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Perspectives for a search of neutrino magnetic moment deep underground
aiilUlmlmumii
l
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proe. Suppl.) 66 (1998) 222-225
Perspectives for a search of neutrino magnetic moment deep underground Presented by R. Bernabei for * Most of the arguments related to the practical feasibility of a search for neutrino magnetic moment deep underground by using an artificial neutrino source are reviewed and briefly discussed.
The competitiveness of a search for "large" (10 -1° - - 10 -11 #B, where/~B is the Bohr magneton) neutrino magnetic moment, pv , deep underground using an artificial neutrino source (ANS) has been already discussed in ref.[1,2]. The relevance of such a measurement regards both the neutrino properties and the investigation of a possible solution of the solar neutrino problem (SNP) [3,4]. Recent works are in favour of a magnetic moment solution of SNP [5], being these processes able to account for all the solar neutrino data including their possible (anti)correlation with solar activity, whose presence has been hypotized in the Homestake results [3,5], and analysed in some details in ref. [6]. In any case, we stress that, regardless of the SNP, an investigation of the electromagnetic properties of the neutrino must be of obvious interest and importance for particle physics. We recall that the available stringent constraints from the analysis of stellar evolution [7], are model-dependent, and that e.g. those obtained from SN1987A observation [8], rise up to _ 10-10/ZB if a model independent analysis is considered. On the other hand, direct experimental constraints on # , come only from reactor experiments [9-11]. They have rather high (1.5 - - 3 MeV) threshold on recoil electron and marginal signal - - estimated by comparing reactor on and *I.R. Barabanov a, P. Belli b, R. Bernabei b, C. J. Dai c, L.K. Ding c, W. Di Nicolantonio a, V.I. Gurentzov a, E.A. Ianovich a, A. Incicchitti a, V.E. Janza,V.N. Kornoukhov e, H.H. Kuang c, J.M. Ma c, F. Montecchia b, I.V. Orekhov a, C.V. Danshin a, D. Prosperi d. a Institute for Nuclear Reasearch, Moskow, Russia; b Dipartimento di Fisica,H Universitd di Roma and LN.F.N. Sezione di Roman, Italy; c IHEP, Chinese Academy, P.O. Boa: 918/3, Beiring 100039, China; d Dipartimento di Fisica, Universit?~ di Roma and LN.F.N. Sezione di Roma, Italy. e Institute of Theoretical and Experimental Physics, Moscow, Russia 0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All fights reserved. PII S0920-5632(98)00041-3
reactor off measurements - - respect to the background. Furthermore, the possible systematics are generally not quantified although they should significantly lower the limits quoted considering only statistical errors. A more effective way to investigate the values #~ < 3 x 10 -1° #B can be achieved by a direct experiment using a de source realized by a ~-decaying isotope of large activity (1-5 MCi) with low Q~ value (100-500 keV), placed deep underground 2 in the center of a large-mass lowradioactive NaI(T1) set-up, working with a few keV energy threshold [12]. The choice of an artificial ~7 source and low energy threshold highly radiopure NaI(T1) set-up will allow to work in an energy region where the weak interax:tion crosssection of ve e scattering, e w , is much smaller than that due to the fie magnetic moment, t r M M . A similar project will be extremely competitive respect to the projects recently proposed to improve the sensitivity of the reactor experiments; in fact, there are some important advantages of an experiment with an artificial ve source over reactor experiments: 1. The proposed reactor experiments are going to use the neutrino energy region less than 1 MeV, but there are no direct experiments in this energy region to determine the neutrino spectrum and the uncertainty is larger in the low energy end where the neutrino magnetic moment interaction 2We recall that measurements with sources of comparable strength and with essentially higher "c-activity have been successfully performed by the (]ALLEX experiment in the (]ran Sasso Laboratory and by the SAGE experiment in the Baksan Laboratory. As regards the source activity, it can be measured e.g. with the method developed at Vserossiiskyi Institut Metrologii Ira. Mendeleeva (San Peterburg) for the SAGE calibration experiment, guaranteing an accuracy of 1%.
223
R. Bernabei/Nuclear Physics B (Proc. Suppl.) 66 (1998) 222-225
is the most important. The results in this energy region could be compared only with theoretical calculation of neutrino spectrum, but its uncertainty is quite large _~15%. On the contrary, the neutrino flux of ANS can be measured with accuracy ~_1% - - or even better if necessary - taking into account the well known parameters of proposed source scheme decay. 2. The neutrino flux from the reactor is limited by 1013 c m - 2 s -1 (in reported experiments it is even 2-3 times less). In the proposed experiment with ANS the effective neutrino flux is 10 times higher and could be increased in future steps. 3. The expected effect of neutrino magnetic m o m e n t interaction in the range of 10 - l ° - 10-11 #B is very small and the detector background must be extremely low. If a reactor experiment is carried on the surface or shallow depth the radioactive isotopes would be produced in the detector by the continuous interaction of hadrons and muon captures of cosmic rays. The process of cosmic ray interaction set a principal limit for the background improvement of detectors at surface or shallow depths. For the sake of completeness we recall also that there is a possibility to use an underground reactor (500 m.w.e.) where one of the best limits for #~ has been achieved [13]; but even in the 1 MeV region the measured background is 6 order of magnitude higher than the effect from weak and magnetic m o m e n t neutrino - - electron scattering, although the experimental detector was built with low background precautions. This measured large background could be ascribed to additional background sources next to the reactor such as radioactive isotope diffusion, low intensity neutron flux activation, air and dirt activation etc. and will practically limit the sensitivity of future experiments. To point out the interest in using very low energy neutrinos we summarize in table 1 - - for #~, =10 -1° #B - - the O'MM values for different #~ energies and different electron energy thresholds, calculated according to ref. [14]. For comparison crw - - evaluated from zero to the m a x i m u m electron energy - - is given in the last column [1,2]. It is evident the importance of using very low energy antineutrinos and detection threshold as
Table 1 O'MM at different ff~ energies, E~,, and different electron energy thresholds, w0, for p~, =10 -1° # s . For comparison crw - - evaluated from zero to the m a x i m u m electron energy - - is given in the last column. The cross-section are given in units of 10-45cm2. E~,/wo (keV) 60 80 100 120 140 160 180 200 210 220 300 500 1000 2000
2
5
10
20
30
crw
3.80 5.08 5.93 6.60 7.14 7.61 8.00 8.35 8.51 8.65 9.61 11.1 12.9 14.7
1.79 2.86 3.72 4.38 4.95 5.37 5.76 6.10 6.26 6.40 7.35 8.82 10.6 12.4
0.27 1.33 2.12 2.76 3.28 3.72 4.11 4.40 4.60 4.74 5.67 7.12 8.94 10.7
0.65 1.24 1.73 2.15 2.51 2.8 3.00 3.13 4.03 5.35 7.24 8.98
0.44 0.90 1.30 1.64 1.95 2.09 2.23 3.10 4.48 6.26 7.99
0.09 0.11 0.15 0.20 0.26 0.31 0.37 0.43 0.46 0.49 0.74 1.40 3.19 6.68
low as possible, together with a well reduced environmental and intrinsic background. As regards the #~ source, it must fit the following requirements: a) the energy region for the #~ has to be 100-500 keV, where the ratio O'MM/O"W is the largest one; b) the isotope activity should be ___ 1-5 MCi and it must be easy enough to produce either by (n, 7) activation or by extraction from used reactor fuel; c) the half-life time of the isotope should be sufficiently long to produce, transport and allow a measurement in the underground laboratory; d) no accompainying 3'rays have to be present (in fact, a high flux of 7rays from the isotope or unavoidable admixtures would require a thick shield, increasing the distance between source and detector and lowering the effect). A careful investigation has been carried out to verify the real possibility of a high strength ANS creation. Details of these studies are discussed in ref. [2,15] considering all the isotopes listed in table 2. The analysis of ref.[2,15], clearly point out that
224
R. Bernabei/Nuclear Physics B (Proc. Suppl.) 66 (1998) 222-225
Table 2 Isotopes taken into account for ANS creation. Isotope ~3p ~5S 45Ca SSFe ~°Sr (9oy) ~2~I 14XCe ~4~Pm ~5~Gd 15~Eu ~SYDy lWTm lWTa lslW lV4Os :~°~Hg
Decay mode 100%/3 100%/3 100%/3 100%EC 100%/3 100%/3 100% EC 100%/3 100%/3 100%/3 100%/3 100% EC 100%fl 100% EC 100%/3 100%/3 100%/3
T1/2 25.3 d 87.4 d 165 d 2.73 y 28.5 y 3.24 d 60.2 d 35.2 d 2.62 y 241 d 4.9 y 144 d 121 d 665 d 75.1 d 6.0 d 46.6 d
Q~-Qsc (keV) 249 167 260 232 546 2282 177 580 225 244 246 366 188 115 432 970 491
E7 (keV) no no 12.4 no no 200 <35 145 <121 <172 <146 <347 <152 no 125 83 279
the only realistic possibility to create at present a suitable high strength ANS is the 147pm, we consider in ref. [1,2]. The 147pm isotope is a pure /~ emitter and decays into 147Sm, being EZ,,o, = 234.7 keV and T1/2 = 2.6234 years. The flspectrum of 147pm is well known from experimental measurements and the corresponding ~e energy spectrum can be easily calculated from it; in particular, the energy region 120 - - 230 keV includes 80% of all ~ . As regards the source production we recall that the yield of 147Pm produced from used reactor fuel is among the largest ones (see ref. [1,2]). The long half-life of 147Pm gives the possibility to produce the required activity in a few months with the help of relatively inexpensive techniques and allows also to wait some time for the decay of short-life isotopes. The 147pm isotope has been commercially produced since 1975 at the "Mayak" plant (Russia) in processes of spent reactor fuel treatment and to produce 5 MCi of 147pm, 100 t of spent reactor fuel should be treated. The scheme of 147pm production has been discussed in ref. [2]. The needed calculations have been performed in order
to determine the needed shield and the admissible concentration of other rare earth long-lived radioactive isotopes in the source, considering an uniform distribution of 5 MCi of 147pm inside cylinder volume in form of Pm203 and a total mass of NaI(T1) of 450 kg uniformely distributed around the shield (25 cm of W and 5 cm Cu). In that case, the expected rate of events from magnetic moment lye interaction and #v= 10 - l ° #B is: N(2-25 keV) = 874 d -1 (1.94 kg -1 d - l ) . In ref. [2], it has been shown that - - even for #v =10 -11 #B - - the shield provides a background 2 orders of magnitude less than the expected effect, being the concentration of the most dangerous rare earth elements guaranteed at ~_ 10 - l ° level in commercially produced 147pm. Different types of low-background detectors have been compared and discussed in ref. [1], pointing out that an highly radiopure large-mass NaI(T1) set-up with extremely low energy threshold down to 2 keV [12], is the most suitable for this experiment. The high radiopurity of the setup can assure a favourable signal/background ratio in the interesting energy region, while a very low energy threshold is very important because the spectrum of recoil electron from the magnetic moment ~e interaction steeply increases in the low energy region. The feasibility of a similar set-up is well supported by the performances of the 115.5 kg NaI(Tl) set-up of the DAMA experiment, where 2 keV energy threshold and rate near threshold between ~-0.5 and ~_ 2 cpd/kg/keV have already been achieved [12]. Although the characteristics of this set-up are already interesting for such an experiment, it is also evident the impact of further developments for NaI(T1) of higher radiopurity and larger mass. Therefore, a parallel development toward ultra-lowbackground NaI(T1) detectors (to approach 10-16 g/g U / T h and to remove long-lived cosmogenic isotopes) is under consideration in order to explore the lowest possible #~ values. An external 4~" muon anticoincidence will be used to remove the events connected with 7rays and neutrons produced by residual muons ("~_0.6/m2/h) in inelastic interactions. The achievable accuracy is summarized in table 3. For the sake of completeness we show there also
R. Bernabei/Nuclear Physics B (Proc. SuppL) 66 (1998) 222-225
225
Table 3 Reachable #. (at 3a level) after 1 year data taking with the 5 MCi source. A method similar to the one used in the reactor experiments when comparing data collected with reactor-on 1 year and reactor-off 1 year is considered. Mass Background Reacheable #. Reacheable #, assuming 0 (cpd/kg/keV) (only stat.) (stat. + 1% sys. error) background 100 kg 1. 2.0x 10-11ttB 5.2x 10-11#B 4.5X 10-12ttB 0.1 1.0X lO-11#S 1.7X 10-11#B 250 kg 1. l a x 10-11#B 5.2× 10-11~B 3.6x 10-12pB 0.1 8.0 X 10-12pB 1.7 X 10-11#B 1 ton 0.1 5.4 X10-12#B 1.6X10-11#B 2.5X10-12#B 0.01 3.0 × 10-12pB 5.3 × 10-12~B
the reachable limits in case a 1% systematic error would be present and in case of zero background (assumption sometimes used in project papers). We stress that the first line of table 3 represents an experimental situation already reached by the DAMA experiment [12], while the subsequent ones are also in the aims of this collaboration. In fact, a new R&:D is already started to realize higher radiopure NaI(T1) using a chemical purification of the powders. In conclusion, a similar experiment can really offer the possibility to discover neutrino magnetic moment or to improve the existing limits at least of about a factor 10. The limit #~ <3 × 10 -12 PB - - at 3c~ level - - could be achieved in case of extending the installation if R ~ D for further purification will be successful. In addition, this experiment will allow also to investigate antineutrinoelectron scattering with high statistical accuracy and to check the predictions of electroweak theory at low energy. REFERENCES
I. Barabanov et al, Astrop. Phys. 5 (1996), 159. 2. I. Barabanov et al, ROM2F/97/21 (1997) to appear on Astrop. Phys. 3. M. B. Voloshin et al., Sov. Phys. JETP 64 (1988), 446; E. K. Akhmedov, Phys. Left. B213 (1988), 64; C.S. Lim and W. Marciano, Phys. Rev. D37 (1988), 1368; E.K. Akhmedov et al., SISSA-172/92/EP; C.W. Kim et al.: "Neutrinos in Physics and Astrophysics", .
Harwood Ac. pub. (1993), cap. 11. 4. F. Reines et al., Phys. Rev. Lett. 37 (1976), 315. 5. E.K. Akhmedov, to appear in the Proc. of the IV Int. Solar neutrino conf., Heidelberg 1997. 6. S.Oakley et al, Astrophysical Journal, 437: 63-66, 1994 December 10. 7. R. Barbieri et al., Phys. Rev. Lett. 61 (1988), 27; J. Lattimer et al., Phys. Rev. Lett. 61 (1988), 23; I. Goldman et al., Phys. Rev. Lett. 60 (1988), 1789. 8. M.B. Voloshin, Phys. Lett. 209B (1988), 360; E. Kh. Akhmedov, Sov. Phys. JETP 68 (1989), 690; M. Leuer and J. Liu, Phys. Lett. 219B (1989), 304; M.B. Voloshin, Nucl. Phys. B19 (1991). 9. C. Broggini et al., Nucl.Phys. B35 (1994), 441. 10. G.S. Vidyakin et al., JETP Lett. 49 (1989), 740. 11. M. Fukugita et al., Phys. Rev. D36 (1987), 3817; G. Raffelt, Phys. Rev. Lett. 64 (1990), 2856. 12. R. Bernabei et al., Phys. Lett. B389 (1996), 757. 13. Pizma v Jetp., V 55, N 4, 212-215 (in Russian). 14. A. V. Kyuldjiev, Nucl. Phys. B243 (1984), 387 and references quoted therein; Idernaj Fisika (Russ) V.12 (1970) 6, pag.1233. 15. V. N. Kornoukhov, pre-print of the Institute of theoretical and experimental physiscs 90-94 (1994).
l
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proe. Suppl.) 66 (1998) 222-225
Perspectives for a search of neutrino magnetic moment deep underground Presented by R. Bernabei for * Most of the arguments related to the practical feasibility of a search for neutrino magnetic moment deep underground by using an artificial neutrino source are reviewed and briefly discussed.
The competitiveness of a search for "large" (10 -1° - - 10 -11 #B, where/~B is the Bohr magneton) neutrino magnetic moment, pv , deep underground using an artificial neutrino source (ANS) has been already discussed in ref.[1,2]. The relevance of such a measurement regards both the neutrino properties and the investigation of a possible solution of the solar neutrino problem (SNP) [3,4]. Recent works are in favour of a magnetic moment solution of SNP [5], being these processes able to account for all the solar neutrino data including their possible (anti)correlation with solar activity, whose presence has been hypotized in the Homestake results [3,5], and analysed in some details in ref. [6]. In any case, we stress that, regardless of the SNP, an investigation of the electromagnetic properties of the neutrino must be of obvious interest and importance for particle physics. We recall that the available stringent constraints from the analysis of stellar evolution [7], are model-dependent, and that e.g. those obtained from SN1987A observation [8], rise up to _ 10-10/ZB if a model independent analysis is considered. On the other hand, direct experimental constraints on # , come only from reactor experiments [9-11]. They have rather high (1.5 - - 3 MeV) threshold on recoil electron and marginal signal - - estimated by comparing reactor on and *I.R. Barabanov a, P. Belli b, R. Bernabei b, C. J. Dai c, L.K. Ding c, W. Di Nicolantonio a, V.I. Gurentzov a, E.A. Ianovich a, A. Incicchitti a, V.E. Janza,V.N. Kornoukhov e, H.H. Kuang c, J.M. Ma c, F. Montecchia b, I.V. Orekhov a, C.V. Danshin a, D. Prosperi d. a Institute for Nuclear Reasearch, Moskow, Russia; b Dipartimento di Fisica,H Universitd di Roma and LN.F.N. Sezione di Roman, Italy; c IHEP, Chinese Academy, P.O. Boa: 918/3, Beiring 100039, China; d Dipartimento di Fisica, Universit?~ di Roma and LN.F.N. Sezione di Roma, Italy. e Institute of Theoretical and Experimental Physics, Moscow, Russia 0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All fights reserved. PII S0920-5632(98)00041-3
reactor off measurements - - respect to the background. Furthermore, the possible systematics are generally not quantified although they should significantly lower the limits quoted considering only statistical errors. A more effective way to investigate the values #~ < 3 x 10 -1° #B can be achieved by a direct experiment using a de source realized by a ~-decaying isotope of large activity (1-5 MCi) with low Q~ value (100-500 keV), placed deep underground 2 in the center of a large-mass lowradioactive NaI(T1) set-up, working with a few keV energy threshold [12]. The choice of an artificial ~7 source and low energy threshold highly radiopure NaI(T1) set-up will allow to work in an energy region where the weak interax:tion crosssection of ve e scattering, e w , is much smaller than that due to the fie magnetic moment, t r M M . A similar project will be extremely competitive respect to the projects recently proposed to improve the sensitivity of the reactor experiments; in fact, there are some important advantages of an experiment with an artificial ve source over reactor experiments: 1. The proposed reactor experiments are going to use the neutrino energy region less than 1 MeV, but there are no direct experiments in this energy region to determine the neutrino spectrum and the uncertainty is larger in the low energy end where the neutrino magnetic moment interaction 2We recall that measurements with sources of comparable strength and with essentially higher "c-activity have been successfully performed by the (]ALLEX experiment in the (]ran Sasso Laboratory and by the SAGE experiment in the Baksan Laboratory. As regards the source activity, it can be measured e.g. with the method developed at Vserossiiskyi Institut Metrologii Ira. Mendeleeva (San Peterburg) for the SAGE calibration experiment, guaranteing an accuracy of 1%.
223
R. Bernabei/Nuclear Physics B (Proc. Suppl.) 66 (1998) 222-225
is the most important. The results in this energy region could be compared only with theoretical calculation of neutrino spectrum, but its uncertainty is quite large _~15%. On the contrary, the neutrino flux of ANS can be measured with accuracy ~_1% - - or even better if necessary - taking into account the well known parameters of proposed source scheme decay. 2. The neutrino flux from the reactor is limited by 1013 c m - 2 s -1 (in reported experiments it is even 2-3 times less). In the proposed experiment with ANS the effective neutrino flux is 10 times higher and could be increased in future steps. 3. The expected effect of neutrino magnetic m o m e n t interaction in the range of 10 - l ° - 10-11 #B is very small and the detector background must be extremely low. If a reactor experiment is carried on the surface or shallow depth the radioactive isotopes would be produced in the detector by the continuous interaction of hadrons and muon captures of cosmic rays. The process of cosmic ray interaction set a principal limit for the background improvement of detectors at surface or shallow depths. For the sake of completeness we recall also that there is a possibility to use an underground reactor (500 m.w.e.) where one of the best limits for #~ has been achieved [13]; but even in the 1 MeV region the measured background is 6 order of magnitude higher than the effect from weak and magnetic m o m e n t neutrino - - electron scattering, although the experimental detector was built with low background precautions. This measured large background could be ascribed to additional background sources next to the reactor such as radioactive isotope diffusion, low intensity neutron flux activation, air and dirt activation etc. and will practically limit the sensitivity of future experiments. To point out the interest in using very low energy neutrinos we summarize in table 1 - - for #~, =10 -1° #B - - the O'MM values for different #~ energies and different electron energy thresholds, calculated according to ref. [14]. For comparison crw - - evaluated from zero to the m a x i m u m electron energy - - is given in the last column [1,2]. It is evident the importance of using very low energy antineutrinos and detection threshold as
Table 1 O'MM at different ff~ energies, E~,, and different electron energy thresholds, w0, for p~, =10 -1° # s . For comparison crw - - evaluated from zero to the m a x i m u m electron energy - - is given in the last column. The cross-section are given in units of 10-45cm2. E~,/wo (keV) 60 80 100 120 140 160 180 200 210 220 300 500 1000 2000
2
5
10
20
30
crw
3.80 5.08 5.93 6.60 7.14 7.61 8.00 8.35 8.51 8.65 9.61 11.1 12.9 14.7
1.79 2.86 3.72 4.38 4.95 5.37 5.76 6.10 6.26 6.40 7.35 8.82 10.6 12.4
0.27 1.33 2.12 2.76 3.28 3.72 4.11 4.40 4.60 4.74 5.67 7.12 8.94 10.7
0.65 1.24 1.73 2.15 2.51 2.8 3.00 3.13 4.03 5.35 7.24 8.98
0.44 0.90 1.30 1.64 1.95 2.09 2.23 3.10 4.48 6.26 7.99
0.09 0.11 0.15 0.20 0.26 0.31 0.37 0.43 0.46 0.49 0.74 1.40 3.19 6.68
low as possible, together with a well reduced environmental and intrinsic background. As regards the #~ source, it must fit the following requirements: a) the energy region for the #~ has to be 100-500 keV, where the ratio O'MM/O"W is the largest one; b) the isotope activity should be ___ 1-5 MCi and it must be easy enough to produce either by (n, 7) activation or by extraction from used reactor fuel; c) the half-life time of the isotope should be sufficiently long to produce, transport and allow a measurement in the underground laboratory; d) no accompainying 3'rays have to be present (in fact, a high flux of 7rays from the isotope or unavoidable admixtures would require a thick shield, increasing the distance between source and detector and lowering the effect). A careful investigation has been carried out to verify the real possibility of a high strength ANS creation. Details of these studies are discussed in ref. [2,15] considering all the isotopes listed in table 2. The analysis of ref.[2,15], clearly point out that
224
R. Bernabei/Nuclear Physics B (Proc. Suppl.) 66 (1998) 222-225
Table 2 Isotopes taken into account for ANS creation. Isotope ~3p ~5S 45Ca SSFe ~°Sr (9oy) ~2~I 14XCe ~4~Pm ~5~Gd 15~Eu ~SYDy lWTm lWTa lslW lV4Os :~°~Hg
Decay mode 100%/3 100%/3 100%/3 100%EC 100%/3 100%/3 100% EC 100%/3 100%/3 100%/3 100%/3 100% EC 100%fl 100% EC 100%/3 100%/3 100%/3
T1/2 25.3 d 87.4 d 165 d 2.73 y 28.5 y 3.24 d 60.2 d 35.2 d 2.62 y 241 d 4.9 y 144 d 121 d 665 d 75.1 d 6.0 d 46.6 d
Q~-Qsc (keV) 249 167 260 232 546 2282 177 580 225 244 246 366 188 115 432 970 491
E7 (keV) no no 12.4 no no 200 <35 145 <121 <172 <146 <347 <152 no 125 83 279
the only realistic possibility to create at present a suitable high strength ANS is the 147pm, we consider in ref. [1,2]. The 147pm isotope is a pure /~ emitter and decays into 147Sm, being EZ,,o, = 234.7 keV and T1/2 = 2.6234 years. The flspectrum of 147pm is well known from experimental measurements and the corresponding ~e energy spectrum can be easily calculated from it; in particular, the energy region 120 - - 230 keV includes 80% of all ~ . As regards the source production we recall that the yield of 147Pm produced from used reactor fuel is among the largest ones (see ref. [1,2]). The long half-life of 147Pm gives the possibility to produce the required activity in a few months with the help of relatively inexpensive techniques and allows also to wait some time for the decay of short-life isotopes. The 147pm isotope has been commercially produced since 1975 at the "Mayak" plant (Russia) in processes of spent reactor fuel treatment and to produce 5 MCi of 147pm, 100 t of spent reactor fuel should be treated. The scheme of 147pm production has been discussed in ref. [2]. The needed calculations have been performed in order
to determine the needed shield and the admissible concentration of other rare earth long-lived radioactive isotopes in the source, considering an uniform distribution of 5 MCi of 147pm inside cylinder volume in form of Pm203 and a total mass of NaI(T1) of 450 kg uniformely distributed around the shield (25 cm of W and 5 cm Cu). In that case, the expected rate of events from magnetic moment lye interaction and #v= 10 - l ° #B is: N(2-25 keV) = 874 d -1 (1.94 kg -1 d - l ) . In ref. [2], it has been shown that - - even for #v =10 -11 #B - - the shield provides a background 2 orders of magnitude less than the expected effect, being the concentration of the most dangerous rare earth elements guaranteed at ~_ 10 - l ° level in commercially produced 147pm. Different types of low-background detectors have been compared and discussed in ref. [1], pointing out that an highly radiopure large-mass NaI(T1) set-up with extremely low energy threshold down to 2 keV [12], is the most suitable for this experiment. The high radiopurity of the setup can assure a favourable signal/background ratio in the interesting energy region, while a very low energy threshold is very important because the spectrum of recoil electron from the magnetic moment ~e interaction steeply increases in the low energy region. The feasibility of a similar set-up is well supported by the performances of the 115.5 kg NaI(Tl) set-up of the DAMA experiment, where 2 keV energy threshold and rate near threshold between ~-0.5 and ~_ 2 cpd/kg/keV have already been achieved [12]. Although the characteristics of this set-up are already interesting for such an experiment, it is also evident the impact of further developments for NaI(T1) of higher radiopurity and larger mass. Therefore, a parallel development toward ultra-lowbackground NaI(T1) detectors (to approach 10-16 g/g U / T h and to remove long-lived cosmogenic isotopes) is under consideration in order to explore the lowest possible #~ values. An external 4~" muon anticoincidence will be used to remove the events connected with 7rays and neutrons produced by residual muons ("~_0.6/m2/h) in inelastic interactions. The achievable accuracy is summarized in table 3. For the sake of completeness we show there also
R. Bernabei/Nuclear Physics B (Proc. SuppL) 66 (1998) 222-225
225
Table 3 Reachable #. (at 3a level) after 1 year data taking with the 5 MCi source. A method similar to the one used in the reactor experiments when comparing data collected with reactor-on 1 year and reactor-off 1 year is considered. Mass Background Reacheable #. Reacheable #, assuming 0 (cpd/kg/keV) (only stat.) (stat. + 1% sys. error) background 100 kg 1. 2.0x 10-11ttB 5.2x 10-11#B 4.5X 10-12ttB 0.1 1.0X lO-11#S 1.7X 10-11#B 250 kg 1. l a x 10-11#B 5.2× 10-11~B 3.6x 10-12pB 0.1 8.0 X 10-12pB 1.7 X 10-11#B 1 ton 0.1 5.4 X10-12#B 1.6X10-11#B 2.5X10-12#B 0.01 3.0 × 10-12pB 5.3 × 10-12~B
the reachable limits in case a 1% systematic error would be present and in case of zero background (assumption sometimes used in project papers). We stress that the first line of table 3 represents an experimental situation already reached by the DAMA experiment [12], while the subsequent ones are also in the aims of this collaboration. In fact, a new R&:D is already started to realize higher radiopure NaI(T1) using a chemical purification of the powders. In conclusion, a similar experiment can really offer the possibility to discover neutrino magnetic moment or to improve the existing limits at least of about a factor 10. The limit #~ <3 × 10 -12 PB - - at 3c~ level - - could be achieved in case of extending the installation if R ~ D for further purification will be successful. In addition, this experiment will allow also to investigate antineutrinoelectron scattering with high statistical accuracy and to check the predictions of electroweak theory at low energy. REFERENCES
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