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Limits on neutrinoless double beta decay in 136Xe with a time projection chamber
Nucleae Physicx B (Proc. Suppl.) 28A(1992) 226-228 North-Holland
L ITS ON NELTTRIN®LESS DOÜBLE BETA DECAY IN issXE WITH A TIME PROJECTION CHAMBER H.T. WDNG °~ F. BOEHM a, P. FISHER `x, I{. GABATHULER ~, H. HENRIKSON °`, D. IMEL °, M.Z. IQBAL °, V. JÖRGEN ", L. MITCHELL ", B. O'CALLAGHAN-HAY °, J. THOMAS °, M. TREICHEL ", J.-C . VUILLEUMIER ", J.-L . VUILLEUMIER " . a
Norman Bridge Laboratory of Physics, California Institute of Technology, Pasadena, California 91125, U.S.A. `~ Paui Schemer Institute, 5234 Villigen-PSI, Switzerland . Institut de Physique, A.-L . Breguet 1, 2000 Neuchätel, Switzerland . (presented by H.T . Wong) A xenon Time Projection Chamber With an active volume of 207 liters has been built to study Ov and 2v double beta decay in issXe . Data were taken in the Gotthard Underground Laboratory, with 5 atm of xenon enriched to 62 .5% in issXe. From 4861 hours of data, no evidence has been found for the 0+ -~ 0+ transition. Half-life limts of T°Y(0+ ~ 0+) > z 2.ï( 5.2) x 1023 years in the mass mechanism mode, and T°"(0+ -~ 0+ ) > 1.9( 3.5) x 1023 years z in the right-handed currents mode, at the 90(68)% C.L., were derived . Upper limits for the i~Iajorana neutrino mass parameter were deduced . Neutrinoless double beta decay provides a sensitive probe for lepton number violation and, in particular, Majorana neutrino mass as well as right-handed weak currens l . The implications of this so far unobserved nuclear decay have stimulated intense experimental efforts in recent years2. We have built a Time Projection Chamber (TPC) to study double beta decay in iasXe (transition energy 2.48 MeV ), in both the Ov and 2v channels 3 4. The schematic
ground Laboratory, with a 3000 meter-water-equivalent overburden which attenuates the muon flux by a factor of lOs. The track reconstruction capability of the TPC provides a powerful means of background rejection . A douPREAMPLIFIERS XY READ OUT
COPPER ANO LEAD SHIELDING
PRESSIXiE GAUGE
~
diagram of the experimental set-up is shown in Figure 1 . The TPC has a cylindrical active volume of 207 litres . The operating pressure is 5 atm, with an admixture of 3.9% methane to increase the drift velocity and to suppress diffusion of the secondary electrons . Xenon enriched to 62.5% aasXe is used, giving a total of 1 .6 x 10 25 issXe atoms in the active volume. There are 168 readout channels, with 3.5 mm pitch, in each of the X and Y axes. The detector has been built with low background materials, and is shielded by 20-30 cm of lead. The experiment is being conducted at the Gotthard Under0920-5632/92/505 .
ANODE AND FIELD WIRES
xENDN
CIRCULAT PIMP/ EH~
GR10
RECOVERY T~K
FIELD SHAPING RING
CATHODE HIGH VOLTAG E TURBO PUMP
ABSORPTION PUMP
FIGVRE ~ A schematic diagram of the Time Projection Chamber with the associated set-up .
©1~2 - Elsevier Science Publishers B.V All rights reserved.
H. T. Wong et al. /Limits on neutrinoless double beta decay in
ble beta decay event is identified as a continuous trajectory with the characteristic "end features": large angle multiple scatterings and increased charge depositions (charge "blobs"), at both ends. A typical "two-electron" event, which exhibits such features, is depicted in Figure
2a. Such events can be distinguished easily from those due to alpha particles, cosmic rays, single electrons and multiple Compton scattering. Figure 2b shows a beta
136Xe
227
decay (that is, a single electron with charge blob at only one end) with the emission of an alpha particle at the same (X,Y) co-ordinate 50 ps later. This event is due to the cascade 214Bi __, 214Po + e- + v,, (Q = 3.28 MeV, T,z = 19.7 min), followed by 214Po --, 21opb + a (Q =
7.8 MeV, T z = 164 us), and is evidence of trace radon emission in the system. This cascade can be singled out by looking for the -100 Ees post-trigger alpha activity after an initial single electron event .
a)
The energy of an event is obtained by integrating the anode signals over the drift time. The energy resolution and calibration has been studied with various gamma sources . A 10-15% variation in charge multiplication across the effective area of the anode plane is observed .
To correct this effect, the anode plane is suVll -divided into 45 squares and a gain variation map is made from a measurement with a 137CS source . The anode signals are then compensated at each time bin, based on the (X,Y) co-ordinate of the event at that time. A notable improvement on the energy resolution of 6.6% FWHM at 1.6 MeV is subsequently achieved .
b)
Z
The "two-electron" spectrum from 4861 hours of data is shown in Figure 3. The measured background level at the Ov range is 0.01 counts keV -1 kg-1 yr-1 . Considering 120 m Y O
FIGURE 2 A typical (a) "two-electron" event, and (b) beta decay with delayed alpha emission, recorded by the TPC . The XZ and YZ projections, as well as the evolution of the anode pulse, are displayed . Black spots correspond to increased charge depositions . The full range is 60 cm for both X and Y, while the Z calibration is 10.9 cm per unit.
100 80
d
60
c
40
6
20 0 1500
2000
3000 2500 Energy ( keV )
3500
4000
FIGURE 3 Energy spectrum for "two-electron" events, from 3380 hours of data. The 90% C.L. exclusion curve for a hypothetical Ov peak at 2481 keV is represented by the solid line in the inset.
228
H.T. Mong et al. lLimits on neutrinoless double beta decay
in
t36Xe
TABLE 1 136Xe experiments, Limits on Majorana neutrino mass parameter from the "sGe and adopting calculations from the Caltech and Heidelberg groups. Isotope (run time) 'Ge (3 years) "'Xe (4861 hours) 136 Xe (projection for 3 years)
T I '((M,,)) / yr (at 90% C.L.) > 1.0 x 10 24 > 2.7 x 1023 (>)5.5 x 1023
a resolution of 6.6% FWHM at the Ov transition energy, and an exponential background from 2000 keV to 2650 keV together with a constant background from 2650 keV to 3000 keV, we obtain a 90% C.L. exclusion curve as indicated by the solid line. Folding in the respective detector (25% and 21%) and analysis efficiencies (81% and
64%) for the mass mechanism and right-handed currents mo,les, we obtain the following 90(68)% C.L. half-life limits for the 0} - O} transition: T°y((m )) > 2.7(5 .2) x 1023 years , and
Tolz (RHC) > 1.9(3.5) x 1023 years . These limits represent a factor of 20 improvement over existing ones'- 6. The limit for the Majorana neutrino mass parameter thus deduced depends on which nuclear matrix element calculation one adopts . To illustrate the range of sensitivities, the limits deduced from the calculations by the Caltech' and Heidelberg' groups are tabulated in Table
1. For comparison, the best 'sGe Ov half-life limit s as well as the projected sensitivity of this experiment with 3 years of data are also shown. Owing to the theoretical uncertainties, it. is essential that double beta decay investigations are pursued in various isotopes (preferrably with different ranges in Z). The experiment described here is at present. the only
(m ) / eV Caltech Heidelberg < 2.6 - 5.1 < 1 .5 < 3.2 - 4.8 < 2.9 (<)2.0 (<)2.2 - 3.7
one which has achieved the OvJ3,3 sensitivities of 'sGe with a different isotope. The simultaneous tracking and calorimetry capabilities of the TPC, using the source as its detector medium, prove to be ideal for double beta decay studies . The technique of using the difference in dE/dx to measure the directionality of low energy electron can be a great asset to other experiments at nuclear energies (like the measurement of the neutrino magnetic moment using reactor neutrinos lo ) . REFERENCES 1. See, for example, F. Boehm and P. Vogel, Physics of Massive Neutrinos Cambridge University Press, Cambridge (1987) . 2. For a recent review, see A . Morales, this volume . 3. H.T. Wong, Ph. D. Thesis, Caltech (1991) . 4. H.T. Wong et al., Phy. Rev . Lett. 67 1218 (1991). 5. A.S. Barabash et al., Phys. Lett. B223(2), 273 (1989) . 6. E. Bellotti et al., Phys Lett. B266, 193 (1991) . 7. J . Engel, P. Vogel and M.R. Zirnbauer, Phys. Rev. C37, 731 (1988) . 8. A. Staudt et al., Europhys. Lett. 13(1), 31 (1990) . 9. I.V. Kirpichnikov, this volume . 10. C. Broggini et al. t o be published in Nucl. Instrum . Methods .
L ITS ON NELTTRIN®LESS DOÜBLE BETA DECAY IN issXE WITH A TIME PROJECTION CHAMBER H.T. WDNG °~ F. BOEHM a, P. FISHER `x, I{. GABATHULER ~, H. HENRIKSON °`, D. IMEL °, M.Z. IQBAL °, V. JÖRGEN ", L. MITCHELL ", B. O'CALLAGHAN-HAY °, J. THOMAS °, M. TREICHEL ", J.-C . VUILLEUMIER ", J.-L . VUILLEUMIER " . a
Norman Bridge Laboratory of Physics, California Institute of Technology, Pasadena, California 91125, U.S.A. `~ Paui Schemer Institute, 5234 Villigen-PSI, Switzerland . Institut de Physique, A.-L . Breguet 1, 2000 Neuchätel, Switzerland . (presented by H.T . Wong) A xenon Time Projection Chamber With an active volume of 207 liters has been built to study Ov and 2v double beta decay in issXe . Data were taken in the Gotthard Underground Laboratory, with 5 atm of xenon enriched to 62 .5% in issXe. From 4861 hours of data, no evidence has been found for the 0+ -~ 0+ transition. Half-life limts of T°Y(0+ ~ 0+) > z 2.ï( 5.2) x 1023 years in the mass mechanism mode, and T°"(0+ -~ 0+ ) > 1.9( 3.5) x 1023 years z in the right-handed currents mode, at the 90(68)% C.L., were derived . Upper limits for the i~Iajorana neutrino mass parameter were deduced . Neutrinoless double beta decay provides a sensitive probe for lepton number violation and, in particular, Majorana neutrino mass as well as right-handed weak currens l . The implications of this so far unobserved nuclear decay have stimulated intense experimental efforts in recent years2. We have built a Time Projection Chamber (TPC) to study double beta decay in iasXe (transition energy 2.48 MeV ), in both the Ov and 2v channels 3 4. The schematic
ground Laboratory, with a 3000 meter-water-equivalent overburden which attenuates the muon flux by a factor of lOs. The track reconstruction capability of the TPC provides a powerful means of background rejection . A douPREAMPLIFIERS XY READ OUT
COPPER ANO LEAD SHIELDING
PRESSIXiE GAUGE
~
diagram of the experimental set-up is shown in Figure 1 . The TPC has a cylindrical active volume of 207 litres . The operating pressure is 5 atm, with an admixture of 3.9% methane to increase the drift velocity and to suppress diffusion of the secondary electrons . Xenon enriched to 62.5% aasXe is used, giving a total of 1 .6 x 10 25 issXe atoms in the active volume. There are 168 readout channels, with 3.5 mm pitch, in each of the X and Y axes. The detector has been built with low background materials, and is shielded by 20-30 cm of lead. The experiment is being conducted at the Gotthard Under0920-5632/92/505 .
ANODE AND FIELD WIRES
xENDN
CIRCULAT PIMP/ EH~
GR10
RECOVERY T~K
FIELD SHAPING RING
CATHODE HIGH VOLTAG E TURBO PUMP
ABSORPTION PUMP
FIGVRE ~ A schematic diagram of the Time Projection Chamber with the associated set-up .
©1~2 - Elsevier Science Publishers B.V All rights reserved.
H. T. Wong et al. /Limits on neutrinoless double beta decay in
ble beta decay event is identified as a continuous trajectory with the characteristic "end features": large angle multiple scatterings and increased charge depositions (charge "blobs"), at both ends. A typical "two-electron" event, which exhibits such features, is depicted in Figure
2a. Such events can be distinguished easily from those due to alpha particles, cosmic rays, single electrons and multiple Compton scattering. Figure 2b shows a beta
136Xe
227
decay (that is, a single electron with charge blob at only one end) with the emission of an alpha particle at the same (X,Y) co-ordinate 50 ps later. This event is due to the cascade 214Bi __, 214Po + e- + v,, (Q = 3.28 MeV, T,z = 19.7 min), followed by 214Po --, 21opb + a (Q =
7.8 MeV, T z = 164 us), and is evidence of trace radon emission in the system. This cascade can be singled out by looking for the -100 Ees post-trigger alpha activity after an initial single electron event .
a)
The energy of an event is obtained by integrating the anode signals over the drift time. The energy resolution and calibration has been studied with various gamma sources . A 10-15% variation in charge multiplication across the effective area of the anode plane is observed .
To correct this effect, the anode plane is suVll -divided into 45 squares and a gain variation map is made from a measurement with a 137CS source . The anode signals are then compensated at each time bin, based on the (X,Y) co-ordinate of the event at that time. A notable improvement on the energy resolution of 6.6% FWHM at 1.6 MeV is subsequently achieved .
b)
Z
The "two-electron" spectrum from 4861 hours of data is shown in Figure 3. The measured background level at the Ov range is 0.01 counts keV -1 kg-1 yr-1 . Considering 120 m Y O
FIGURE 2 A typical (a) "two-electron" event, and (b) beta decay with delayed alpha emission, recorded by the TPC . The XZ and YZ projections, as well as the evolution of the anode pulse, are displayed . Black spots correspond to increased charge depositions . The full range is 60 cm for both X and Y, while the Z calibration is 10.9 cm per unit.
100 80
d
60
c
40
6
20 0 1500
2000
3000 2500 Energy ( keV )
3500
4000
FIGURE 3 Energy spectrum for "two-electron" events, from 3380 hours of data. The 90% C.L. exclusion curve for a hypothetical Ov peak at 2481 keV is represented by the solid line in the inset.
228
H.T. Mong et al. lLimits on neutrinoless double beta decay
in
t36Xe
TABLE 1 136Xe experiments, Limits on Majorana neutrino mass parameter from the "sGe and adopting calculations from the Caltech and Heidelberg groups. Isotope (run time) 'Ge (3 years) "'Xe (4861 hours) 136 Xe (projection for 3 years)
T I '((M,,)) / yr (at 90% C.L.) > 1.0 x 10 24 > 2.7 x 1023 (>)5.5 x 1023
a resolution of 6.6% FWHM at the Ov transition energy, and an exponential background from 2000 keV to 2650 keV together with a constant background from 2650 keV to 3000 keV, we obtain a 90% C.L. exclusion curve as indicated by the solid line. Folding in the respective detector (25% and 21%) and analysis efficiencies (81% and
64%) for the mass mechanism and right-handed currents mo,les, we obtain the following 90(68)% C.L. half-life limits for the 0} - O} transition: T°y((m )) > 2.7(5 .2) x 1023 years , and
Tolz (RHC) > 1.9(3.5) x 1023 years . These limits represent a factor of 20 improvement over existing ones'- 6. The limit for the Majorana neutrino mass parameter thus deduced depends on which nuclear matrix element calculation one adopts . To illustrate the range of sensitivities, the limits deduced from the calculations by the Caltech' and Heidelberg' groups are tabulated in Table
1. For comparison, the best 'sGe Ov half-life limit s as well as the projected sensitivity of this experiment with 3 years of data are also shown. Owing to the theoretical uncertainties, it. is essential that double beta decay investigations are pursued in various isotopes (preferrably with different ranges in Z). The experiment described here is at present. the only
(m ) / eV Caltech Heidelberg < 2.6 - 5.1 < 1 .5 < 3.2 - 4.8 < 2.9 (<)2.0 (<)2.2 - 3.7
one which has achieved the OvJ3,3 sensitivities of 'sGe with a different isotope. The simultaneous tracking and calorimetry capabilities of the TPC, using the source as its detector medium, prove to be ideal for double beta decay studies . The technique of using the difference in dE/dx to measure the directionality of low energy electron can be a great asset to other experiments at nuclear energies (like the measurement of the neutrino magnetic moment using reactor neutrinos lo ) . REFERENCES 1. See, for example, F. Boehm and P. Vogel, Physics of Massive Neutrinos Cambridge University Press, Cambridge (1987) . 2. For a recent review, see A . Morales, this volume . 3. H.T. Wong, Ph. D. Thesis, Caltech (1991) . 4. H.T. Wong et al., Phy. Rev . Lett. 67 1218 (1991). 5. A.S. Barabash et al., Phys. Lett. B223(2), 273 (1989) . 6. E. Bellotti et al., Phys Lett. B266, 193 (1991) . 7. J . Engel, P. Vogel and M.R. Zirnbauer, Phys. Rev. C37, 731 (1988) . 8. A. Staudt et al., Europhys. Lett. 13(1), 31 (1990) . 9. I.V. Kirpichnikov, this volume . 10. C. Broggini et al. t o be published in Nucl. Instrum . Methods .