Neutron Background Measurements in the DEVIS Experiment

Transcription

1 ISSN , Instruments and Experimental Techniques, 2010, Vol. 53, No. 5, pp Pleiades Publishing, Ltd., Original Russian Text V.A. Belov, E.V. Brakhman, O.Ya. Zeldovich, A.K. Karelin, V.V. Kirichenko, A.S. Kobyakin, O.M. Kozodaeva, A.V. Kuchenkov, T.N. Tsvetkova, 2010, published in Pribory i Tekhnika Eksperimenta, 2010, No. 5, pp NUCLEAR EXPERIMENTAL TECHNIQUE Neutron Background Measurements in the DEVIS Experiment V. A. Belov, E. V. Brakhman, O. Ya. Zeldovich, A. K. Karelin, V. V. Kirichenko, A. S. Kobyakin, O. M. Kozodaeva, A. V. Kuchenkov, and T. N. Tsvetkova Institute for Theoretical and Experimental Physics, ul. Bol shaya Cheremushkinskaya 25, Moscow, Russia Received February 24, 2010 Abstract The neutron flux has been measured in the experimental area of the Institute for Theoretical and Experimental Physics in the course of the DEVIS tracking experiment aimed at searching for the double β decay of Xe, since events imitating double β decay may be produced in neutron interactions with the detector materials. Based on the results of measurements with a neutron source, the chemical composition of the materials used in the setup has been refined, and the accuracy of the Monte Carlo simulation of the setup has been checked. Additional sources of neutron background have been found, and their activity has been estimated. DOI: /S INTRODUCTION The DEVIS experiment aimed at searching for double β decay of 136 Xe is being conducted at the Institute for Theoretical and Experimental Physics (ITEP). Since the two-neutrino decay channel has not yet been observed for this isotope, a search for it is still a relevant task. The experimental setup consists of a time projection chamber (TPC) placed in a magnetic field. The overall view of the setup is presented in Fig. 1, and its detailed description can be found in [1]. The main difficulty encountered in searching for rare processes is the problem of background. The DEVIS experiment is conducted using the difference method: the results obtained with the gas mixture enriched to 94% in 136 Xe isotope and with the mixture depleted of this isotope to 2.4% are subtracted one from the other. This helps exclude the contribution of the major part of background processes due to cosmic rays (the detector is located on the Earth s surface) and radiation loading of the detector. The results of these measurements were presented in [2]. A substantial excess of events has been observed in the measurement of 136 Xe, which conflicts with the rate for double β decay of 136 Xe determined earlier in [3, 4]. An unconventional mechanism of background processes responsible for different contributions to the total counting at different isotopic concentrations of xenon in the chamber was considered in [5]. It is based on the fact that the muon and neutron components of cosmic rays produce short-lived radioactive isotopes in the setup, the qualitative and quantitative compositions of which depend on the isotopic composition of the xenon mixture. Subsequent decay of 137 Xe, 134 I, and 136 I isotopes in the setup may generate two-electron events resembling the double β decay of 136 Xe. Though the composition and intensity of the background muon component on the surface are known well [6], the neutron component is nevertheless strongly dependent on the local conditions. Therefore, measurements of the neutron background at the site of the DEVIS setup had to be taken in order to determine more precisely the contribution of the neutron component to the total counting presented in [5]. MEASURING PROCEDURE Neutron counters of the SN-01 type produced by the Scientific Production Center Aspect (Dubna, Moscow oblast) were used to measure the neutron background. They are shaped as 3-cm-diameter 90-cm-long tubes filled with a gas mixture of 3 He isotope and argon at partial pressures of 2.0 atm. The measuring instrument consists of three tubes of this type located parallel to each other in a common plane with a separation of ~4 cm. The counters detect only thermal neutrons in the reaction n + 3 He p + 3 H kev. The cross section of this reaction is 5350 barn. The reaction products lose their energy mostly by ionization of 40 Ar; as a result, the charge proportional to the energy deposited in the gas is collected on the wire of the counter (see [7]). The ionization signal from the counters passed through the preamplifier circuit was thereafter summed and fed to the Nokia multichannel analyzer. The partial spectrum containing pulses from the counters was determined using the analyzer; in subsequent measurements, pulses were counted only in this spectral range. The measurements were taken for two detector modifications. In the first case, the counters were inserted into a polyethylene (C 2 H 4 ) n slab with dimensions of cm and a density 0.94 g/cm 3 (i.e., the counters were covered ) to measure the integrated spectrum by moderation of fast neutrons to thermal energies; in the second, the poly- 629

2 630 BELOV et al. Magnet coils Neutron source H E Multiwire proportional detectors 2900 Xe CH 4 CH E Detector chamber (fiberglass plates) Neutron counters Lead Fig. 1. Overall view of the setup. ethylene was removed and the naked counters measured only the thermal part of the neutron spectrum. CALIBRATION OF THE COUNTERS An IBN-238 Pu Be neutron source with a certified activity of 2.8 GBq as of 1962 was used to calibrate the counters. The source was a capsule with a radioactive preparation, enclosed in a 11-cm-diameter 22-cmlong steel cylinder filled with paraffin. In view of the Pu half-life T 1/2 = 87.7 yr, the neutron yield today is expected to be neutrons/s. The yield measured by the counters is neutrons/s. Therefore, the discrepancy, which is <14%, can be explained by the counter efficiency and the uncertainty in the quantitative composition of the source. The counting rates were measured at different relative positions of the counters and the polyethylene moderator slabs. The counting rate was also measured when the counters were placed under the DEVIS setup near the central chamber, whereas the neutron source was located on the top of the chamber. In this case, the readings of both naked and covered (covered with polyethylene) counters were measured. The results of these measurements were then compared to the results of computer simulation. The counters, the source, and the setup were simulated using the GEANT package [8] with a set of data of neutron cross sections G4NDL 3.12, and the results

3 NEUTRON BACKGROUND MEASUREMENTS IN THE DEVIS EXPERIMENT 631 of this simulation were analyzed with the ROOT 5.18 package [9]. The spectrum of neutrons emitted by a Pu Be mixture was taken from [10]. Since the capsule with the preparation was enclosed in paraffin, the detailed spectrum shape was not so important; therefore, the spectrum was smoothed in the simulation. The spectrum loaded into the GEANT is presented in Fig. 2. The simulation and comparison with the experiment was thereafter performed for different relative positions of the source, the counters, and the polyethylene that covers them. In the tested configurations, polyethylene layers with different thickness were located on opposite sides of the detector, and the source was located at different distances; the counting rate without the polyethylene was also measured. The results of all these measurements for the source and the counters agreed with the simulation results with an accuracy of 15% or better. The results for some arrangements of the counters and the source are presented in Table 1. An experiment was additionally conducted, in which the polyethylene was replaced by a piece of fiberglass plastic that is the chamber material in the main setup. Its composition was determined more precisely during the experiment; fiberglass and epoxy C 15 O 2 H 16 were found to be its compounds. The composition of this fiberglass is presented below according to the data in [11] Material Weight fraction, % SiO 2 54 Al 2 O 3 15 B 2 O 3 10 CaO 17 MgO 4 It is apparent that boron having a large absorption cross section for slow neutrons is present in the fiberglass in rather big quantities and, therefore, has a significant effect on the spectrum of neutrons passing thought the setup. The results of simulations with the old composition of the fiberglass plastic (boron-free) Neutrons/keV E, MeV Fig. 2. Simulated spectrum of the source. and with the refined composition are presented in Table 1. The refined composition ensures better coincidence of the experimental data with the simulation results. Since the setup contains ~2 t of the fiberglass plastic and its composition has a significant effect on the neutron fluxes inside and near the setup, it is the refined composition with boron added that has been used in the simulation. The measurement of the counting rate of the counters located under the chamber with the neutron source located above the chamber at the surface of the active shielding inside the magnet (see Fig. 1) was simulated thereafter. The results of the simulation and experiments coincided with a good accuracy. BACKGROUND MEASUREMENTS After the counters were calibrated, the neutron fluxes were measured at different points in the experimental area and the setup. The counting rate at different points in the experimental area appeared to vary only slightly, which was evidence that neutron sources were absent in the experimental area. In addition, the neutron counting occurred to be constant along the height of the chamber, and, when the counters were Table 1. Counts over 1 h for different configurations of the neutron counters Operating mode and position of the counters Measurements (without boron) (with boron) Naked near the source on the table Covered under the source on the table Naked under the chamber; the source over the chamber Covered under the chamber; the source over the chamber

4 632 BELOV et al. Table 2. Counts over 1 h for different configurations of the neutron counters (experiment and simulation) Operating mode and position of the counters Measurements (without the accelerator) (old background) (new background) Naked under the chamber on the floor in the experimental area under the chamber (the reference point) Covered on the floor in the experimental area at the side of the support displaced in the horizontal plane, the difference was 25% or less. As the basic measurements for further investigations, we considered the results obtained when the counters were placed inside the magnet immediately under the chamber outside the active shielding and on the floor of the experimental hall at a certain distance from the setup. The measurements were taken both with the naked counters that basically detected thermal neutrons and with the covered counters enclosed in a neutron-moderating polyethylene envelope to measure the integrated spectrum. In the course of these measurements, it was found out that the counting rates of thermal neutrons inside and outside the magnet differed by factors of 3 4, whereas the integrated counting rates differed only by a factor of 1.5. The ITEP has an accelerator storage facility based on the U-10 accelerator. The distance from the accelerator target and the beam forming point to our setup is ~300 m. The influence of this distance on the neutron fluxes in the experimental area was estimated. To do this, the neutron counting was separately analyzed when the accelerator was in operation and in the off state. The measurements showed that operation of the accelerator increased the neutron background severalfold and that the specific value of the measured background level was strongly dependent on the operating mode of the accelerator; in addition, its operation had the major effect on the fast neutron flux. In processing of the results of the main measurements in the DEVIS experiment, all data were divided into time intervals with the accelerator in the on and off states; in this case, no increase in events was observed when the accelerator was in operation. Table 2 presents the counting rates of the naked and covered counters in the magnet under the chamber and on the floor far from the setup when the accelerator was in the off state. It also contains the external background level simulated using the spectrum and the intensities of the muon and neutron fluxes described in [5]. According to these data, the total intensity of the flux through a horizontal area element is 1 particle/(cm 2 min) for muons and particles/(cm 2 s) for neutrons. The simulation results are presented in Table 2 in the old background column. In the simulation, the neutron flux was considered to be isotropic within the angles and the standard angular distribution ~cos 2 θ was used for muons. The counting rates obtained thereby appear to be much lower than the experimental data. Taking into account that the muon flux intensity at sea level is known rather well and that the neutron data strongly depend on some features of the surroundings, the neutron flux intensity and possible changes in the relationship between the partial neutron spectra used in the simulation were estimated based on the data for the counters located on the floor in the experimental area and under the chamber. It turned out that, whatever changes were introduced, it was nevertheless impossible to bring the results of simulation and experiments into correspondence for all four measurements (the naked and covered counters under the chamber and on the floor in the experimental area). The presence of an additional neutron source near the setup was taken as the working hypothesis. Based on the results from the covered counters located far from the setup, we determined the external neutron background flux that could not be associated with the additional source. As compared to the value used earlier, it increased by a factor of ~1.33, reaching a value of particles/(cm 2 s). Nevertheless, such an increase in the neutron flux could not explain the neutron background level measured under the chamber. The measurements with the covered counters placed on the floor separately from the setup and closed on the top or on the bottom with sheets of fiberglass plastic (which is used as the chamber material and can act as the neutron moderator and absorber) showed that such a shielding should reduce the counting relative to the case when there was no fiberglass plastic near the counters. Therefore, for the counters located under the chamber that essentially act as the large neutron moderator and absorber, the number of neutrons per time unit is expected to be no greater than at this distance from the chamber.

5 NEUTRON BACKGROUND MEASUREMENTS IN THE DEVIS EXPERIMENT 633 However, in the experiment, the measured counting rate of the integrated flux increased 1.5 times. The flux of cosmic muons trapped in the material of the setup cannot be responsible for this result, since, according to the results of simulation, its contribution is 400 and 40 counts/h for the covered and naked counters, respectively. An additional investigation of the materials and items surrounding the setup was carried out in order to find an explanation for the substantial difference in the counting rate. The floor under the setup was covered with ~57-kg 1-cm-thick lead sheets to protect the setup against γ rays from sources that could be located on the floor of the experimental area. When searching for an additional radiation source, we found out that this lead exhibited a high neutron activity. When removed and stacked far from the setup, the lead sheets with the counters placed on the top of them exhibited almost linear dependence of the counting rate on the number of sheets in the stack. In this case, each additional layer increased the total counting by 700 counts/h. The neutron flux emitted by the lead was simulated. In the simulation, the angular distribution of neutrons was considered to be isotropic, and the points of their production were uniformly distributed in the bulk of the material. Since it was for the covered counters that a significant excess of events was observed in the experiment, the neutrons had to exhibit a high-energy spectrum; therefore, the neutron spectrum identical to that of the neutron source was used in the estimation (see Fig. 2). The simulation showed that such a counting rate corresponded to specific neutron activity of the lead A Pb = neutrons/(kg s). If the total mass of the lead under the setup is 1260 kg, this activity provides a weighty contribution to the neutron background near the detector. Nevertheless, the simulation showed that the contribution of the lead to the readings of the covered counters located under the chamber was only 320 counts/h. This can be attributed to the fact that the lead sheets are distributed over a large area under the chamber and that the contribution to the readings of the counters is strongly reduced by the geometrical factor. The additional measurements taken using the covered counters located under the setup on the floor far from the TPC center after the lead sheets were removed from the floor showed that the background level increased only slightly relative to the measurements taken far from the setup. Therefore, we can consider that the entire background from non-lead neutron sources located on the floor of the setup is a component of the total neutron background used in the earlier simulation. The other set of measurements was performed under the setup near its center, in the vicinity of the vertical support that carried the chamber (see Fig. 1). It turned out that the neutron background measured near the setup using the covered counters was substantially increased. The counting rate was ~6000 counts/h at the concrete floor, 7500 counts/h close to the vertical part of the support, and 6800 counts/h at some distance from the support at a greater height above the chamber. Therefore, the metal that is the material of the support is also contaminated with neutron sources. Unfortunately, it is impossible to take a sample of the support material and analyze it in independent measurements in the same manner as it was made in the case of the lead. Therefore, the remaining excess in the number of neutrons under the chamber was ascribed to the source uniformly distributed over the volume of the vertical part of the support, its angular distribution was considered to be isotropic, and the spectrum was assumed to be identical to that of the IBN-238 source (see Fig. 2), since a significant excess in events was again observed in the integrated spectrum. The vertical part of the support had a mass of 465 kg, and its specific activity determined using this method was neutrons/(kg s). The simulated readings of both naked and covered counters placed under the setup are presented in column new background of Table 2. It is apparent that, after all refinements, the obtained neutron and muon fluxes and spectra provide a good description of the actual neutron background in the setup. CONCLUSIONS A set of measurements was performed using SN-01 neutron counters in order to determine more precisely the parameters of the external neutron background that affected the intensity of the processes imitating double β decay events. The calibration was made using the Pu Be neutron source, and the chemical composition of the constructional materials of the setup was determined more precisely. The neutron background level was measured thereafter at several points of the setup and at different operating modes of the accelerator. The estimates for the neutron background intensity in the experimental area were refined, and new neutron background sources were found. The resultant fluxes of muons and external neutrons are as follows: I µ = 1 particle/(cm 2 min) and I n = particles/(cm 2 s). ACKNOWLEDGMENTS We thank M.V. Danilov for his interest in our experiment. This work was supported by the Russian Foundation for Basic Research, grant nos _a and _b. REFERENCES 1. 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