- Email: [email protected]
The multielement proportional chamber — a promising tool for double beta decay experiments
454
THE MULTIELEMENT PROPORTIONAL BETA DECAY EXPERIMENTS
Nuclear
Instruments and Methods in Physics Research
B17 (1986) 454-457 North-Holland, Amsterdam
CHAMBER
- A PROMISING
TOOL FOR DOUBLE
P. KUBINEC, J. MASARIK, 1. MEL0 and P. POVINEC Comenius Universiry. Faculty of Malhematics and Physics. Departmenr oj Nuclear Physics, 842 15 Bratislava, CSSR
The extension of the multielement proportional chamber techniques for double beta decay experiments is discussed. Results of computer modelling of double beta decay events in the multielement proportional chamber are described. If xenon under the pressure of 3 MPa is used as the gas filling, the optimum element radius of 1.0 cm is found. The operating characteristics of a proportional chamber consisting of 19 elements are presented.
1.
Introduction
Low-level proportional counters are widely used in various branches of pure and applied research. such as the measurement of cosmogenic radionuclides ( 3H, 14C, “At-. 39Ar, “Kr, etc.) in terrestrial and extraterrestrial samples, low-level anthropogenic radioactivity measurements (3H, 14C, 85Kr, etc.), studies of rare decay processes, etc. (see, for example, ref. [1,2]). For traditional proportional counters used in lowlevel counting the possibilities for further improvement of their characteristics have already been exhausted. It has been shown [3) that good low-level detectors should have a high detection efficiency, a high volume efficiency and a low background. On the basis of these requirements multielement proportional chambers (MEPC) have been developed [4,5]. The simple single wire counter has been replaced by seven or more element counters of the same dimensions arranged in a hexagonal form, separated from each other by cathode wires. The MEPC design solves all the problems encountered in the construction of low-level detectors. The counter diameter is considerably decreased. which means a higher gas pressure can be used at the same working voltage. This geometry also fulfils requirements on long-term stability [6), as the effect of electronegative impurities on the counter characteristics is much weaker. Another important advantage of this design is the essential decrease of the counter background using a system of internal and external anticoincidences [7]. MEPC together with multichannel electronics operating on-line with a minicomputer opens up new possibilities in lowlevel counting of several radionuclides IS]. It has been suggested to use MEPC in experiments searching for double beta decay and for investigation of other high order rare processes [7,9,10]. This is because a MEPC enables one to use large-volume gas samples and to reach a very low background and long-term stability. 0168-583X/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
The double beta decay is one of the rarest processes that may occur in nature. The increasing interest in the study of this process is determined by the expectation arising from grand unified theories, which suppose that several conservation laws may be violated. This may lead to such interesting phenomena as lepton number nonconservation, possible right-handed admixtures in the weak lepton current, and the mass and charge conjugation properties of the electron neutrino. According to the currently accepted laws of physics, the double beta decay may occur as a second order weak interaction process with emission of 2 beta particles and 2 neutrinos. Thanks to the abundant final state with 4 leptons, occurrence of this process is rather suppressed. If the neutrino is a Majorana particle, a decay mode with emission of 2 beta particles is possible, producing a neutrinoless final state. Thanks to only 2 leptons in the final state, this mode enjoys a considerable phase-space advantage over the previous one. From the experimental point of view a certain sign of the physically attractive neutrinoless decay is the appearence of two electrons with a sharp total transition energy, since the kinetic energy of the recoiling nucleus is negligible. This fact can be used for experimental discrimination of the neutrinoless decay over the 2 Y decay. Although the double beta decay is the oldest open problem in nuclear physics with a large number of experiments carried out in the past, no direct positive experimental result is available at present: Therefore new experiments exploiting new techniques are needed. A new scope of activity seems to lie in exploiting the possibilities of the MEPC technique, described also in ref. I9-12}, and in exploiting drift chambers for low-level counting work [13-17). Among the prospective double beta decay emitters a few isotopes of rare gases can be found ( s6Kr, ‘“Xe, 136Xe). These isotopes enjoy a great advantage over the others: they can be used both as a sample and as the detector medium-gas filling for the MEPC. ‘36Xe is the
P. Kubinec et al. / MEPC for double p decay experiments
best candidate because of the highest transition energy of the double beta decay (2478 * 5 keV). We have been concentrating at present in collaboration with Prof. A.A. Pomansky’s group of the Baksan Neutrino Observatory on developing a large-volume MEPC filled with xenon. In this paper results of computer modelling of double beta decay events in the MEPC are discussed, and operational characteristics of a MEPC consisting of 19 elements are presented.
2. Computer modelling of neutrinoless double beta decay events in a MEPC The radius and length of the elements, as well as the pressure of the xenon gas filling, are considered to be free parameters. The calculations have been performed under the following assumptions: 9 Each decay event has been generated in the central element by the Monte Carlo method. ii) The total transition energy E,, has been distributed randomly according to the formula
455
Table 1 Mutual electron correlations [%] Forr=lcm,p=3MPa 1, J I=1
2.1
J=l 2 3 4 5
2
3
4
5
10.1 9.9
9.5 17.3 6.7
8.1 13.9 5.3 0.2
6.7 6.1 0.5 0.0 0.0
2
3
4
5
15.3 14.4
13.4 22.8 4.5
10.3 9.9 0.8 0.0
3.1 1.2 0.0 0.0 0.0
4
5
For r = 1.25 cm, p = 3 MPa. 1, J I=1 4.1
J=l 2 3 4 5
Forr=2cm,p=3MPaand/orr=1.5cm,p=4MPa 1, J
p(E)=E(E2-m2c4)1’2(E,,-E)
I=1 9.5
J=l
Directions of emitted electrons have been distributed according to the function (1 - cos 9). This case corresponds to the assumption of the massive Majorana neutrino existence. iii) The path of the created electron with kinetic energy E, in the MEPC has been approximated by a semiempirical formula R =
0.412~(‘.265-0.0954 e
In E,)
(2)
valid for the energy interval 0.01 MeV d E, < 2.5 MeV. This approximation is rather simplified (later we shall use more rigorous one), however, it helps in the first orientation in the problem. Under these assumptions we have computed correlation probabilities of electron pair registration in several elements for a given element radius and a gas pressure. The results are summarized in table 1. The correlation (I, .Z) means that one electron reaches an element lying in the Zth ring from its origin, while the other reaches an element in the .Zth ring. Thus, for example, the correlation (1, 1) corresponds to the case when both electrons do not leave the element of their origin; the correlation (1, 2) corresponds to the case when one electron is stopped in the element of its origin, while the other electron is stopped in an adjacent element, etc. The most favourable decay events are considered when an electron pair hits 3 or 4 different adjacent elements. Events with a smaller number of elements are difficult to distinguish from the background, and hits with more than 4 elements require complicated elec-
2 3 4 5
2
3
34.1 24.9
17.7 11.6 0.2
1.5 0.5 0.0 0.0
0.0 0.0 0.0 0.0 0.0
tronic processing. From this point of view, the most favourable correlations are (1, 3), (2, 2), (2, 3) and (1, 4). We can see from table 1 that in this approximation the most favourable configuration for the element radius is r = 1.25 cm and for the gas pressure p = 3 MPa, providing the total registration efficiency of 61%. In the more rigorous approximation the items i) and ii) of the previous assumptions have stayed the same, but the semi-empirical formula (2) in the item iii) has been replaced by the following simulation of the interaction of beta electrons with the gas filling of the MEPC: -collisions of electrons with atoms of the gas filling are described by the Rutherford formula (corrections originating in employing the relativistic Mott formula are at the level of 0.1% and can thus be neglected), -energy losses of electrons obey the familiar Bethe-Bloch formula, -between individual Rutherford scatterings electrons propagate along straight lines (fully defined by the previous scattering) having been influenced only by energy losses given by the Bethe-Bloch formula. Fig. 1 shows a cross section of a MEPC consisting of 19 elements with three different paths of an electron III. RARE DECAYS
P. Ksbinec et al. / MEPC for delve
456
jzldecay experimenis
Table 2 Percentage probabilities of the registration of emitted electron pairs in various element ~n~gurat~ons ( p = 3 MPa) Configuration
Radius of the element (cm) 0.5
all other cases
0.15
1.0
1.25
1.5
3.0
4.4
13.3
18.7
19.6
10.0
20.0
26.7
39.0
40.4
8.7
13.3
20.0
13.7
16.0
4.0
6.3
6.7
9.0
8.8
3.0
7.0
5.3
5.0
4.2
5.0
7.3
10.3
6.0
4.8
4.3
7.0
s.0
2.1
1.6
0.7
3.7
2.0
0.3
0.0
2.0
1.3
0.7
0.3
0.0
1.3
1.0
0.3
0.0
0.0
0.3
a.0
0.0
0.0
0.4
0.3
0.7
0.0
0.0
0.0
56.4
26.0
9.1
5.3
4.2
Fig. 1. Cross section of the multielement proportional chamber. Three paths of a beta electron originating in the centre of the central element (as computed by the Monte Carlo method) are shown.
emitted from the centre of the element with the kinetic energy of I MeV, projected on the G-plane of the MEPC. The radius of the element was taken as Y= 1 cm and the pressure of xenon gas filling as p = 3 MPa. Results of calculations are summarized in table 2. The numbers correspond to the percentage probabilities of registration of electron pairs in various element configurations. The results have been computed under fixed values of the xenon pressure of 3 MPa and the length of elements of 60 em. The following element radii have been used: 0.75 cm, 1.0 cm, 1.25 cm and I.5 cm. We can see that the total probabilities of regis~ation of electron pairs in 2, 3 and 4 different adjacent elements are practicalIy constant (76,2%, 76.0% and 77.0% for the radii of I.5 cm, 1.25 cm and 1.0 cm, respectively), However, as we have pointed out, the most favourable events are considered when electron pairs hit 3 or 4 elements. Table 2 shows that the relevant total probabilities of these events reach the values of 35.8% 37.0% and 50.3% for the element radii of 1.5 cm, 1.25 cm and 1.0 cm, respectively. Therefore we can conclude that for 4 element hits and the xenon pressure of 3 MPa the most profitable element radius of the MEPC is 1.0 cm. in the case of 5 or 6 element hits the optimum element radius is 0.5 cm.
A MEPC consisting of 19 elements with the radius of 1.0 cm has been constructed and tested. The counter
P. Kubinec er al. / MEPCjor
F 4
m 800
-
-
1 i_ E
I
t
L-
0
I 3.5
I
c*
1
I
-2
m
457
double p decoy experimenrs
background obtained by applying the anticoincidences between element counters. A further decrease of the MEPC background is studied by a simultaneous application of particle range discrimination with the pulse height discrimination and the time analysis of signals. A large-volume (10 litre) MEPC with 61 elements designed for double beta decay experiments is under construction. Using enriched ‘36Xe at 3 MPa as a gas filling of the MEPC operating in the Baksan underground laboratory we expect about 200 decays/year if the half-life of 136Xe is 10” yr. The experiment is planned to run for several years.
3
I
I
L
I L5
LO
We are indebted to J. Franko, PleSko for technical assistance.
R. Janik
and
M.
V IkVI
Fig. 2. Background characteristics of the MEPC methane at a pressure of 0.1 MPa and placed within made of 10 cm of lead. 1 - integral background shield, 2 - integral background in the shield, 3 anticoincidence background in the shield, 4 - internal anticoincidence background in the shield.
filled with the shield out of the - external + external
body is made of a stainless steel tube that works simultaneously as the outer cathode of the multiwire ring counter and the vacuum insulation of the chamber. The ring counter protects the element counters against penetrating background radiation. The inner cathode between the ring counter and the element counters is made of copper foil, thickness of 35 pm. The element counters are made of wires with diameters of 100 pm which form the cathodes. The anodes situated in the centres of the hexagonal counters have diameters of 50 pm. Gold coated molybdenum wires have been used for cathodes and anodes. The sensitive volume of the MEPC is 3.1 litres. The cross section of the MEPC is shown in fig. 1. The electronics enables a separate pulse registration from the counter elements and the ring counter, or it is possible to combine the anodes of element counters into 3 groups in such a way that no adjacent elements are grouped to the same output. This arrangement enables to use a simple 4 channel analyser. The background characteristics of the MEPC are shown in fig. 2. The background of the MEPC placed in a simple shield made of 10 cm of modern lead has been reduced from 320 cpm to 1.5 cpm. The MEPC was filled with methane at a pressure of 0.1 MPa. The external anticoincidence background represents the background obtained by applying the anticoincidences between the ring counter and integral element counters. The internal anticoincidence background represents the
References measurements and ap111Proc. Conf. on Low-radioactivity plications, eds.. P. Povinec and S. UsaEev (SPN, Bratislava, 1977). PI Proc. 2nd Conf. on Low-level counting, ed., P. Povinec (VEDA, Bratislava, 1982). High sensitivity counting 131D.E. Watt and D. Ramsden, techniques (Pergamon, Oxford, 1964). 141P. Povinec. Nucl. Instr. and Meth. 101 (1972) 613. ISI P. Povinec, J. Szarka and S. UsaEev, Nucl. Instr. and Meth. 163 (1979) 369. I61 P. Povinec. Nucl. Instr. and Meth. 163 (1979) 363. I71 P. Povinec, Nucl. Instr. and Meth. 176 (1980) 111. I81 P. Povinec in Proc. 2nd Conf. on Low-level counting, ed., P. Povinec (VEDA. Bratislava, 19R2) vol. 2, p. 5. 191V.V. Kuzminov. V.M. Novikov and A.A. Pomansky. Proc. Symp. Nucl. Electronics, Bratislava (JINR. Dubna, 1984) p. 350. [lOI E. Belloti, 0. Cremonesi, E. Fiorini, C. Liguori, A. Pullia, P.P. Sverzellati and L. Zanotti. ibid., p. 353. V.M. Novikov, A. A. Pritichenko, A.A. IllI V.V. Kuzminov, Pomansky, P. Povinec and R. Janik, these Proceedings (Low-level Counting), Nucl. Instr. and Meth. 817 (1986) 452. E. Bellotti, C. Cattadori, D. Camin, 0. WI A. Alessandrello, Cremonesi, F. Fiorini, C. Liguori, A. Pullia, L. Rossi, S. Ragazzi, P.P. Sverzellati and L. Zanotti, these Proceedings (Low-level Counting), Nucl. Instr. and Meth. 817 (1986) 411. AIP I131 M.V. Moe and A.A. Hahn, Science underground, Conf. Proc.. eds.. M.M. Nieto et al. (AIP, New York, 1983) p. 374. chamber. AIP Conf. Proc., I141 A. Forster, Time projection ed., J.A. Macdonald (AIP, New York, 1983) p.56. [I51 Yu.K. Akimov et al., JINR preprint (Dubna, 1981). 1161S.D. Boris et al.. ITEF preprint 47 (Moscow. 1982). 1171E. Belloti, 0. Cremonesi, F. Fiorini, C. Liguori, S. Ragaui, L. Rossi, L. Transpedini and L. Zanotti, CERN Preprint EP/1418 (Geneva, 1983).
111. RARE
DECAYS
THE MULTIELEMENT PROPORTIONAL BETA DECAY EXPERIMENTS
Nuclear
Instruments and Methods in Physics Research
B17 (1986) 454-457 North-Holland, Amsterdam
CHAMBER
- A PROMISING
TOOL FOR DOUBLE
P. KUBINEC, J. MASARIK, 1. MEL0 and P. POVINEC Comenius Universiry. Faculty of Malhematics and Physics. Departmenr oj Nuclear Physics, 842 15 Bratislava, CSSR
The extension of the multielement proportional chamber techniques for double beta decay experiments is discussed. Results of computer modelling of double beta decay events in the multielement proportional chamber are described. If xenon under the pressure of 3 MPa is used as the gas filling, the optimum element radius of 1.0 cm is found. The operating characteristics of a proportional chamber consisting of 19 elements are presented.
1.
Introduction
Low-level proportional counters are widely used in various branches of pure and applied research. such as the measurement of cosmogenic radionuclides ( 3H, 14C, “At-. 39Ar, “Kr, etc.) in terrestrial and extraterrestrial samples, low-level anthropogenic radioactivity measurements (3H, 14C, 85Kr, etc.), studies of rare decay processes, etc. (see, for example, ref. [1,2]). For traditional proportional counters used in lowlevel counting the possibilities for further improvement of their characteristics have already been exhausted. It has been shown [3) that good low-level detectors should have a high detection efficiency, a high volume efficiency and a low background. On the basis of these requirements multielement proportional chambers (MEPC) have been developed [4,5]. The simple single wire counter has been replaced by seven or more element counters of the same dimensions arranged in a hexagonal form, separated from each other by cathode wires. The MEPC design solves all the problems encountered in the construction of low-level detectors. The counter diameter is considerably decreased. which means a higher gas pressure can be used at the same working voltage. This geometry also fulfils requirements on long-term stability [6), as the effect of electronegative impurities on the counter characteristics is much weaker. Another important advantage of this design is the essential decrease of the counter background using a system of internal and external anticoincidences [7]. MEPC together with multichannel electronics operating on-line with a minicomputer opens up new possibilities in lowlevel counting of several radionuclides IS]. It has been suggested to use MEPC in experiments searching for double beta decay and for investigation of other high order rare processes [7,9,10]. This is because a MEPC enables one to use large-volume gas samples and to reach a very low background and long-term stability. 0168-583X/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
The double beta decay is one of the rarest processes that may occur in nature. The increasing interest in the study of this process is determined by the expectation arising from grand unified theories, which suppose that several conservation laws may be violated. This may lead to such interesting phenomena as lepton number nonconservation, possible right-handed admixtures in the weak lepton current, and the mass and charge conjugation properties of the electron neutrino. According to the currently accepted laws of physics, the double beta decay may occur as a second order weak interaction process with emission of 2 beta particles and 2 neutrinos. Thanks to the abundant final state with 4 leptons, occurrence of this process is rather suppressed. If the neutrino is a Majorana particle, a decay mode with emission of 2 beta particles is possible, producing a neutrinoless final state. Thanks to only 2 leptons in the final state, this mode enjoys a considerable phase-space advantage over the previous one. From the experimental point of view a certain sign of the physically attractive neutrinoless decay is the appearence of two electrons with a sharp total transition energy, since the kinetic energy of the recoiling nucleus is negligible. This fact can be used for experimental discrimination of the neutrinoless decay over the 2 Y decay. Although the double beta decay is the oldest open problem in nuclear physics with a large number of experiments carried out in the past, no direct positive experimental result is available at present: Therefore new experiments exploiting new techniques are needed. A new scope of activity seems to lie in exploiting the possibilities of the MEPC technique, described also in ref. I9-12}, and in exploiting drift chambers for low-level counting work [13-17). Among the prospective double beta decay emitters a few isotopes of rare gases can be found ( s6Kr, ‘“Xe, 136Xe). These isotopes enjoy a great advantage over the others: they can be used both as a sample and as the detector medium-gas filling for the MEPC. ‘36Xe is the
P. Kubinec et al. / MEPC for double p decay experiments
best candidate because of the highest transition energy of the double beta decay (2478 * 5 keV). We have been concentrating at present in collaboration with Prof. A.A. Pomansky’s group of the Baksan Neutrino Observatory on developing a large-volume MEPC filled with xenon. In this paper results of computer modelling of double beta decay events in the MEPC are discussed, and operational characteristics of a MEPC consisting of 19 elements are presented.
2. Computer modelling of neutrinoless double beta decay events in a MEPC The radius and length of the elements, as well as the pressure of the xenon gas filling, are considered to be free parameters. The calculations have been performed under the following assumptions: 9 Each decay event has been generated in the central element by the Monte Carlo method. ii) The total transition energy E,, has been distributed randomly according to the formula
455
Table 1 Mutual electron correlations [%] Forr=lcm,p=3MPa 1, J I=1
2.1
J=l 2 3 4 5
2
3
4
5
10.1 9.9
9.5 17.3 6.7
8.1 13.9 5.3 0.2
6.7 6.1 0.5 0.0 0.0
2
3
4
5
15.3 14.4
13.4 22.8 4.5
10.3 9.9 0.8 0.0
3.1 1.2 0.0 0.0 0.0
4
5
For r = 1.25 cm, p = 3 MPa. 1, J I=1 4.1
J=l 2 3 4 5
Forr=2cm,p=3MPaand/orr=1.5cm,p=4MPa 1, J
p(E)=E(E2-m2c4)1’2(E,,-E)
I=1 9.5
J=l
Directions of emitted electrons have been distributed according to the function (1 - cos 9). This case corresponds to the assumption of the massive Majorana neutrino existence. iii) The path of the created electron with kinetic energy E, in the MEPC has been approximated by a semiempirical formula R =
0.412~(‘.265-0.0954 e
In E,)
(2)
valid for the energy interval 0.01 MeV d E, < 2.5 MeV. This approximation is rather simplified (later we shall use more rigorous one), however, it helps in the first orientation in the problem. Under these assumptions we have computed correlation probabilities of electron pair registration in several elements for a given element radius and a gas pressure. The results are summarized in table 1. The correlation (I, .Z) means that one electron reaches an element lying in the Zth ring from its origin, while the other reaches an element in the .Zth ring. Thus, for example, the correlation (1, 1) corresponds to the case when both electrons do not leave the element of their origin; the correlation (1, 2) corresponds to the case when one electron is stopped in the element of its origin, while the other electron is stopped in an adjacent element, etc. The most favourable decay events are considered when an electron pair hits 3 or 4 different adjacent elements. Events with a smaller number of elements are difficult to distinguish from the background, and hits with more than 4 elements require complicated elec-
2 3 4 5
2
3
34.1 24.9
17.7 11.6 0.2
1.5 0.5 0.0 0.0
0.0 0.0 0.0 0.0 0.0
tronic processing. From this point of view, the most favourable correlations are (1, 3), (2, 2), (2, 3) and (1, 4). We can see from table 1 that in this approximation the most favourable configuration for the element radius is r = 1.25 cm and for the gas pressure p = 3 MPa, providing the total registration efficiency of 61%. In the more rigorous approximation the items i) and ii) of the previous assumptions have stayed the same, but the semi-empirical formula (2) in the item iii) has been replaced by the following simulation of the interaction of beta electrons with the gas filling of the MEPC: -collisions of electrons with atoms of the gas filling are described by the Rutherford formula (corrections originating in employing the relativistic Mott formula are at the level of 0.1% and can thus be neglected), -energy losses of electrons obey the familiar Bethe-Bloch formula, -between individual Rutherford scatterings electrons propagate along straight lines (fully defined by the previous scattering) having been influenced only by energy losses given by the Bethe-Bloch formula. Fig. 1 shows a cross section of a MEPC consisting of 19 elements with three different paths of an electron III. RARE DECAYS
P. Ksbinec et al. / MEPC for delve
456
jzldecay experimenis
Table 2 Percentage probabilities of the registration of emitted electron pairs in various element ~n~gurat~ons ( p = 3 MPa) Configuration
Radius of the element (cm) 0.5
all other cases
0.15
1.0
1.25
1.5
3.0
4.4
13.3
18.7
19.6
10.0
20.0
26.7
39.0
40.4
8.7
13.3
20.0
13.7
16.0
4.0
6.3
6.7
9.0
8.8
3.0
7.0
5.3
5.0
4.2
5.0
7.3
10.3
6.0
4.8
4.3
7.0
s.0
2.1
1.6
0.7
3.7
2.0
0.3
0.0
2.0
1.3
0.7
0.3
0.0
1.3
1.0
0.3
0.0
0.0
0.3
a.0
0.0
0.0
0.4
0.3
0.7
0.0
0.0
0.0
56.4
26.0
9.1
5.3
4.2
Fig. 1. Cross section of the multielement proportional chamber. Three paths of a beta electron originating in the centre of the central element (as computed by the Monte Carlo method) are shown.
emitted from the centre of the element with the kinetic energy of I MeV, projected on the G-plane of the MEPC. The radius of the element was taken as Y= 1 cm and the pressure of xenon gas filling as p = 3 MPa. Results of calculations are summarized in table 2. The numbers correspond to the percentage probabilities of registration of electron pairs in various element configurations. The results have been computed under fixed values of the xenon pressure of 3 MPa and the length of elements of 60 em. The following element radii have been used: 0.75 cm, 1.0 cm, 1.25 cm and I.5 cm. We can see that the total probabilities of regis~ation of electron pairs in 2, 3 and 4 different adjacent elements are practicalIy constant (76,2%, 76.0% and 77.0% for the radii of I.5 cm, 1.25 cm and 1.0 cm, respectively), However, as we have pointed out, the most favourable events are considered when electron pairs hit 3 or 4 elements. Table 2 shows that the relevant total probabilities of these events reach the values of 35.8% 37.0% and 50.3% for the element radii of 1.5 cm, 1.25 cm and 1.0 cm, respectively. Therefore we can conclude that for 4 element hits and the xenon pressure of 3 MPa the most profitable element radius of the MEPC is 1.0 cm. in the case of 5 or 6 element hits the optimum element radius is 0.5 cm.
A MEPC consisting of 19 elements with the radius of 1.0 cm has been constructed and tested. The counter
P. Kubinec er al. / MEPCjor
F 4
m 800
-
-
1 i_ E
I
t
L-
0
I 3.5
I
c*
1
I
-2
m
457
double p decoy experimenrs
background obtained by applying the anticoincidences between element counters. A further decrease of the MEPC background is studied by a simultaneous application of particle range discrimination with the pulse height discrimination and the time analysis of signals. A large-volume (10 litre) MEPC with 61 elements designed for double beta decay experiments is under construction. Using enriched ‘36Xe at 3 MPa as a gas filling of the MEPC operating in the Baksan underground laboratory we expect about 200 decays/year if the half-life of 136Xe is 10” yr. The experiment is planned to run for several years.
3
I
I
L
I L5
LO
We are indebted to J. Franko, PleSko for technical assistance.
R. Janik
and
M.
V IkVI
Fig. 2. Background characteristics of the MEPC methane at a pressure of 0.1 MPa and placed within made of 10 cm of lead. 1 - integral background shield, 2 - integral background in the shield, 3 anticoincidence background in the shield, 4 - internal anticoincidence background in the shield.
filled with the shield out of the - external + external
body is made of a stainless steel tube that works simultaneously as the outer cathode of the multiwire ring counter and the vacuum insulation of the chamber. The ring counter protects the element counters against penetrating background radiation. The inner cathode between the ring counter and the element counters is made of copper foil, thickness of 35 pm. The element counters are made of wires with diameters of 100 pm which form the cathodes. The anodes situated in the centres of the hexagonal counters have diameters of 50 pm. Gold coated molybdenum wires have been used for cathodes and anodes. The sensitive volume of the MEPC is 3.1 litres. The cross section of the MEPC is shown in fig. 1. The electronics enables a separate pulse registration from the counter elements and the ring counter, or it is possible to combine the anodes of element counters into 3 groups in such a way that no adjacent elements are grouped to the same output. This arrangement enables to use a simple 4 channel analyser. The background characteristics of the MEPC are shown in fig. 2. The background of the MEPC placed in a simple shield made of 10 cm of modern lead has been reduced from 320 cpm to 1.5 cpm. The MEPC was filled with methane at a pressure of 0.1 MPa. The external anticoincidence background represents the background obtained by applying the anticoincidences between the ring counter and integral element counters. The internal anticoincidence background represents the
References measurements and ap111Proc. Conf. on Low-radioactivity plications, eds.. P. Povinec and S. UsaEev (SPN, Bratislava, 1977). PI Proc. 2nd Conf. on Low-level counting, ed., P. Povinec (VEDA, Bratislava, 1982). High sensitivity counting 131D.E. Watt and D. Ramsden, techniques (Pergamon, Oxford, 1964). 141P. Povinec. Nucl. Instr. and Meth. 101 (1972) 613. ISI P. Povinec, J. Szarka and S. UsaEev, Nucl. Instr. and Meth. 163 (1979) 369. I61 P. Povinec. Nucl. Instr. and Meth. 163 (1979) 363. I71 P. Povinec, Nucl. Instr. and Meth. 176 (1980) 111. I81 P. Povinec in Proc. 2nd Conf. on Low-level counting, ed., P. Povinec (VEDA. Bratislava, 19R2) vol. 2, p. 5. 191V.V. Kuzminov. V.M. Novikov and A.A. Pomansky. Proc. Symp. Nucl. Electronics, Bratislava (JINR. Dubna, 1984) p. 350. [lOI E. Belloti, 0. Cremonesi, E. Fiorini, C. Liguori, A. Pullia, P.P. Sverzellati and L. Zanotti. ibid., p. 353. V.M. Novikov, A. A. Pritichenko, A.A. IllI V.V. Kuzminov, Pomansky, P. Povinec and R. Janik, these Proceedings (Low-level Counting), Nucl. Instr. and Meth. 817 (1986) 452. E. Bellotti, C. Cattadori, D. Camin, 0. WI A. Alessandrello, Cremonesi, F. Fiorini, C. Liguori, A. Pullia, L. Rossi, S. Ragazzi, P.P. Sverzellati and L. Zanotti, these Proceedings (Low-level Counting), Nucl. Instr. and Meth. 817 (1986) 411. AIP I131 M.V. Moe and A.A. Hahn, Science underground, Conf. Proc.. eds.. M.M. Nieto et al. (AIP, New York, 1983) p. 374. chamber. AIP Conf. Proc., I141 A. Forster, Time projection ed., J.A. Macdonald (AIP, New York, 1983) p.56. [I51 Yu.K. Akimov et al., JINR preprint (Dubna, 1981). 1161S.D. Boris et al.. ITEF preprint 47 (Moscow. 1982). 1171E. Belloti, 0. Cremonesi, F. Fiorini, C. Liguori, S. Ragaui, L. Rossi, L. Transpedini and L. Zanotti, CERN Preprint EP/1418 (Geneva, 1983).
111. RARE
DECAYS