·Table of Contents
·Workshop - Neutron workshop
Facilities for Neutron Radiography in Europe: Performance, Applications and future UseE.H. Lehmann, Paul Scherrer Institut, Switzerland
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:
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/
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.
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.
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.
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.
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.
The Josef Stefan Institute near Ljubljana is hosting the TRIGA reactor station where one thermal beam line is in use for radiography purposed.
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.
Neutron flux level:
in most cases experimentally obtained by Au foil activation
Cadmium-Ratio: measured by standard methods [12,13] or estimated by geometric consideration of the beam collimator design
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).
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 .
4.1. 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 Ubeam 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 .
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 .
|Fig 7: Beam area at the end position of NR beam lines|
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  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.|
|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.
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