unique opportunities provided by the combination of a short-pulse neutron source at LANSCE and high resolution neutron counting detector [8] with fast quad Timepix readout [9].

yet at FP5 and it will be performed in the near future. The isotope concentration can be measured from the analysis of the transmission spectra, allowing to map the isotope distribution within the sample. The attenuation

(a) Thermal neutron radiography and neutron resonance attenuation imaging The integrated intensity of pulsed neutron beamlines is usually lower than that of a reactor source and therefore conventional full spectrum transmission radiography is not among the unique capabilities of the LANSCE neutron source. However, the spatial resolution of neutron imaging can be well characterized by what can be achieved in the radiographic image, such as one shown in Figure 1a, acquired for the full spectrum available at the Flight Path 5 (FP5) beamline. The spatial resolution is not as high as reported previously ~15 μm for the experiments conducted at Paul Scherrer Institute [10], but relatively small features on a few hundred μm scale can still be resolved. The unique capability to conduct isotope-specific imaging with the same experimental setup is demonstrated in Figure 1b. The sharp increase in the attenuation due to resonant absorption at isotope-specific energies is utilized in that measurement, where neutrons of epithermal energies are grouped around resonance energies for the foils of several materials. It should be emphasized that the images of different energies are measured simultaneously by sorting neutrons of different energies recovered from their time of flight. The distribution of various isotopes in the samples can be measured by this method providing their resonance energies can be resolved. Currently our detection system is capable of resolving neutron energies up to ~30 keV, however the presence of a relatively high background signal may limit the upper energies which still can be imaged at LANSCE at the present time [11]. There was no optimization of shielding for the epithermal energies conducted

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(b) Figure 1. (a) Full spectrum neutron transmission images of test objects accumulated at Flight Path 5. A 5 cm lead filter was installed in the beam to reduce the gamma and fast neutron flash at the time of spallation. On the left: small steel screws, plastic washers, needle, steel nut with a zip tie; On the right: wrist watch. Both images are normalized by the open beam image. (b) Resonance absorption images of the mask consisting of foils of different materials. The images of a particular resonance energy are selected to demonstrate the material- (in fact isotope-) specific imaging of the neutron resonance attenuation imaging technique. Unless the resonance energies overlap (e.g. some overlap of U-238 and W resonances is seen at the left bottom image), the materials are mostly transparent to the epithermal neutrons and only are visible at the specific energies. All the images of different energies are taken simultaneously.


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Figure 2. The attenuation of 155 μm thick Ta foil calculated from the tabulated cross section values and the measured attenuation as a function of neutron energy. The absolute values of experimental data are not normalized properly as no precise measurement of the background spectra was performed at that experiment. The presence of the background signal at these energies reduces the measured attenuation of the sample and should be properly calibrated for the accurate quantification of the isotope thickness or isotope concentration in the sample.

at resonance energies increases with the amount of the particular material in the neutron path. Owing to the large absorption cross-sections of thousands of barn for typical absorption resonances, with proper analysis software being developed at the present time it should be possible to recover the isotope concentrations on a sub-1% level for the materials which exhibit resonances at energies below keV. The typical attenuation spectra for a thin Ta foil shown in Figure 2. The agreement between our measured and theoretical spectra (up to a constant value, which has to be found by taking into account the contribution of background signal and gamma emission by the sample) provides the basis for the future accurate quantification of isotope concentration in the samples.

attenuation coefficient for thermal neutrons is nearly equal for both materials. However, the image obtained by grouping neutrons at the resonance energies of 238U where transmission of other materials is relatively high and only 238U has large attenuation, reveals the homogenous distribution of 238U-238 within the assembly, Figure 4b. The contrast in that image is practically only due to the presence of atoms of 238U seen through the steel cladding and W inclusions. The effective thickness of 238U within the assembly can be recovered from the measured transmission spectra as mentioned earlier, although we are still working on the accurate quantification algorithms and the means to reduce and characterize the background signal contribution. The same way we separate the presence of W inclusions in the assembly by imaging only the neutrons around resonance energy of W. Figure 4c depicts the placement of W pieces within the assembly, where the W pieces are clearly visible. A tomographic reconstruction is possible if sufficient number of such projections can be acquired. Obviously the neutron statistics is not as high as in case of thermal radiography due to the limited detection efficiency and to the fact that only a small fraction of neutrons is used to form these images. The placement of W

Energy resolved imaging of mockup fuel assembly To demonstrate the unique capability of our energy resolved neutron radiography experiments, a set of 5 pellets consisting of urania sintered with W pieces was imaged in the epithermal and thermal/cold energies. The experimental setup is shown in Figure 3, with the fuel assembly encapsulated in a steel cladding. The conventional thermal transmission radiography shown in Figure 4a reveals the presence of cracks and voids in the pellets as wells as the internal placements of the pellets in the cladding, but there is no information of the presence of W inclusions and on density distribution of the urania itself, as expected from the fact that

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Figure 3. The photograph of the experimental setup. The neutron counting detector with MCP and Timepix readout with active area of 28 mm is installed in the direct beam. The samples (a set of three mockup fuel assemblies in that photograph) were placed at 15–25 mm distance to the active area to prevent image blurring by the beam divergence. Fast frame rate of the detector with timing of each neutron within the frame enabled detection of neutrons in a wide range of energies.

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Figure 4. Neutron transmission images of a mockup fuel assembly acquired at different neutron energies (compare the X-ray and proton radiography results published in reference [12]). The transmission of the same fuel assembly shows substantial differences at various neutron energies. (a) Thermal neutron transmission image. The steel cladding and urania pellets are visible with voids and cracks in the fuel clearly resolvable. (b) Transmission image acquired at the resonance energies of 238U. Only uranium is strongly absorbing at the resonance energies, while other materials are mostly transparent at these energies. (c) Ratio of thermal transmission image to the image acquired at W resonance energies. This image shows the cladding together with the W inclusions, which were deliberately introduced into the mockup fuel elements. (d) Ratio of image acquired at the 238U resonances to the image acquired for the resonances of W. Both Urania and Tungsten inclusions are clearly visible in that image, with no cladding seen in the image. The dashed rectangle indicates the area for the cross section across image (a) shown in Figure 6.

pieces within urania pellets is also shown in Figure 4d, where no cladding is visible as only resonances of U and W are used in the images. Not only the isotope distribution can be recovered from that experiment, as mentioned earlier, but some information on the crystallographic structure of pellets and the cladding can be found from the transmission spectra in the thermal range. The transmission spectra of a small area of the cladding wall and one of the pellets are shown in Figure 5. A wide range of existing methods utilizing the Bragg edge neutron transmission can be extended to the transmission neutron analysis, such as the reconstruction of strain, phase, texture, crystallinity and other parameters from the measured spectra [4, 12], providing the neutron statistics and accuracy of measurements are sufficient. The shape of the assembly can even be partially reconstructed in some cases from the single radiographic projection. The cross section through assembly of Figure 4a shown in Figure 6 indicates that it is likely of a cylindrical shape.

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Figure 5. The thermal neutron transmission spectra of the sample shown in Fig. 4. The spectra of cladding steel at the edge of the sample as well as the spectra of the center area including both Bragg edges of steel cladding and Urania are shown. The intensity of the beam at FP5 at the strongest edges (~4 A) is low and other beamlines with colder moderator will be beneficial for the reduction of integration time.

Figure 6. Cross section through the image shown in Figure 4. The shape of the transmission curve indicates that the fuel pellet has cylindrical geometry.

It is likely that such experiments will require relatively long integrations in the near future. However, the unique information provided by such measurement can justify the need for such experiments which still can be conducted in a reasonable time on hour(s) scale.

Conclusion The unique combination of a short-pulse neutron source and a high resolution neutron counting detectors provides some novel non-destructive testing techniques which can be conducted only in a few places in the world. The energy resolved neutron radiography spanning a very broad range of neutron energies from tens


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of keV to sub meV energies of neutrons can provide exclusive information on the samples. The presence of a broad range of neutron energies in every neutron pulse enables the simultaneous studies of various aspects of samples. The unique characteristics of short-pulse spallation neutron source at LANSCE in combination with broad experience of its scientists working with various neutron interrogation techniques and novel high resolution detection devices enable a unique suite of novel non-destructive testing techniques which will be improved and extended to the new applications in the near future.

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Acknowledgements

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This work was partially funded by the Department of Energy under STTR Grant No. DE-SC0009657 and was performed under the auspices of the US Department of Energy by Los Alamos National Security, LLC under contract DE-AC52-06NA25396. The detector was developed within the Medipix collaboration at the University of California.

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