The severe shortage in supply of the standard neutron converter ³He created serious problems in providing large area detectors for detection of thermal neutrons. ³He is a by-product of military tritium production which is no longer produced since the Cold War ended in beginning of the 1990s. The world's annual available amount of ³He is not enough to meet the demand by neutron scattering instruments – especially at neutron spallation sources - and for civil defense.
Alternative detection technologies for neutron scattering instruments with large active areas (2 to 50m²) are mandatory. Here, the substitution of the traditional ³He neutron converter is the dominant challenge for the technological design of new types of neutron detectors.
Goals and objectives
Promising alternatives that are attended as part of the Detector technology and Systems Platform are a) detectors with improved scintillation materials and b) 10BF3 (boron trifluoride) detectors with optimized gas gain, accompanied by the elaboration of required safety measures and c) the development of 10B-film converter detectors.
Currently there are two types of scintillation materials used. The default materials are 6Li glass in Anger cameras and 6LiF/ZnS plates or structures read out with an orthogonal array of wavelength shifting fiber (WLSF). Both scintillation materials do currently not meet the requirements. Reasons are the high gamma sensitivity of 6Li glass together with high material costs and the high opacity and therefore a rather low detection efficiency of 6LiF/ZnS. Existing scintillator mixtures of 6LiF/ZnS use converter material with grain sizes of a few microns in diameter. To increase the efficiency and the light output, new mixtures have to be developed applying methods of today’s available nanotechnologies enabling the production of nanocrystalline materials with grain sizes ranging from 1 to 30nm. The resulting lower light scattering within the scintillator increases the transparency of the material, hence thicker layers and thus higher detection efficiencies will be possible.
When using boron trifluoride 10BF3 the smaller cross section for neutron conversion of 10B as compared to ³He must be compensated. This would be possible by using higher gas pressures; however, this is foiled by the strong electronegativity of BF3 gas or traces of admixtures which limit operation pressures to less than 2bar. Thus, an important aspect is an enlargement of the detection depth by maintaining the time-of-flight resolution good enough for experiments. This is possible by dividing the detector in separately readable cells which also shortens the drift path of the released electrons avoiding too high attachment. Losses of electrons could also be compensated by positively acting gas admixtures which having e.g. lower ionization thresholds. Another important element is the development of appropriate safety systems, taking account of the toxicity of BF3 , and allows a low-risk production of large area detectors and their application in user-operated instruments.
The HZG is developing a neutron detector with solid layers of the neutron converter 10B. The concept of HZG is to incline the converter layers to very small angles, as small as 2.5° with respect to the incident neutron, and thus increasing the absorption thickness to ~30µm while keeping the thickness for escaping reaction particles as small as ~1µm. This project is embedded in the Design Update of the European Spallation Neutron Source ESS which is under construction in Lund/Sweden.
The aim of this portfolio topic is the development of a readout technology for such detectors at previously unattainable data rates. The concept takes advantage of ASICs developments which are also topic in the current portfolio collaboration. This extremely fast data readout has the potential to be applied in synchrotron radiation detection, too, by exchanging the neutron converter by a high-Z material.
|FZJ||Investigation of samples of LiF/ZnS scintillators in different ratios with nanocrystalline ZnS (active area ~1cm²); characterization of light emission and efficiency and optimization of mixing ratios.|
|HZB||Building of a prototype detector filled with 10BF3 to optimize gas amplification processes; development of safety systems for BF3-filled detectors.|
|HZG||Integrating of a readout technology into detectors with 10B thin film converters at previously not achievable data rates, basing on ASIC developments within portfolio activity.|