A universal stand for the measurements of scintillators has been developed. There were conducted the studies on the radiation hardness of organic plastic scintillators UPS-923A, SCSN-81, SC-301 and SC307 based on polystyrene, and scintillators BC-408 and EJ-260 based on polyvinyl toluene on the stand.
Modern spectrometers are designed to be operated under intense radiation field conditions. Elementary particle detectors included in these setups degrade under the influence of ionizing radiation. The determination of the radiation resistance of detector materials is the most important task that researches can come across. A wide variety of materials, which are used to develop such detectors requires numerous measurements. One of the most important aspects of these studies involves the establishment almost-real life operating conditions.
Cosmic radiation is the only constant source of relativistic particles for the stand. To measure a numerous number of scintillators at a low flux density within short timeframes, we need to conduct simultaneous multiple measurements Therefore, a multi-channel universal stand was developed. The stand is constructed of nine-channel optical modules. The signals from the optical modules are sent directly to the 64-channel analog-to-digital converter (ADC), then digitized and recorded by the data acquisition system on a PC. Such a stand allowed using up to 7 such modules and measure up to 63 scintillators simultaneously.
The block-scheme of the measuring stand is shown in fig. 1 [
Each optical module is connected to a nine-channel high-voltage power supply board. Base voltages up to 90 V are supplied to the board via the system bus. There are nine voltage regulators on the board, which are controlled by a slow control system device. Tailored bias voltages are transmitted to all nine channels of the optical module through a short flat cable.
The temperature of the photodetectors is controlled by a thermal sensor installed on the module. It is used for the thermoregulation of the SiPM operating point displacement. The software of the system device changes the output voltage in accordance with the temperature change.
The triggering signal for the ADC is the coincidence of signals from two monitor counters that highlight the region of muon passage. Monitor counter is a 100x100x10 mm3 scintillator, viewed with FEU-85. The scintillator is mated to the FEU-85 using a light guide.
On the developed experimental stand it is possible to measure single plastic scintillators 30x30x3mm3 (fig. 3, a) as well as block-samples (fig. 3, b). The block-scintillators are assembled out of nine single cells 30x30x3 mm3, glued together with epoxy glue with a reflective additive TiO2. Each single scintillator sample has a dimple in the center. The dimple is a place for the photodetector and a lens which collects the light directly on the photodetector.
Scintillators based on polystyrene (SCSN-81 и UPS-923A) and scintillators based on
polyvinyl toluene (BС-408 и EJ-260) were prepared by ISMA (Kharkiv) [
To obtain the value of the output signal in absolute units – photoelectrons (p.e.), it is necessary to convert the values of the signal amplitude from relative units expressed in ADC channels. To do this, all channels were calibrated to a single level of light flux with a reference scintillator. The light yield (LY) of the reference scintillator was measured in fig. 5, b. Recalculation was tailored and performed using gauge coefficients for each channel of the measuring module (total 18 SiPMs). The total error of all measurements does not exceed 5% (fig. 4).
Measurements were carried out before and after irradiation of the studied scintillators. The measurement results of the scintillators before irradiation have a following form (Fig. 6, 7). The overall spread of absolute LY values is determined by the positioning of the scintillators relative to the SiPMs. It should be noted that the number of photoelectrons depends on the size of the SiPM and the size of the scintillator’s dimple. For the 6.2 mm radius dimple, which we use (fig. 3, a), we had about 20 p.e. for UPS-923A samples (fig. 6) and about 22 p.e. for BC-408 samples (fig. 7).
The measurement results of the samples before and after irradiation are presented in tab. 1 (columns 4 and 7). The relative light yield (RLY) of all scintillators was normalized to the LY of BC408 in the ESR reflector before irradiation (tab. 1, column 5) and after irradiation (tab. 1, column 9)
The irradiation was carried out in two different experiments: CMS at CERN [
Type BC-408
EJ-260 UPS-923A
SC-301 SC-307 BC-408
EJ-260 UPS-923A SCSN-81
2.
Reflector
ESR ESR ESR ESR ESR Tyvek Tyvek Tyvek Tyvek 3. #
The LY of the Tyvek-wrapped scintillators (tab. 1, lines 6-8) are two times less than the equivalent ESR-wrapped scintillators (tab. 1, lines 1-3).
EJ-260 scintillators have the highest radiation hardness. The EJ-260 scintillators with a longwavelength emission (490 nm) showed no changes of the LY at CMS experiment (table. 1, line 7). Meanwhile these scintillators irradiated in IBR-2 reactor had a small decrease of LY (table. 1, lines 2).
Scintillators with similar emission wavelength have the similar degradation behavior. So the scintillators with emission in blue area (420 nm), lose more than 50% of LY after the absorbed dose of 0.8 MRad (tab. 1, lines 1,3,4,5).
At the CMS experiment there is a similar behavior for blue scintillators. In contrast to the IBR-2, loss of LY at CMS is less. Irradiation conditions in the IBR-2 reactor are more aggressive than at CMS, which lead to an additional decrease in light output.
The LY for block-samples (tab. 2) does not differ from the LY of single samples (tab. 1, lines 1,3,6). Block-samples demonstrate similar radiation hardness relative to single scintillators.
Tyvek
Relative LY after irradiation
The developed stand allowed us to measure a large number of scintillators in a short-time period. Measurements were performed continuously before and after irradiation of scintillators. The stand is easy to operate, portable and reliable in operation. The obtained results are used to establish a radiation-resistant CMS Hadron Endcap Calorimeter at CERN.