Tasks & Partners
Micro pattern structures like MSGCs have been shown to emit 0.2-0.4 secondary photons per electron when operated in a charge amplifying proportional mode in gas mixtures of 3He-CF4. Photon yields of up to 106 photons per neutron have been measured in the wavelength region 220nm < < 750nm. Various potential charge amplifying structures like “standard” MSGCs, semitransparent TIO-MSGCs recently developed at Tokyo University, GEM-detectors or MWPCs shall be investigated in the contributing labs with respect to their physical properties of secondary light production. These properties include spectral response, photon yield and scintillation decay time.
The influence of the gas composition, total gas pressure, CF4 partial pressure and gas purity will be studied in detail as a function of the individual electrode structures. In task 22.2 Prof. Takahashi‟s group from Tokyo University will contribute as non-European collaborator.
The use of an internal light reflector transparent to electrons or the application of external optical elements like dispersers, collectors or lenses could help to enhance the photon yield detected by the optical readout devices. Task 22.2 addresses the evaluation of suitable options and their possible use in a GSPC device.
The photon yield increases with CF4 partial pressure, the neutron detection efficiency increases with 3He pressure. To achieve the envisaged performance, the GSPC has to be operated at high gas= pressure which requires a suitable pressure vessel with an optical window. Mechanical design studies that allow a modular device to be constructed with a minimum dead space between individual modules will be performed
Different readout systems are envisaged for the detection of the scintillation light and the neutron position determination. Monte Carlo calculations taking into account photon yield, optical window, detector quantum efficiency, readout pixel size and pixel distance as well as geometrical parameters shall be performed to optimise the design of the scalable demonstrator detector. Based on the preceding studies and the results obtained within task 22.3 a small scalable demonstrator detector shall be constructed and evaluated in the neutron beam to show the feasibility of the concept.
There are a number of different types of photomultiplier devices that can be used to read out the scintillation light from GSPCs. The photocathode response needs to be matched to the 3He-CF4 emission spectrum and the advantages of using quartz, blue or red PMTs will be quantified. A readout system will be explored using single cathode photomultiplier tubes. Detector performance using this readout system will be determined. In parallel with this work readout devices based on position sensitive PMTs (PS-PMTs) and multi anode PMTs (MA-PMTs) will also be developed. PS-PMTs are single cathodes devices which determine the position of photons incident on their photocathodes using a resistor chain readout. They have the advantage of only requiring four electronics channels to read out each PMT. Position resolution to a large extent is proportional to the magnitude of the light input. Multi anode PMTs are pixelated into typically 16, 64 or 256 channels which can be read out individually. The number of electronics channels is significantly higher than for single cathode or PS-PMTs, but the small pixel size offers the possibility of higher position resolution.
In addition to PMT based readout methods a number of other possibilities will be explored. These will include the use of innovative light detecting devices such as avalanche photodiodes, silicon photomultipliers or hybrid PMTs.
Initially the properties of the GSPCs and their corresponding readout systems will be analysed using digitiser systems which capture individual detector / readout responses for each neutron in digital format for detailed off-line analysis. A number of these systems (e.g. Acqiris) are available at several of the partner laboratories. This information can be used for system optimisation and the development of suitable algorithms required for electronics for real time data processing. The results of these studies will act as input parameters for the development of suitable detector electronics including voltage divider networks, front end electronics and signal processing hardware.
According to the Anger camera principle the position resolution obtained by an analysis of the light distribution on the PMT pixels is considerably smaller than the pixel size. In order to carry out such an analysis the fast preamplifier signals of the illuminated pixels have to be shaped, digitized and correlated to identify neutron events and determine the impact position. In view of the envisaged scalability of the demonstrator detector to a 20 cm 20 cm detector the readout electronics may require a considerable number of pixels. Furthermore, in order to apply the Anger camera principle for position recognition when using MA-PMTs, gain variations of a factor 2-3 between individual pixels need to be compensated. A compact high performance, modular and cost effective front end electronics will be developed, which accounts for the gain spread of individual pixels and the specific signal response of the GSPC structure under investigation. Based on the results of the digitiser analysis dedicated signal processing hardware will be developed to carry out neutron identification and position reconstruction in real time. This hardware will also carry out setup and data collection from the front end modules and provide data transport to the PC.
The results obtained in this task will be used in the construction of the scalable demonstrator detector, readout system and appropriate electronics.
ILL – Institut Laue-Langevin
LIP – Laboratório de Instrumentação e Física Experimental de Partículas. Portugal
STFC – Science and Technology Facilities Council
FZJ- Forschungszentrum Jülich
CNR – National Research Council, Italy
TUM – Technischen Universität München
ToU – University of Tokyo