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Facilities for Neutron Radiography in Europe: Performance, Applications and future Use E.H. Lehmann, Paul Scherrer Institut, Switzerland Contact

ABSTRACT

A summary about the European facilities for neutron radiography is presented, based on collected information of a Working Group "Standardisation" within the European COST collaboration 524 [1]. An extension of this database is intended by other published values with the aim to provide best suited options for customers from research and industry if methods in neutron radiography should be efficiently applied.
Based on the recent developments in detector technology and system control there are several new approaches for practical applications as quantitative non-destructive studies, computed neutron tomography and real-time imaging. This article tries to underline the possibilities which are presently available in Europe.

1. Introduction

Neutron radiography (NR) has a tradition in Europe for decades since the first strong neutron sources for research purpose became available [2-3]. This have mainly been research reactors constructed and implemented in the fifties and sixties of the 20^th century. Until the end of the eighties, the most common detector for radiography with neutrons has been X-ray film applied together with a neutron-to-radiation (or light)- converter. This technical approach is described in very detail in several handbooks [4,5] and in reports presented at the World Conferences on Neutron Radiography [6-11].
In the last years, some new detector conceptions have importantly improved the ability of neutron radiography by the availability of digital systems which can be used very efficiently by their much higher sensitivity compared to film, their large dynamic range and linearity and the various options for digital image post-procession.
In this way, also weak neutron sources as low power research reactors, accelerator driven and even mobile sources get the possibility for efficient neutron radiography work. Nevertheless, the quality of neutron imaging depends very strongly on the beam properties regarding collimation, beam size, gamma background and not least neutron flux level.
This paper intends to give a summary of available information about different facilities in Europe dealing with neutron radiography, mainly based on measured system parameters but also on proposed design parameters for new facilities. There is a limitation to those systems of institutes collaborating within the COST action 524 "Neutron radiography for the detection of defects in materials" of the European Community [1]. The author knows that some other facilities exist in Europe (in Belgium, Denmark, Norway and Germany) and hopes to attract them to the COST action in the near future.

2. Short description of the NR facilities

Until now, there are 11 European member countries of the COST collaboration 524, whereas Switzerland and Russia play a special role being not explicitly funded as COST members. The table 1 gives a summary of participating countries, their NR facility sites and the radiography stations available. Obviously, there are large differences by the number of available radiography systems, the involved research groups and the system properties (as will be shown below). Therefore, it seems to be very important to share the knowledge and the possibilities for utilisation of other radiography stations within the COST action. This is efficiently supported by "short term scientific missions" among the participants.

Comment for IT: Facility not yet completed Comment for CZ: Facility not in operation and available now for the COST action Comment for RU, CZ: Russia and Switzerland are not COST countries, but they contribute on its own budget

Table 1: Summary of NR systems available in the COST action 524

In detail, the following radiography beam lines are available in the different member countries:

France
Two groups are active in the COST action, CEA Saclay operating systems at the research reactors ORPHEE, OSIRIS and ISIS and the company SODERN providing and operating mobile sources based on accelerator tubes with the D-D fusion reaction for neutron generation.
Details: http://www.sodern.fr/SODERN/neutronicsa.htm, http://www.cea.fr/

Germany
This country is represented by two groups, one from the Technical University Munich operating a station at the FRM-1 reactor, the other at the Hahn-Meitner-Institute working at a cold neutron beam line of the BER-II reactor. In the near future, the Munich facility will be replaced by a station to be built up at the new reactor FRM-II, proposed to go into operation in 2001.
Details: http://www.frm2.tu-muenchen.de,http://www.hmi.de/

Hungary
This group utilises beam lines at the KFKI institutes research reactor of the Russian WWS type (light water cooled, tank design). Because of the high level of the neutron flux, several applications were successfully implemented which are based on real-time inspection.
Details:http://www.kfki.hu/~baranyai/rtlnr.htm

Austria
The experimental basis for neutron research, including neutron radiography, is the Atominstitute in Vienna. There a TRIGA research reactor is in operation running at 250 kW and two beam lines are suited for neutron radiography.
Details: http://www.ati.ac.at/mainframe_4_ger.html

Switzerland
Since 1997 at the spallation neutron source SINQ of the Paul Scherrer Institut a radiography station is in routine operation (NEUTRA) at a thermal beam line. In 1999, a second station (NCR) was implemented at a cold guide line, mainly devoted to studies for biological samples but also attractive for several radiography investigations due to its alternative properties.
Details: http://www1.psi.ch/www_sinq_hn/welcome_sinq.html

Italy
The TRIGA reactor at the ENEA institute in Cassacia near Rome is the experimental basis for radiography in Italy. In strong collaboration with the University of Bologna the set up of a CCD camera based detector system was developed and will be installed for routine work, including tomography with neutrons.
Details: http://wwwerg.casaccia.enea.it/

Slovenia
The Josef Stefan Institute near Ljubljana is hosting the TRIGA reactor station where one thermal beam line is in use for radiography purposed.
Details: http://www-rcp.ijs.si/index-e.html

The remaining COST members Spain, United Kingdom and Czech Republic have presently no radiography systems in operation or available respectively. The active work is performed by scientific missions at other sites in Europe where beam time is provided based on scientific proposals.
Russia was accepted as COST member, however without direct financial support from the European Commission (similar to CH). Because of the difficult economic situation in this country, the efficiency of the contribution in the COST action depends very strongly on the support from domestic sources (including Russian industry). The knowledge about the Russian facilities is limited and data about it will not be included into this report.

3. Performance parameters for the description of the system properties

In addition to the applied detector system, the properties of the used neutron source and the special formatted beam line will limit the application range and obtainable information (image quality, quantitative data, time sequence of images). Because there is a wide range of different facilities to be described, some basic parameters have to be defined which should characterise the systems and gives the possibility for comparisons.
The following parameters were agreed within the Working Group "Standardisation" of the COST action. The information from the different sites were contributed by answering to a special questionnaire provided by the Working Group.

Neutron flux level:
in most cases experimentally obtained by Au foil activation

Collimation-Ratio:
Cadmium-Ratio: measured by standard methods [12,13] or estimated by geometric consideration of the beam collimator design

Cadmium-Ratio:
measured by Au activation with and without a Cd-cover around the activation foils

Gamma background and the neutron/gamma ratio:
the gamma component of the beam could be measured by a neutron insensitive detector or by gamma devices behind a efficient neutron shielding (the induced activity in the detector by the neutron field must be avoided).

Beam size:
this can be describe either by an efficient diameter (for circular beams) or a beam area.

Because some of the parameters are competing each other (high collimation against intensity and gamma content respectively) it was tried to summarise some parameters to a joint value:

Source strength (SS):
It represents the number of neutrons penetrating the outlet window of the beam line just in front of the objet - detector assembly. This number will be obtained by the multiplication of the neutron flux intensity and the beam area and is given in neutrons per second.

Source quality (SQ):
To get a highly collimated beam, it is necessary to reduce the number of neutrons emitted primarily from the source by either small apertures D or large distances from the source area in order to select the most parallel ones. In this way, high L/D values are indications how parallel the selected neutron comes to the detector plane.

Because the neutron field intensity is following a 1/R² -law it is reasonable to describe the source quality as SQ = SS * (L/D)² .

4. Description and comparison of the neutron radiography beam lines

All the previously mentioned parameters will be presented by figures in this chapter if the necessary data are available for the specific facility. The missing data are described by empty space in the figures. For world-wide comparison to other stations the leading Japanese facility at the JAERI research reactor JRR-3M [14] will be used.

4.1. Neutron flux level
This value describes the performance of the system very globally because the necessary exposure time for image production or the possibility for the implementation of time resolving investigations are strongly related to it. The data are presented by Fig. 1 in a logarithmic scale.

Fig 1: Neutron beam intensities of European NR beam lines (a Japanease one is given as reference

Most of the beam lines have a thermal spectrum, cold neutrons are provided at the CEA-ORPHEE station, the PSI-NCR facility and partly by the TUM-channel 2. At the HMI-beam line the cold neutrons are further mono-chromatised and the beam intensity becomes very low therefore.
The cold neutron guides at CEA and PSI have the highest intensity but limited beam sizes (see below). They are used for practical applications with extended samples in a horizontal scanning mode.

4.2. L/D ratio
This important parameter describes the beam collimation and will limit the obtainable spatial resolution by the inherent blurring independently from the detector properties. This unsharpness U_beam can easily be related to the distance between the object and the detector plane d and the L/D ratio.



The summary of L/D values for the different beam lines is given in Fig. 2. Most of the values are experimentally obtained, the remaining are based on the design parameters of the collimator assembly.

Fig 2: Comparison of L/D values for the different beam lines of neutron radiography stations in Europe (a Japanese one is presented as reference)

Fig 3: The spatial resolution given by the beam collimation at the different beam lines is related to the inherent detector resolution of three radiography methods. Most of the facilities cannot use completely the detector performances due to the inherent unsharpness of the beam.

The very wide range in beam formation properties depends much on the layout of the individual facility, the general performance goals and the practical applications established at these institutes.
The consequences of a limited beam collimation are described by Fig. 2, where the inherent unsharpness of the beam is correlated to the spatial resolution given by some typical radiography detector systems. This behaviour is described by 3 curves for 1, 2 and 5 cm distances between the object and detector plane. It shows which facilities are able to use the features of 3 radiography detectors (X-ray film, imaging plates, CCD + scintillator) regarding their spatial resolution. The distances are mostly given by the sample dimension, which means that a badly collimated beam will describe the front and backward layer with different blurring.
In order to utilise the possibilities of high resolving detectors those systems have highest advantage where the L/D ratio is very large. Future systems should pay attention especially to this parameter.

4.3. Cadmium ratio
This value describes the content of epithermal contributions in the neutron spectrum. There are two aspects concerning these neutrons: the disadvantage by perturbing the thermal interactions with a sample by neutron moderation (especially if it contains hydrogen), the advantage of the efficient use of epithermal resonance reactions in In as neutron detector when strong thermal absorbers should be transmitted [15].
Therefore, it is not easy to decide which aspect is more important for the applications at a NR facility. It depends really on the demands of the experiment and has to be considered individually.

Fig 4: The Cd-ratio describes the content of thermalised neutrons compared to those with higher energies. For some beam lines the information is missing or can it be neglected.

4.4. Gamma radiation background
Caused by the emission during the free neutron generation process (fission, spallation, fusion), there are contributions of gamma radiation within the neutron beam too (absolute values in Fig. 5). It depends on the layout of the beam line, the used moderator (hydrogen or deuterium) and the installed filters in the beam line how dominant this gamma background will be.
Depending on the applied detector, the high background can give disadvantageous fog in the images. This holds for film and especially for imaging plates, but much less for neutrons sensitive scintillators coupled to camera systems.
If the gamma field is collimated as good as the neutron field, the beam line could be used for gamma radiography too. In this way, the disadvantage of a high gamma background can be reversed. However, this approach is very seldom in use. Mostly, the facilities are optimised for a low gamma background accepting a neutron flux reduction by filters (Bi, Pb). The neutron to gamma ratio is described by Fig. 6.

Fig 5: Gamma dose rates for NR beam lines (some data were not available or values can be neglected)

Fig 6: Ratio between the neutron and gamma flux levels in the NR beams (for facilities with unknown gamma fields no value is presented)

4.5. Beam size
The neutron field at the end of a beam line used for radiography image production can be very important for the range of practical applications. Most of the beams have a circular shape, but neutron guide lines have commonly a rectangular cross-section. This shape has to fit with the size of the detector area. The beam area is therefore only an indication of the performance in first order of magnitude.
The guides for cold neutrons have relatively small fields of view but high intensity. Therefore they are used in a scanning mode where the sample is moved together with the detector horizontally across the beam. The homogeneous illumination is guaranteed by the stability of the source or by a monitor controlled movement [16].

Fig 7: Beam area at the end position of NR beam lines

4.7.Performance comparison
As shown in the previous figures, there are large differences in the system properties among the different facilities. Sometimes a compensation is possible between parameters and it seems reasonable to summarise some value to a much more global one.
The so-called source strength SS was introduced in chapter 3 as a unification of flux and beam area. The results are presented in Fig. 8, also in a logarithmic scale. The relation between strong beams with low cross-section are higher evaluated in the SS values.

However, the quality of images depends strongly on the right collimation. According [17] we used the source quality SQ parameter to emphasis this aspect with values given in Fig. 9.

Fig 8: The source strength parameter SS should help for better comparison between different facilities

Fig 9: The source quality value SQ is taking into account the properties of the beam lines concerning beam collimation too.

5. Conclusions

This overview on existing NR facilities operated by COST action 524 members helps to understand the abilities for neutron radiography in Europe. Compared to the conditions outside Europe (especially Japan and North America) there is really a competing situation given by the leading facilities in Hungary, France and Switzerland. The proposed station at the new reactor facility FRM-2 (TU Munich) will enhance the present situation.
Nevertheless, the less powerful stations have good possibilities for practical work due to the increased performances of detector systems available nowadays.

6. Outlook to further NR activities

The European NR community should be well equipped for future work at their different facilities. Based on new developments on the detector side it will be possible to reach better performances regarding quantification, time resolution and image reconstruction. Fortunately, the increasing computer power and disk space will help to solve the challenging problems in digital imaging with neutrons.

Fig 10: The uptake of water from a basin below the soil sample can be visualised and quantified by digital imaging and image post-processing. The distribution is the water amount only, the soil sample was completely "removed". The experiment was done at the NEUTRA station at PSI, Switzerland.

An example is presented in Fig. 10 which is illustrating the moisture movement in a soil sample obtained after subtraction of the sample background in the image. Such procedures are only possible if a fixed digital detector is applied and the tools for image post-processing are available. Much faster processes in bulk samples should be possible to measured in the near future.

References

1. http://www.cost524.com/
2. J. Thewlis, R. Derbyshire, Harwell-Report A.E.R.E. M/TN 37 (1956)
3. J. Stade, Neutron Radiography with Electron Linear Accelerator, Materialprüfung 19 (1977) Nr. 10, pp 436-439
4. P. von der Hardt, H. Röttger (ed.), Neutron Radiography Handbook, D.Reidel Publ. Comp., 1981
5. J.C. Domanus (ed.), Practical Neutron Radiography, Kluwer Academic Publ., 1992
6. Proc. 1^st World Conf. on Neutron Radiograhy, San Diego, 1981
7. Proc. 2^nd World Conf. on Neutron Radiograhy, Paris, 1986
8. Proc. 3^rd World Conf. on Neutron Radiograhy, Osaka, 1989
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10. Proc. 5^th World Conf. on Neutron Radiograhy, Berlin, 1996
11. Proc. 6^th World Conf. on Neutron Radiograhy, Osaka, 1999
12. ASTM-Standard E803-81
13. H. Kobayashi, Proc. of the first Asian symposium on research reactors, Rikkyo University Tokyo, 1986, pp. 885-892
14. K. Kanda, S. Fujine, The status of the neutron radiography research activities in Japan, Proc. 5^th World Conf., Berlin 1996, DGfzP
15. E. Lehmann, P. Vontobel, Utilisation of epithermal neutrons for investigations with strong neutron absorbers and nuclear fuel elements, Annual Report PSI 1998, Volume VI, p.70
16. G, Kühne, R. Erne, The neutron capture radiography (NCR) facility is operating now, Annual Report PSI 1999, Volume VI, p.70
17. H. Kobayashi, Beam formation and characteristic for NR, 6^th World Conf. on NR, Osaka 1999, under publication

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