A three-detector experimental setup for studying the angular distribution of orthopositronium annihilation γ-quanta in a weak magnetic field (<1 kG) using the method of prompt and delayed coincidences is described. The anisotropy of the angular distribution of γ rays from the 3γ decay of polarized positronium has been investigated. It is theoretically shown that this anisotropy, caused by time oscillations of the orthopositronium spin in a magnetic field, occurs when the orthopositronium decay rate in the target is comparable to the frequency of positronium spin oscillations in the magnetic field and that it reaches its maximum when these values are equal.

A method for detecting thorium by its decay products has been tested on PRIPYAT six-crystal 4π γ-ray coincidence spectrometers. This method makes it possible to measure large-volume samples of any shape with a higher sensitivity and a shorter acquisition time than when an HPGe spectrometer with a comparable cost is used.

The new measuring possibilities of 6-crystal gamma-rays spectrometers of “Pripyat” type are reported. Distinctive features of these spectrometers are the 4π registration geometry and large sensitive volume (up to 5 l). Principle of operation of the new registration system and the processing methods of spectra using the corresponding software are described. The minimal volume activities measured by using modernized spectrometers of “Pripyat” type are presented (137Cs, 226Ra, 232Th < 1 Bq/l, 40К < 10 Bq/l).

Spectra-angular distribution of the orthopositronium 3γ-decay was investigated for the first time in an asymmetric geometry of annihilation, using multidetector spectrometer ARGUS. In order to analyse the experimental data, a method for selection of the three-photon annihilation events and background reduction, built on the well-known algorithm of Dalitz plane, was developed. The experimentally obtained distribution is found to be in a good agreement with prediction of quantum electrodynamics, calculated by the computer Monte-Carlo modelling. The technique realised in this experiment is going to be used for testing quantum electrodynamics of bound state.

Processes of the summation of cascade gamma -quanta usually neglected, when registering the annihilation energy spectrum for the determination of the three-photon annihilation probability P-3 gamma of positronium in samples, are studied. The deviations of the actual P-3 gamma value from that determined without allowance made for the summation processes are 56% and 25% for NaI(Tl) and Ge detectors placed at the distance of 3 cm from a positron source, respectively.

The results of experimental test of CPT invariance by observing the angular correlations in the polarized orthopositronium atoms are described. Multidetector setup and a new technique for selection of annihilation events were used. The final measured asymmetry is found to be A 0.0008 +/- 0.00091, that leads to the resulted value of angular correlation coefficient C-n = 0.014 +/- 0.019.

Differential nonlinearity and a high background are the main difficulties in experiments on observation oscillations of orthopositronium spin in condensed media. A new technique to overcome these difficulties is considered.

A method for event selection takes into account all features of orthopositronium-atom annihilation. The spectral and angular distributions of positronium three-photon decay was investigated with the 4π-spectrometer ARGUS. The filtration program for the events corresponding to the three-photon positronium decay was developed, and positronium 3γ-annihilation was simulated by the Monte Carlo method. The measured spectra agree well with the simulation. The technique developed will be used to test the quantum electrodynamics of bound state.

The processes of cascade gamma-quanta summation, disregarded when measuring the energy spectrum of 22 Na in samples by a one-detector spectrometer, are studied, along with the effect of these processes on the change in the area bounded by the 511-keV annihilation line in the calculation of the probability P 3γ of three-photon annihilation of positronium. An increase in the number of annihilation quanta in “positronium-forming” samples and their redistribution to the low-energy spectral region are responsible for the difference between the areas of the photopeak of the accompanying nuclear quantum, with respect to which the spectra are normalized, and of the corresponding peak in the aluminum standard. The departure of the real value of P 3γ from its value, determined without taking into account the summation processes on a highly effective scintillation NaI(T1) detector and a semiconducting Ge detector, amounts to 56 and 25%, respectively, for a distance of 3 cm between the positronium source and the detector surface.

We describe a setup for experimental testing of the CPT-invariance by observing the angular correlations in the decay of polarized orthopositronium atoms. Annihilation events are detected by five NaI(T1) sensors of 150-mm diameter and 100-mm thickness, located at the vertices of a regular pentagon. The sensor signals are detected in the double-coincidence circuits gated by signals with different time delays from the positron escape. The setup has a time resolution of 8.0 ns. In order to reduce the effect of the long-term instability and the instrumental asymmetry on the experimental results, the direction of positron incidence on a SiO 2 target was reversed with a program-driven period during data acquisition. The value of the asymmetry, measured with due account of statistical and systematic errors, is 0.0008±0.00091, which is twice better as compared with the previous experiment.

We describe a simple method for proximate monitoring:of the serviceability and time resolution estimation for the AP Gamma YC 32-crystal gamma-coincidence 4 pi-spectrometer. The method is based on measuring simultaneously, for all detector combinations, the count rates of double random coincidences between two gamma-quanta from a one-cascade monoenergetic source, when gamma-rays are detected by their photopeaks. The spectrometer is described briefly, and evaluation results of this method are presented. It is shown that the spectrometer-testing time:is reduced by several orders of magnitude.

A method for determining the concentration of radon decay products using a six-crystal coincidence 4 pi gamma-spectrometer is described. This technique provides a tenfold decrease in the duration of measurements as compared to that in a single-crystal scintillation spectrometer, in addition to reduction of the time and effort required for the sample preparation and calibration work in various sample geometries. Another advantage on the proposed method is the ability of conducting measurements in areas contaminated with a Cs-137 radionuclide.

In the paper we describe the capabilities of the multidimensional (multidetector) spectroscopy of photon cascades. Attention is paid to a considerable increase in the resolution of multidetector spectrometers compared to one-detector spectrometers. We give a derivation of the radiation intensity detection limit, which relates all the basic characteristics of a multidetector spectrometer, and present examples of calculation of spectrometer characteristics.

We have designed and built an automated spectrometer of delayed coincidences to study a new phenomenon, namely monofrequent oscillations in the angular distribution of gamma-quanta due to 3 gamma-decay of positronium in a weak magnetic field. The spectrometer detection unit consists of three NaI(Tl) scintillation detectors 100 mm long and 150 mm in diameter. The time resolution of the facility is 7 nsec. A magnetic field is generated by permanent samarium-cobalt magnets. A powdered target that generates positronium is placed in a vacuum-tight flask with a positron source made from Na-22. Experimental results are given and a data-processing technique which compensates for the differential nonlinearity of the time-to-analog converter is proposed.

The observation method of gamma-quantum angular distribution oscillations in positronium annihilation in magnetic field is essentially improved. Oscillation parameters in the experiment are in agreement with theoretical estimations.

The existence of the new physical phenomena: time oscillation of the gamma-quantum angular distribution under orthopositronium annihilation in a weak magnetic field is experimentally confirmed and the method of its observation is essentially improved.

The paper describes a magnetic system generating a fild up to 1 kG for experiments involving oscillations in the angular distribution of quanta due to the annihilation of orthopositronium in a magnetic field and lists its parameters. The magnetic field's nonuniformity over a working volume of 5.10(-2) cm3 is under 0.6%, the long-term instability of the field induction is under 0.1%.