Translation. Region: Russian Federation –
Source: Novosibirsk State University – Novosibirsk State University –
A rig for studying the radiation aging of solid-state photomultipliers (SPMT) was created by scientists from Novosibirsk State University together with their colleagues from the Budker Institute of Nuclear Physics SB RAS. The rig they developed is designed to operate at the boron neutron capture therapy (BNCT) facility, which is located at the INP SB RAS. It is integrated into the BNCT facility, expanding its capabilities. The rig is designed to study the radiation aging of SPMT. BNCT makes it possible to irradiate the devices under study with fast neutrons, and the rig, in turn, allows one to observe how this process affects their parameters. The first tests of the rig were conducted in November last year.
Solid-state photomultiplier tubes (SSPMs) are a type of photodetector that are widely used in science. They can register single photons that result from the interaction of particles with the substance through which they pass. Since there are many such processes (scintillation, Cherenkov radiation, bremsstrahlung, etc.), the scope of application of SSPMs is very diverse. Almost every modern detector employs hundreds and thousands of SSPMs.
— Under the influence of radiation — in our case, fast neutrons — the material is destroyed. In fact, neutrons destroy the structure of bonds in the semiconductor (usually silicon), from which the TFMTs are made. On the other hand, inside any detector operating at its collider, neutrons are also formed during the collision of counter beams of particles, and, therefore, along with the “useful” particles that the TFMTs are used to register, they undergo radiation aging. As a result, free charge carriers are formed, forming a dark current, and the TFMT at some point simply stops working. Therefore, it is necessary to know the permissible level of radiation at which they can be used. At the same time, the task of physicists is to make detectors such that their systems effectively register particles and at the same time are as little exposed to the harmful effects of radiation exposure as possible, — said the leading engineer of the interfaculty group of advanced developments of the Department of General Physics of the Physics Faculty of NSU, senior researcher at the Institute of Nuclear Physics named after G.I. Budker Institute of Nuclear Physics SB RAS Viktor Bobrovnikov.
In 2022, scientists from NSU and INP SB RAS spent a month at the BNCT facility studying the effect of radiation on the optical transparency of the fiber used in the calibration system of the electromagnetic calorimeter of the CMS detector operating at the Large Hadron Collider at the European Organization for Nuclear Research (CERN). Part of this fiber is located quite close to the collision site of hadron beams, so it begins to darken – “age” under the influence of radiation. Studies conducted by scientists from NSU and INP SB RAS showed that the transparency of the fiber degrades by 25-30% at a dose corresponding to 3 years of CMS operation per experiment. The CMS calorimeter calibration team was completely satisfied with the result obtained. In this experiment, the researchers used the equipment and measurement methodology proposed by foreign colleagues. The experience gained was used to create our stand for studying TFMTs.
The TFMT research setup consists of three main elements. The first is the light distribution system from the source (laser) to the TFMTs under study. It is necessary because all equipment must be located in a radiation-protected area (control room) to prevent damage to the equipment, while the TFMTs are directly exposed to radiation. The second element is a heat and cold chamber. Sometimes it is called a “climate chamber”. It allows you to set a certain temperature for the TFMT from -20 to 55 degrees. Temperature in this case is an important parameter, since the previously mentioned TFMT dark current (or noise) depends on it. If this noise is high enough, it can completely drown out the useful TFMT signal. Also, a “climate chamber” is necessary for researchers because the ambient temperature is quite unstable, and for repeatability of experiments to study the TFMT response, it is necessary to work in one temperature mode under strictly identical conditions. In addition, researchers are interested in conducting research outside room temperature in order to better understand the capabilities of the TFMT. The third important component of the stand is the data collection system. It is needed for digitalization and subsequent recording of signals from the studied TFEU, laser parameters, microclimate parameters in the TFEU location, signals from sensors measuring the stability of the laser source and the transparency of the optical fiber, and so on.
— The solutions implemented in the stand are already used to one degree or another in various installations. The uniqueness lies in the process of irradiating the TFEU itself. Along with the simultaneous measurement of the TFEU parameters, we can evaluate the level of radiation dose. This gives us a rare opportunity to thoroughly study the level of radiation exposure to the TFEU. Such an opportunity is completely absent when conducting similar studies on reactors; in the end, you will only receive an answer about the initial and final state of your device without understanding how its parameters changed during irradiation, — explained Viktor Bobrovnikov.
The stand was tested in November last year. A significant amount of data was obtained, which is currently being processed, but scientists are already noting that the effect of radiation aging of the TFEU has become quite obvious and it remains to complete the analysis to fully understand the whole picture.
— We plan to upgrade the stand taking into account the experimental experience gained. It is impossible to take everything into account at once — some of the features are revealed directly in the process of work. In the conducted irradiation session, we worked with rather old TFEMs, which are now practically not used, but are quite suitable for “testing” the measurement technique in real conditions. Now we have three types of TFEMs, currently used in real experiments. One of them is used in the electromagnetic calorimeter “shashlik” of the MPD detector of the NIKA experiment (Dubna, Moscow). We and our colleagues are interested in knowing the response of these TFEMs to irradiation. So, we have extensive plans, at least for the next 2-3 years, — said Viktor Bobrovnikov.
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