Imaging with cold neutrons

Imaging with cold neutrons

Nuclear Instruments and Methods in Physics Research A 651 (2011) 161–165 Contents lists available at ScienceDirect Nuclear Instruments and Methods i...

620KB Sizes 2 Downloads 52 Views

Nuclear Instruments and Methods in Physics Research A 651 (2011) 161–165

Contents lists available at ScienceDirect

Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

Imaging with cold neutrons E.H. Lehmann n, A. Kaestner, L. Josic, S. Hartmann, D. Mannes Spallation Neutron Source Division, Paul Scherrer Institute, CH-5232 Villigen, Switzerland

a r t i c l e in f o

abstract

Available online 17 December 2010

Neutrons for imaging purposes are provided mainly from thermal beam lines at suitable facilities around the world. The access to cold neutrons is presently limited to very few places only. However, many challenging options for imaging with cold neutrons have been found out, given by the interaction ˚ behavior of the observed materials with neutrons in the cold energy range (3–10 A).

Keywords: Neutron imaging Beam properties Micro-tomography Energy selection Bragg edges

For absorbing materials, the interaction probability increases proportionally with the wavelength with the consequence of more contrast but less transmission with cold neutrons. Many materials are predominantly scattering neutrons, in particular most of crystalline structural materials. In these cases, cold neutrons play an important role by covering the energy range of the most important Bragg edges given by the lattice planes of the crystallites. This particular behavior can be used for at least two important aspects—choosing the right energy of the initial beam enables to have a material more or less transparent, and a direct macroscopic visualization of the crystalline structure and its change in a manufacturing process. Since 2006, PSI operates its second beam line for neutron imaging, where cold neutrons are provided from a liquid deuterium cold source (operated at 25 K). It has been designed to cover the most current aspects in neutron imaging research with the help of high flexibility. This has been done with changeable inlet apertures, a turbine based velocity selector, two beam positions and variable detector systems, satisfying the demands of the individual investigation. The most important detection system was found to be a micro-tomography system that enables studies in the presently best spatial resolution. In this case, the high contrast from the sample interaction process and the high detection probability for the cold neutrons combines in an ideal combination for the best possible performance. Recently, it was found out that the energy selective studies might become a research field in its own sing the Bragg edge behavior and its modification to contribute to material research by the direct visualization of textures and the observation of stress and strain. This topic is still in the beginning but has some important relevance for the design of future beam lines for imaging at the pulsed spallation sources. Considering the neutrons to be waves, the cold energy range is important to push and to investigate phase effects in detail with high spatial resolution. Although a lot of studies have been done in this respect previously, there is enough space to study refraction at the edges, diffraction and total reflection with the best possible accuracy, and to figure out when and why neutrons interfere. Phase contrast methods like grating interference methods have to be implemented as a user option, which enables one to define their future application range. & 2010 Elsevier B.V. All rights reserved.

1. Introduction Most of the facilities for neutron imaging are coupled to a source of thermal neutrons. The access to cold neutrons is a new aspect that came into the game at only few places recently. Now we have some experience in imaging with cold neutrons and promising new aspects can be handled in this way. This article gives an overview on experiences from neutron imaging studies, mainly based on

n

Corresponding author. Tel.: + 41 56 310 2963; fax: + 41 56 310 3131. E-mail address: [email protected] (E.H. Lehmann).

0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.11.191

investigations at the ICON facility at PSI. Furthermore, a prospect is delivered for future developments in the field of neutron imaging using cold neutrons.

2. Cold neutrons—definition and sources Neutrons as uncharged particles cannot be influenced in their motion and energy by magnetic or electric fields but only by nuclear collisions within a moderator material, which is characterized by its temperature. Under equilibrium conditions, the molecular motion in the moderator material corresponds just

162

E.H. Lehmann et al. / Nuclear Instruments and Methods in Physics Research A 651 (2011) 161–165

with the averaged energy of the neutrons migrating within the moderator. The energy can be defined according to the relation

60

E ¼ kB T

50

ICON

ð1Þ

2

E ¼ h2 =ð2ml Þ

ð2Þ

Cold neutrons are defined to have wavelengths longer than 30 nm, corresponding to 3 A˚ (an even more common length scale unit within the neutron research community). Fig. 1 shows the core region of the FRM-2 reactor, where a cold source is an insert vessel with 20 l of liquid deuterium, located inside

FUNSPIN BOA

40

Counts

Here the Boltzmann’s constant kB is the essential parameter. In units of electron volt (eV), cold neutrons are defined to have energy below 15 meV. Often the wavelength scale is used instead, which is based on the relation

NEUTRA 30 20 10 0 0

1

2

3

4

5

6

7

8

9

10

11

Wavelength [Å] Fig. 3. Wavelength neutron spectra of different beam lines at SINQ, with either thermal ˚ or cold neutrons; as per definition cold neutrons have a wavelength more than 3 A.

Fig. 1. Top view onto reactor core with D2 cold source and beam port of ANTARES imaging facility.

the main heavy water moderator. The beam ports are arranged in a manner such that an optimal cold flux can be extracted while the gamma radiation is suppressed by the tangential alignment. The NI facility ANTARES [1] is fed by the cold neutrons where some thermal and epithermal neutrons also contribute to the entire spectrum. The layout of the spallation neutron source SINQ is shown in Fig. 2. Similarities with the FRM-2 design are visible, indicating the same optimization criteria for the best neutron utilization. The imaging beam line ICON [2] has a direct view to the cold deuterium source, similar to ANTARES, and thermal neutrons from the area behind the cold source insert also contribute to the resulting spectrum. Fig. 3 shows the neutron spectra of beam lines at SINQ, including those that are mentioned later in the text. There is some discussion about the beam line layout with respect to the use of neutron guides. On the one hand, a guide system enables high neutron intensity and can increase the cold contributions in the spectrum by suppressing misleading higher energies when curved. On the other hand, it will increase the beam divergence and has locally varying spectra. Gaps in the beam line will directly be visible in the open beam distribution. CONRAD at HZB [3] works in that manner.

3. New aspects for imaging using cold neutrons We identified at least six different aspects of the use of cold neutrons for the purpose of imaging. These investigations are in a preliminary stage because the experience in this new field is still limited. As will be shown below, the field of cold neutron imaging has a high potential for interesting future research and a lot of practical applications.

3.1. Cross-sections for cold neutrons (contrast/transmission)

Fig. 2. Central part of spallation neutron source SINQ, showing D2 cold source and beam ports for ICON and BOA beam lines.

Neutrons are either absorbed or scattered by the interacting material. The absorption cross-section increases proportionally with wavelength l and the contrast increases accordingly for strong neutron absorbers like the detector materials Gd, 10B and 6Li. Therefore, cold neutrons have to be used preferentially in order to increase the spatial resolution [4] in ultra-thin scintillation screens. Also the scattering probability increases towards longer wavelength, however less strongly than for the real absorbers. Fig. 4 compares the beam attenuation for water in a thermal and a cold

E.H. Lehmann et al. / Nuclear Instruments and Methods in Physics Research A 651 (2011) 161–165

beam. The higher slope for the cold neutrons has importance, e.g. in the more sensitive quantification of very thin layers of water.

163

For the utilization and investigation of Bragg scattering properties, cold neutrons are therefore most important.

3.3. Energy selection—different options

3.2. Bragg edges of structural materials Structural materials like major metals (Fe, Ni, Al, Cu, Zr, etc.) have a quasi homogeneous poly-crystalline structure and a specific neutron scattering behavior that dominates over their absorption properties. Fig. 5 delivers the cross-section data [5] of some of the mentioned materials, derived from theoretical lattice cell parameters and the linked Bragg scattering law. For real metallic samples, these data are valid in first order only because texture and other structural properties will cause a deviating behavior even on the macroscopic scale. The major point in Fig. 5 with the pronounced Bragg edges is the fact that they can be found preferentially in the cold energy range.

If neutron imaging is performed with a broad cold spectrum the properties of the crystalline materials average over all the involved energies. A preferential scattering behavior at lattice planes of the crystallites will not become visible therefore. A better access to the Bragg edges is only possible when the energy bands are narrowed, which means mono-energetic neutrons are provided. There are at least four options available and are tested to provide cold neutrons with a small band width for imaging purposes

 turbine type energy selector (PSI),  double-crystal mono-chromators (CONRAD, ANTARES),  narrowing setup (by M. Tamaki) with single crystals pairs (PSI) and

 time-of-flight techniques, pulsed sources preferred (ISIS). Although the number of neutrons in the remaining tight energy band is much smaller than in a white beam, the exposure time in energy selective imaging remains within some minutes and are still practical even for tomography studies. Energy selective neutron imaging has been used for the following important aspects:

 To enhance or reduce the contrast when measurements are





Fig. 4. Neutron transmission of water layers with neutrons of ICON and NEUTRA beam lines. Data are obtained from Ref. [12].

performed above and below a Bragg edge of a structural material. Choosing the right energy enables to set a material more transparent while a higher contrast occurs for another one. The quantification of material content is more precise if a small energy band is used only. This is of particular importance if beam hardening effects should be suppressed in tomography studies. It has already been demonstrated that textures become directly visible on the macroscopic scale [6] in some cases of steel welds

3.0 Ni

Σ(λ) [cm-1]

2.5 2.0

Fe bcc

1.5 Cu

1.0 0.5 0.0

λ [Α]

1.0 ICON NEUTRA

Pb

Zr

Al

0.5

0.0 1

2

3

4

5

6

7

8

9

λ [Α] Fig. 5. Cross-section data of several structural materials, characterized by Bragg edges, mainly in cold region.

164



E.H. Lehmann et al. / Nuclear Instruments and Methods in Physics Research A 651 (2011) 161–165

and rolled Al plates. We are searching for more of such examples by narrowing the specific energy bands. It is still a vision with some prospect to be able to visualize directly the strain in metallic structures by selecting the right energy band. With the inherent high spatial resolution, such an approach will be a complementary technique compared to the common diffraction methods with spatial resolution in the mm range and a slow scanning procedure.

3.4. How to distinguish water and ice by cold neutron imaging It is of high interest for many applications to get easy knowledge when and how the transfer from water to ice and the opposite happens in various environments. A prominent recent case is the PEM fuel cell under cold startup conditions. Studying the attenuation coefficients of water and ice indepen˚ effective attenuation dently over the energy range from 2 to 6 A, coefficients can be derived according to the relation (with water/ice layer thickness d and the intensities in front and behind the sample, I and I0, respectively)   I d ð3Þ Seff ¼ ln 0 I Although thickness d was not controlled during the experiments at the CONRAD beam line using the double-mono-chromatizer setup, it was individually stable over the whole energy scans. Therefore, the absolute increase are lesser, but the differing slopes for water or ice in Fig. 6 is of importance. So, the ratio between the attenuation at different wavelength ˚ increases up to about 10% at 6 A. Another experiment was performed at ICON recently, where the phase transfer was directly observed with neutrons of either 3 or ˚ and the ratio in Fig. 6 was verified. The results during the phase 6 A, transfer are shown in Fig. 7 for the 6 A˚ measurements. It can be concluded that the water–ice phase transfer can be observed better at cold neutrons because the cross-sections are more different despite the density change. Further studies have to be made to complete this picture in imaging of the water/ice transformation. 3.5. Refraction/reflection at edges It was shown in 1998 [7] already by experiments with a pinhole geometry and a relatively large distance between sample and

4.5

Σ [cm-1]

4.0

Fig. 7. Reflection and refraction at a mirror with m¼ 4.6 when sample is turned by angle of 0.31 with respect to beam direction using cold white beam at ICON.

detector that ‘‘edge enhancement’’ takes place with loss of intensity inside the object region and a gain outside the object region. At this time, the image data obtained under the described conditions were attributed to ‘‘phase contrast’’ in a sense of Fresnel interference caused by spatially coherent neutrons. However, similar such enhancement has been found with samples in closer distance between sample and detector and with much larger aperture of the beam [8], mainly with cold neutrons. A high resolution of the detector was needed to visualize the edge effects in the right manner. As claimed by Strobl et al. [9], the reason for these effects is to be seen more in refraction than in diffraction or interference. Probably, some reflection on surfaces might contribute to the edge effects too. It is well known that a refraction index can be derived also for neutrons 2

n ¼ 1

3.5

3.0 ICE WATER

2.5 2.0

2.5

3.0

4.0 4.5 3.5 Wavelength [Å]

5.0

5.5

6.0

Fig. 6. Energy dependent experimental attenuation cross-sections of water and ice measured at CONRAD beam line, HZB. Both curves deviate in cold region with higher slope for water than for ice. This should be a useful property to distinguish both states better in future.

Nl Ub 2p

ð4Þ

Here, atomic density N, wavelength l and scattering length b have to be taken into account. Refraction index n describes the change in the beam direction after passing an interacting material. This deviation is of the order of 10  6 only, enough to be seen with a high resolution detector as the micro-setup [4] at ICON. Total reflection should take place if the following condition is satisfied for the glancing incident angle g: rffiffiffiffiffiffi Nb ð5Þ g rarc cosðnÞ ¼ l

p

As relations (4) and (5) indicate, the effects are wavelength dependent and increase towards longer wavelength. Therefore, cold neutrons are preferred to study refraction and reflection of materials under certain conditions in more detail. The aim is either to use edge effects to enhance the visibility or to suppress it if disturbing the images.

E.H. Lehmann et al. / Nuclear Instruments and Methods in Physics Research A 651 (2011) 161–165

For this purpose we aligned a small piece (20 mm  20 mm) of a neutron super-mirror (m¼4.6) at 5 cm distance from the microsetup detection system. The observation was done by turning the angle in 0.11 steps from 51 towards 51. The reflected neutron component is clearly visible (see e.g. Fig. 7) moving from the mirror to the outside region more and more. However, there is an additional fringe on the other side of the sample, which must clearly be attributed to refraction at the edge. It can be stated that both refraction and reflection can contribute to edge effects depending on the material properties and beam conditions. 3.6. Polarization option in imaging Neutrons as elementary particles have a nuclear spin of 1/2, which occurs in two states ( 1/2 and +1/2) in equal ratio. In addition, neutrons carry a magnetic moment, which makes them sensitive to magnetic fields and magnetic phenomena. Sorting out one spin state means polarization of the neutrons and the collective behavior of these polarized neutrons can be used best to study magnetic effects that would be averaged out in a unpolarized beam. Imaging with polarized neutrons is a new field with some pioneering experiments [10] done recently. It was shown that the Larmor precession in magnetic field B by angle y can be used to induce an image contrast where the distribution of the magnetic field can be observed directly Z glm Bds ð6Þ y¼ h where m is the rest mass of the neutron and h is Planck’s constant. To use this approach efficiently, polarized but also monoenergetic neutrons are required because the superposition of neutrons with different wavelengths and therefore different precession angles would average out themselves. Cold neutrons are clearly favorites for an optimal use of this promising method in the future.

165

4. BOA as future imaging option at PSI A project was started in 2009 at PSI in order to modify the beam line FUNSPIN [11], which has been used for nuclear physics experiments with cold neutrons into a multiple purpose test beam line, called BOA. One of the future utilization options is neutron imaging, where the softer wavelength spectrum compared to ICON, the higher intensity but in particular the polarization of more than 90%, will be important. For the moment, only a simple CCD based imaging system has been installed, but further installation of a turbine selector, guiding fields and analyzer devices accompanied with infra-structure for inducing magnetic effects will follow. It is intended to perform competitive studies of magnetic imaging in the near future.

References ¨ [1] E. Calzada, B. Schillinger, F. Grunauer, Nucl. Instr. and Meth. A 542 (2005) 38. ¨ [2] G. Kuhne, et al., Swiss Neutron News 28 (2005) 20 /http://sgn.web.psi.ch/sgn/ snn/snn_28.pdfS. [3] A. Hilger, N. Kardjilov, M. Strobl, W. Treimer, J. Banhart, Phys. B: Condens. Matter 385–386 (2006) 1213. ¨ [4] E. Lehmann, G. Frei, G. Kuhne, P. Boillat, Nucl. Instr. and Meth. A 576 (2–3) (2007) 389. [5] J.R. Grenada, Z. Naturforsch. 39a (1984) 1160. [6] W. Kockelmann, et al., Nucl. Instr. and Meth. A 578 (2007) 421; M.D.I. Manke, N. Kardjilov, A. Hilger, M. Strobl, J. Banhart, New J. Phys. 11 (2009) 043013. [7] D.L. Jacobson, B.E. Allman, P.J. McMahon, K.A. Nugent, D. Paganin, M. Arif, S.A. Werner, Appl. Radiat. Isot. 61 (4) (2004) 547. ¨ [8] M. Strobl, N. Kardjilov, A. Hilger, G. Kuhne, G. Frei, I. Manke, Nucl. Instr. and Meth. A 604 (3) (2009) 640. [9] Markus Strobl, Wolfgang Treimer, N. Kardjilov, Andre´ Hilger, Nucl. Instr. and Meth. A 542 (1–3) (2005) 383. [10] M. Dawson, I. Manke, N. Kardjilov, A. Hilger, M. Strobl, J. Banhart, New J. Phys. 11 (2009) 04013. [11] J. Zeima, et al., Nucl. Instr. and Meth. A 539 (2005) 622. [12] D. Kramer, Mass transport aspects of polymer electrolyte fuel cells under twophase flow conditions, Dissertation TU Freiberg, 2007.