- Email: [email protected]
HELLAZ : a low energy neutrino spectrometer
___ EB
a
SUPPLEMENTS Nuclear
ELSEVIER
HELLAZ A. Sarrata,
Physics
B (Proc. Suppl.) 95 (2001) 177-180
: a low energy neutrino on behalf
of the HELLAZ
www.elsevier.nl/locate/npe
spectrometer
collaboration
aLaboratoire de Physique Corpusculaire et Cosmologie 11, place Marcelin Berthelot, 75000 Paris, France
- Collgge de France,
The determination of neutrino properties, especially their mass, is one of the central problems of present day HELLAZ is a project of experiment dedicated to this subject by measuring the solar neutrino spectrum.
particle physics.
1. Introduction An improved knowledge of neutrino properties is one of the main goals of particle physics. Results of solar neutrino experiments show two solar neutrino crucial problems : all measured fluxes are lower than solar models predictions, and there are discrepancies between these measurements. These two facts can be seen as an indication for new neutrino physics via neutrino oscillations (see [l] and references there in). A good way to study this problem in a selfconsistent manner is to measure separately the various contributions of the solar neutrino spectrum with the same reaction in a single detector. The task of HELLAZ R&D is to study the feasibility of such an experiment. 2. HELLAZ
So, we just have to measure T, and 6 to deduce the neutrino energy. The main advantages of this interaction are the theoretically exactly known cross-section, the possibility of real time detection and the absence of threshold. The principle
He at 20 bar. 273 K
principle
The main goal of HELLAZ [2] is to obtain the low energy part of the solar neutrino spectrum We therefore have to use an interaction which allows one to retrieve the neutrino energy with a good accuracy, sufficient to separate pp from 7Be neutrinos. A good candidate for this is the elastic electron - neutrino scattering. The kinematics is that of the elastic scattering :
where m,, T,, and p, are the mass, kinetic energy and momentum of the scattered electron and 8 is the direction of the electron with respect to that of the neutrino (i.e. the direction of the Sun). 0920-5632/01/F% - see front matter 8 2001 Elsevier Science B.V PI1 SO920-5632(01)01079-9
Diffusion (r = 140 pm-).
Figure
1. Detection
principle
1” .!’
V drift = 1.1 mm/ p.s
in HELLAZ.
of HELLAZ is shown in fig. 1. A neutrino scattered off an electron which produce a ionisation track in a TPC filled with helium. Under the action of an electric field of a few 100 V. cm-‘, the track drifts towards an end-cap detector on which each ionisation electron or at least most of them is detected separately. The geometry and the conditions in the TPC must satisfy several requirements. CompAll rights reserved
178
A. Sarm~/Nuclear
Pltwics
B (Proc.
ton scattering events induced by photons are indistinguishable1 from neutrino events, so we must use a zero radioactive background gas as target in the TPC. Helium is a good candidate because it is easy to purify with boil off techniques. To recover the initial direction of the scattered electron, the lowest energy (- 100 keV) track length has to be at least about 5 cm long. As a consequence, we have to work with an helium density N 3 x 10m3g . cmT3 (i.e. 20 bar at 273 IS). With this set up we will have small multiple scattering and small difIusion so it will be less diicult to find the track direction. We will also have a small drift velocity, and therefore a larger time separation between electrons. This make the single electron detection easier. Our last requirement is that the neutrino events rate has to be N 15 events per day. The neutrino - electron scattering cross-section and the predicted neutrino fluxes imply a number of target electrons N 2 x 103’ therefore a 2000 m3 volume of He at this density. Regarding the single electron detection, we need a detector with a high gain (- 106) and a high detection rate, of order 100 MHz to obtain a good efficiency detection. We investigate the MICROMEGAS detector whose properties seem to fulfill these requirements.
SuppI.)
95 (2001)
177-IS0
than 10 ns. Due to HELLAZ requirements, we have to obtain the same results at 20 bar. The first tests were performed with several mixtures of helium + methane. With this mixture, the maximum achievable gain N lo4 is about two order of magnitudes too low to allow single electrons detection. A simulation from Biagi 141allows to compute gain as a function of electric field, gap length and medium composition. As can be seen from figure 2, this simulation predicts that the maximum
3. Results
1
The R&D was focused on two essential fields, a hardware part for the detection of single electrons, and a software part to develop efficient algorithms to find the track direction and to simulate background reduction. 3.1. Harware A first series of tests was performed to detect single electrons with the MICROMEGAS detector [3] at normal pressure and temperature, using a mixture of helium + 6 % isobutane as quencher. Under these conditions, we obtained single electrons detection with an efficiency of 98 % and we can detect pulses with a time separation lower lsee section 3.2 for a statistical kind of eventa
rejection method of this
%
qumeher
Figure 2. Computed gains for &butane and methane in helium at 20 bar for a 50pm gap.
gain achievable with methane is about 2 x 104. This is what we obtained experimentally. It also shows that it seems possible to reach gain higher than lo6 if using isobutane as quencher. Our latest tests show that the gain with isobutane as quencher is larger than with methane, but it is actually not possible to achieve the maximum predicted gain. Indeed, the gain simulation doesn’t take into account breakdowns, and those
A. Sarrat/Nuclear
Physics B (Proc. Suppl.) 95 (2001) 177-180
appear before reaching the maximum gain. However with this mixture, we obtained gain of order 105, not too far from what we need. 3.2. Software The software part of the R&D is subdivided into three tasks, a simulation of the efficiency of the MICROMEGAS, the development of algorithms to find the direction of the track, and a simulation to test the reconstruct neutrino spectrum. When passing through the electronics, the signal of one electron could be lost due to several causes of inefficiency. First of all, we need a discriminator to have a digital response extracting signal from background. Since the gain during the avalanche is highly variable, a fraction of the electrons have a signal lower than the threshold and cannot be detected. Moreover, these discriminators work at 100 MHz and have a subsequent dead time of about 10 ns. As a consequence, if two or more electrons arrive on the same strip within 10 ns, only the first one will be detected. In order to affect an arrival time to each electron, we use a TDC with a frequency N 1 GHz. This means that we will affect the same arrival time to signals separated by less than 1 ns and thus there will be an ambiguity of position in such cases (and these electrons can not be used to find the track direction). The results of this simulation have shown that with a dead time of w 10 ns and a step N 1 mm, we can detect more than 2/3 of the electrons which is satisfying. One of the most difficult challenges in HELLAZ is to find the angle at the origin of the track with a good accuracy, typically N 35 mrad _= 2’. To find this angle, we fit the beginning of the track, the length of the fit being N 1 - 2 cm, depending on the energy and on the drift length of the scattered electron. Indeed, this length has to be not too short compared to the transverse extension of the cloud in order to keep a privileged direction. It must be as
179
long as possible in order to increase the statistic. But it must not be too long, because of multiple scattering which causes accidents on the track. Some results obtained at several energies and drift lengths are displayed in figure 3. The errors
0.3
..,
,
.
.
.
.
.
.
.
(
.
.
.
#
.
.
.
.
&
d
,
0
.
0
..‘*..‘-*a’.*.‘..* 0.2
0.4 E
0.6
0.8
1
(Mev)
Figure 3. Angular resolution from Monte-Carlo simulation (dashed curve) for different energies and drift lengths. The solid curve is a fit of the Monte-Carlo results.
are relatively big, especially at low energy, but as we will see, it is already possible to obtain a good spectrum with this angular accuracy. The last part of the software R&D concerns the simulation of the radioactive background and the reconstruction of the neutrino spectrum. We generate radioactive background in the TPC using the following assumptions : - we suppose an extreme purity of helium so that the background generated in the target of the TPC (- 1600 per day) just comes from the i4C of the quencher. - background from 23sU and 232Th decay chains is generated in the body of the TPC with a rate of order 10000 per day.
180
A. Sarrat/Nuclear
Physics B (Proc. Suppl.)
While using elastic scattering, we have no means to discriminate radioactive background from neutrino events. However multiple Compton events can be eliminated as a neutrino would only have one single interaction in the TPC. A fiducial volume and energy cut will also allow to reduce the background. But the main technique to reduce background is a delayed coincidence with the Sun position. Indeed, our signal comes from the Sun, and the kinematic of the elastic scattering is such that all the events will be in a direction opposed to the Sun. Thus the signal at time Tnow will be : Sreal =
%easured
-
&5 si
(2)
95 (2001)
177-150
1400
100-900
cm drift lenght
5 neutrino sources 5 years 1200
loooomdioactive b-hmndI*
loo0
800
600
2=6
where Si is the signal measured i hours before Tnow* With this procedure, the backgroung which is supposed to be isotropic in space and time will naturally be suppressed. The results of a Monte-Carlo simulation (fig. 4) shows that we can already have a noise reduction up to 10000 background per day. To obtain this reconstructed neutrino spectrum, we use the angular resolution parametrization from figure 3. We can see that the neutrino energy resolution is already sufficient to separate pp from ‘Be neutrinos. We can estimate that resolution (- 6%) at 800 keV using the width of the ‘Be line.
400
200
0
t:
0
Figure 4. Reconstruct background reduction.
neutrino spectrum
after
4. Conclusion There were three crucial problems under study in this R&D concerning the detection of single electrons, software tools developments to ilnd both the scattered electron direction and radioactive background level. We obtained good results for single electron detection at normal pressure with a detection efficiency of order 98 %. At the rated pressure of HELLAZ, we still do not have a sufficient gain to achieve this detection, but we are not too far from what we need. As it can be seen from fig. 4, we think we obtain a sufficient accuracy for the direction of the track and we can extract signal from noise in order to separate 7Be line from pp neutrinos. This was done with a realistic radioactive background level.
These points are still under investigation and will be improved in the future. It seems it is now time to look for a collaboration in order to pass from this fundamental R&D to an applied one, by building a two cubic meter prototype with low radioactive materials. REFERENCES M. Gonzalez-Garcia, hepph/0010136. P. Gorodetzky, A. de Bellefon, J. Dolbeau, T. Patzak, P. Salin, A. Sarrat and J.-C. Vanel, Nucl. Phys. B (Proc. Suppl.) 87 (2000) 506. T. Patzak et al., Nucl. Instr. and Meth. A 434 (1999) 358. Private Communication.
a
SUPPLEMENTS Nuclear
ELSEVIER
HELLAZ A. Sarrata,
Physics
B (Proc. Suppl.) 95 (2001) 177-180
: a low energy neutrino on behalf
of the HELLAZ
www.elsevier.nl/locate/npe
spectrometer
collaboration
aLaboratoire de Physique Corpusculaire et Cosmologie 11, place Marcelin Berthelot, 75000 Paris, France
- Collgge de France,
The determination of neutrino properties, especially their mass, is one of the central problems of present day HELLAZ is a project of experiment dedicated to this subject by measuring the solar neutrino spectrum.
particle physics.
1. Introduction An improved knowledge of neutrino properties is one of the main goals of particle physics. Results of solar neutrino experiments show two solar neutrino crucial problems : all measured fluxes are lower than solar models predictions, and there are discrepancies between these measurements. These two facts can be seen as an indication for new neutrino physics via neutrino oscillations (see [l] and references there in). A good way to study this problem in a selfconsistent manner is to measure separately the various contributions of the solar neutrino spectrum with the same reaction in a single detector. The task of HELLAZ R&D is to study the feasibility of such an experiment. 2. HELLAZ
So, we just have to measure T, and 6 to deduce the neutrino energy. The main advantages of this interaction are the theoretically exactly known cross-section, the possibility of real time detection and the absence of threshold. The principle
He at 20 bar. 273 K
principle
The main goal of HELLAZ [2] is to obtain the low energy part of the solar neutrino spectrum We therefore have to use an interaction which allows one to retrieve the neutrino energy with a good accuracy, sufficient to separate pp from 7Be neutrinos. A good candidate for this is the elastic electron - neutrino scattering. The kinematics is that of the elastic scattering :
where m,, T,, and p, are the mass, kinetic energy and momentum of the scattered electron and 8 is the direction of the electron with respect to that of the neutrino (i.e. the direction of the Sun). 0920-5632/01/F% - see front matter 8 2001 Elsevier Science B.V PI1 SO920-5632(01)01079-9
Diffusion (r = 140 pm-).
Figure
1. Detection
principle
1” .!’
V drift = 1.1 mm/ p.s
in HELLAZ.
of HELLAZ is shown in fig. 1. A neutrino scattered off an electron which produce a ionisation track in a TPC filled with helium. Under the action of an electric field of a few 100 V. cm-‘, the track drifts towards an end-cap detector on which each ionisation electron or at least most of them is detected separately. The geometry and the conditions in the TPC must satisfy several requirements. CompAll rights reserved
178
A. Sarm~/Nuclear
Pltwics
B (Proc.
ton scattering events induced by photons are indistinguishable1 from neutrino events, so we must use a zero radioactive background gas as target in the TPC. Helium is a good candidate because it is easy to purify with boil off techniques. To recover the initial direction of the scattered electron, the lowest energy (- 100 keV) track length has to be at least about 5 cm long. As a consequence, we have to work with an helium density N 3 x 10m3g . cmT3 (i.e. 20 bar at 273 IS). With this set up we will have small multiple scattering and small difIusion so it will be less diicult to find the track direction. We will also have a small drift velocity, and therefore a larger time separation between electrons. This make the single electron detection easier. Our last requirement is that the neutrino events rate has to be N 15 events per day. The neutrino - electron scattering cross-section and the predicted neutrino fluxes imply a number of target electrons N 2 x 103’ therefore a 2000 m3 volume of He at this density. Regarding the single electron detection, we need a detector with a high gain (- 106) and a high detection rate, of order 100 MHz to obtain a good efficiency detection. We investigate the MICROMEGAS detector whose properties seem to fulfill these requirements.
SuppI.)
95 (2001)
177-IS0
than 10 ns. Due to HELLAZ requirements, we have to obtain the same results at 20 bar. The first tests were performed with several mixtures of helium + methane. With this mixture, the maximum achievable gain N lo4 is about two order of magnitudes too low to allow single electrons detection. A simulation from Biagi 141allows to compute gain as a function of electric field, gap length and medium composition. As can be seen from figure 2, this simulation predicts that the maximum
3. Results
1
The R&D was focused on two essential fields, a hardware part for the detection of single electrons, and a software part to develop efficient algorithms to find the track direction and to simulate background reduction. 3.1. Harware A first series of tests was performed to detect single electrons with the MICROMEGAS detector [3] at normal pressure and temperature, using a mixture of helium + 6 % isobutane as quencher. Under these conditions, we obtained single electrons detection with an efficiency of 98 % and we can detect pulses with a time separation lower lsee section 3.2 for a statistical kind of eventa
rejection method of this
%
qumeher
Figure 2. Computed gains for &butane and methane in helium at 20 bar for a 50pm gap.
gain achievable with methane is about 2 x 104. This is what we obtained experimentally. It also shows that it seems possible to reach gain higher than lo6 if using isobutane as quencher. Our latest tests show that the gain with isobutane as quencher is larger than with methane, but it is actually not possible to achieve the maximum predicted gain. Indeed, the gain simulation doesn’t take into account breakdowns, and those
A. Sarrat/Nuclear
Physics B (Proc. Suppl.) 95 (2001) 177-180
appear before reaching the maximum gain. However with this mixture, we obtained gain of order 105, not too far from what we need. 3.2. Software The software part of the R&D is subdivided into three tasks, a simulation of the efficiency of the MICROMEGAS, the development of algorithms to find the direction of the track, and a simulation to test the reconstruct neutrino spectrum. When passing through the electronics, the signal of one electron could be lost due to several causes of inefficiency. First of all, we need a discriminator to have a digital response extracting signal from background. Since the gain during the avalanche is highly variable, a fraction of the electrons have a signal lower than the threshold and cannot be detected. Moreover, these discriminators work at 100 MHz and have a subsequent dead time of about 10 ns. As a consequence, if two or more electrons arrive on the same strip within 10 ns, only the first one will be detected. In order to affect an arrival time to each electron, we use a TDC with a frequency N 1 GHz. This means that we will affect the same arrival time to signals separated by less than 1 ns and thus there will be an ambiguity of position in such cases (and these electrons can not be used to find the track direction). The results of this simulation have shown that with a dead time of w 10 ns and a step N 1 mm, we can detect more than 2/3 of the electrons which is satisfying. One of the most difficult challenges in HELLAZ is to find the angle at the origin of the track with a good accuracy, typically N 35 mrad _= 2’. To find this angle, we fit the beginning of the track, the length of the fit being N 1 - 2 cm, depending on the energy and on the drift length of the scattered electron. Indeed, this length has to be not too short compared to the transverse extension of the cloud in order to keep a privileged direction. It must be as
179
long as possible in order to increase the statistic. But it must not be too long, because of multiple scattering which causes accidents on the track. Some results obtained at several energies and drift lengths are displayed in figure 3. The errors
0.3
..,
,
.
.
.
.
.
.
.
(
.
.
.
#
.
.
.
.
&
d
,
0
.
0
..‘*..‘-*a’.*.‘..* 0.2
0.4 E
0.6
0.8
1
(Mev)
Figure 3. Angular resolution from Monte-Carlo simulation (dashed curve) for different energies and drift lengths. The solid curve is a fit of the Monte-Carlo results.
are relatively big, especially at low energy, but as we will see, it is already possible to obtain a good spectrum with this angular accuracy. The last part of the software R&D concerns the simulation of the radioactive background and the reconstruction of the neutrino spectrum. We generate radioactive background in the TPC using the following assumptions : - we suppose an extreme purity of helium so that the background generated in the target of the TPC (- 1600 per day) just comes from the i4C of the quencher. - background from 23sU and 232Th decay chains is generated in the body of the TPC with a rate of order 10000 per day.
180
A. Sarrat/Nuclear
Physics B (Proc. Suppl.)
While using elastic scattering, we have no means to discriminate radioactive background from neutrino events. However multiple Compton events can be eliminated as a neutrino would only have one single interaction in the TPC. A fiducial volume and energy cut will also allow to reduce the background. But the main technique to reduce background is a delayed coincidence with the Sun position. Indeed, our signal comes from the Sun, and the kinematic of the elastic scattering is such that all the events will be in a direction opposed to the Sun. Thus the signal at time Tnow will be : Sreal =
%easured
-
&5 si
(2)
95 (2001)
177-150
1400
100-900
cm drift lenght
5 neutrino sources 5 years 1200
loooomdioactive b-hmndI*
loo0
800
600
2=6
where Si is the signal measured i hours before Tnow* With this procedure, the backgroung which is supposed to be isotropic in space and time will naturally be suppressed. The results of a Monte-Carlo simulation (fig. 4) shows that we can already have a noise reduction up to 10000 background per day. To obtain this reconstructed neutrino spectrum, we use the angular resolution parametrization from figure 3. We can see that the neutrino energy resolution is already sufficient to separate pp from ‘Be neutrinos. We can estimate that resolution (- 6%) at 800 keV using the width of the ‘Be line.
400
200
0
t:
0
Figure 4. Reconstruct background reduction.
neutrino spectrum
after
4. Conclusion There were three crucial problems under study in this R&D concerning the detection of single electrons, software tools developments to ilnd both the scattered electron direction and radioactive background level. We obtained good results for single electron detection at normal pressure with a detection efficiency of order 98 %. At the rated pressure of HELLAZ, we still do not have a sufficient gain to achieve this detection, but we are not too far from what we need. As it can be seen from fig. 4, we think we obtain a sufficient accuracy for the direction of the track and we can extract signal from noise in order to separate 7Be line from pp neutrinos. This was done with a realistic radioactive background level.
These points are still under investigation and will be improved in the future. It seems it is now time to look for a collaboration in order to pass from this fundamental R&D to an applied one, by building a two cubic meter prototype with low radioactive materials. REFERENCES M. Gonzalez-Garcia, hepph/0010136. P. Gorodetzky, A. de Bellefon, J. Dolbeau, T. Patzak, P. Salin, A. Sarrat and J.-C. Vanel, Nucl. Phys. B (Proc. Suppl.) 87 (2000) 506. T. Patzak et al., Nucl. Instr. and Meth. A 434 (1999) 358. Private Communication.