A two-dimensional thermal-neutron detector developed for a small-angle diffractometer by the Petersburg Nuclear Physics Institute is described. The detector, with a sensitive area of 600 × 600 mm2, is based on a multiwire proportional chamber. A gas mixture containing 3He is the neutron converter. A new technology for manufacturing electrodes has been developed and used to increase both the gas purity and the lifetime of the detector without refilling its working volume under the experimental conditions. Data acquisition is carried out by the readout system based on the method of cathode-strip data readout to an LC delay line, which is located inside the detector. The detector is operable in a vacuum owing to its design.

A 64-channel readout and data-acquisition module is described in detail. It consists of an H12700 multianode photomultiplier tube, four PADIWA preamplifier boards, and a TRB v3 card that perform the functions of a time-to-digital converter and a data concentrator. The software modules that are necessary for operation of the prototype are described. The inter-channel delays are calibrated. The drift of individual delays does not exceed 0.5 ns for the entire measurement time. The spectra of the “time over threshold” (ToT) are investigated. The influence of periodic noise pickups and the need to improve circuit designs are revealed. The timing properties of the wavelength shifter and its effect on the detection efficiency for Cherenkov rings are investigated. The most intense component is characterized by a decay time of 1.1 ns and there are components with characteristic times of 3.8 and 45 ns. The influence of single-electron spectrum features on the detection efficiency for photoelectrons and the probability of false hits are determined. The total time resolution of 131 channels is 1.1 ns (FWHM). The results make it possible to use the investigated system of readout and data acquisition in the CBM experiment. Nevertheless, the elimination of the revealed shortcomings will provide the efficiency margin and improve the reliability of the system during long-term operation.

Supersource of ultracold neutrons on the basis of superfluid helium is under construction in PNPI NRC KI. It must provide UCN density 2-3 orders of magnitude higher than existing sources. For the new source we propose an experiment on search for neutron–antineutron oscillations based on the storage of ultracold neutrons in a material trap. The sensitivity of the experiment mostly depends on the trap size and the amount of UCN in it. The results of simulations of the designed experimental scheme show that the sensitivity can be increased by ∼ 10–40 times compared to sensitivity of previous experiment depending on the model of neutron reflection from walls.

We propose an experiment on search for neutron–antineutron oscillations based on the storage of ultracold neutrons (UCN) in a material trap. The sensitivity of the experiment mostly depends on the trap size and the amount of UCN in it. In Petersburg Nuclear Physics Institute (PNPI) a high-intensity UCN source is projected at the WWR-M reactor, which must provide UCN density 2-3 orders of magnitude higher than existing sources. The results of simulations of the designed experimental scheme show that the sensitivity can be increased by ∼ 10–40 times compared to sensitivity of previous experiment depending on the model of neutron reflection from walls.