- Email: [email protected]
Electron capture branching ratio measurements in an ion trap for double beta decay experiments at TITAN
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 4643–4646 www.elsevier.com/locate/nimb
Electron capture branching ratio measurements in an ion trap for double beta decay experiments at TITAN T. Brunner a,b,*, M. Brodeur a, C. Champagne a, D. Frekers c,a, R. Kru¨cken b, A. Lapierre a, P. Delheij a, R. Ringle a, V. Ryjkov a, M. Smith a, I. Tanihata a, J. Dilling a b
a TRIUMF, 4004 Wesbrook Mall, Vancouver, Canada Physik Department E12, Technische Universita¨t Mu¨nchen, James-Franck-Strasse, Garching, Germany c Universita¨t Mu¨nster, IKP, Wilhelm–Klemm–Strasse 9, Mu¨nster, Germany
Available online 7 June 2008
Abstract Double beta decay (bb) is a nuclear decay mode expected to appear in at least two varieties, the double-neutrino (2m) and the zeroneutrino (0m) mode. The 0mbb-decay is of particular interest as it requires the neutrino to be a Majorana particle. The search for such a decay is presently being carried out or planned in a number of experiments, such as EXO, MAJORANA, GERDA, CUORE, COBRA, NEMO-III and SNO+. The 0m-decay rate depends on the neutrino mass but, unfortunately, also on a rather complex nuclear matrix element, making the extraction of the mass heavily dependent on the underlying theoretical nuclear model. However, all theoretical models can readily be tested against the 2m mode, which, unlike its 0m counterpart, only involves simple Gamow–Teller nuclear matrix elements. These elements can be determined experimentally either through charge-exchange reactions or, for the ground-state transition, through the electron capture (EC) or single b-decay of the intermediate odd–odd nucleus. The present program is geared towards the measurement of the EC branching ratios (BR). In most cases, these ratios are poorly known or not known at all, because EC is usually suppressed by several orders of magnitude compared to the b-decay counterpart due to energy considerations. Traditional methods for measuring these ratios have so far suffered from overwhelming background generated by these high-energy electrons. Recently, a unique background-free method for measuring EC branching ratios was proposed using the TITAN ion trap at the TRIUMF ISAC (Isotope Separator and ACcelerator) radioactive beam facility. The measurements will make use of the EBIT (Electron Beam Ion Trap) operating in Penning mode where electrons from the b-decay will be confined by the magnetic field. K-shell X-rays from EC will be detected by seven X-ray detectors located around the trap, thus providing orders of magnitude background suppression and thus ideal low-BR measurement environment. Ó 2008 Elsevier B.V. All rights reserved. Keywords: TITAN; EBIT; Electron capture; Double beta decay; Branching ratio; Majorana neutrinos; Nuclear matrix element
1. Introduction Recent large-scale neutrino experiments like SNO and SuperK confirmed evidence for neutrino oscillations [1] that can only occur if the neutrino is a massive particle. It is found that the neutrino eigenstates ml are a superposition of the three neutrino mass eigenstates mi that are con*
Corresponding author. Address: TRIUMF, 4004 Wesbrook Mall, Vancouver, Canada. E-mail address: [email protected] (T. Brunner). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.05.105
nected by the elements Ui of the Pontecorvo–Maki–Sakata unitary neutrino matrix [2] j < mml > j ¼
3 X
U li mi ;
l ¼ e; l; s:
ð1Þ
i¼l
These m-oscillation experiments allow the determination of mixing angle hij as well as the squared mass difference of different neutrino flavors dm2 [2]; but by using this relative mass scale the absolute neutrino mass mm cannot be determined [1].
4644
T. Brunner et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4643–4646
Measurements of bb-decays such as the still to be confirmed Heidelberg–Moscow experiment [3] would provide the mass of neutrinos. Double beta decays occur in even– even nuclei where single beta decays are energetically not possible [4]. Within the Standard Model (SM), the 2mbb-decay is an allowed second order weak process with half lives of more than 1019 yr. In the case of 0mbb, two neutrons simultaneously decay into two protons and two electrons without the emission of neutrinos. This process violates lepton number conservation and is therefore forbidden in the SM [4]. Several collaborations, EXO, Majorana, NEMO-III and CUORE [5], are investigating this decay which, in the case 25 of 76Ge has a half life longer than T 0m 1=2 > 1:57 10 yr on a 90% C.L. [6]. If this decay is observed the neutrino character has to be that of a Majorana particle where the particle is its own anti-particle. In this decay the electron-neutrino mass can be determined directly from the 0mbb half life and the nuclear matrix element FN 1=2 j < mme > j ¼ ðF N T 0m eV: 1=2 Þ
ð2Þ
Either the shell-model or the proton–neutron quasi-particle random phase approximation (pn-QRPA, see overview in [1,4]) are used to calculate FN. The latter description uses an adjustable particle–particle parameter gPP that defines part of the many-body Hamiltonian for all single and double beta decay calculations [1,7]. 2mbbdecays seem to be rather sensitive to gPP whereas 0mbb-decays are rather insensitive. Hence, 2mbb-decays are used to determine this parameter by fitting calculated nuclear matrix elements to experimental half lives. Later, this gPP is used to determine the nuclear matrix elements needed to describe the 0mbb process [8]. Unfortunately, the gPP determined by this method does not accurately reproduce the nuclear matrix elements for single beta decays and electron capture transitions. In fact, there seems to be two compensating errors: too large EC nuclear matrix elements are compensated by too small b nuclear matrix elements so that, in general, gPP reproduces the 2mbb half life. In order to back up the pn-QRPA theory and to determine FN it is important to know the nuclear matrix elements of the involved nuclei. A detailed discussion is provided in [1]. One way to gain this information is by measuring the b and electron capture branching ratios. Currently, the electron capture branching ratio is not well known due to its small value of the order of 103 and less. Conventional techniques have reached a limit of sensitivity, so a new experiment using a novel approach is being set up at TITAN (TRIUMF Ion Trap for Atomic and Nuclear physics) to determine this BR.
line beam of nuclei is cooled and bunched by a gas-filled linear radio-frequency quadrupole [11] and then delivered to several traps. A variety of measurements can be performed, such as high-precision mass spectroscopy, laser spectroscopy, and X-ray spectroscopy [12]. The central component of the EC–BR measurements is the EBIT [13] that will be used in Penning mode, hence without electron beam (Fig. 1). The trap is generated by two super-conducting coils in a Helmholtz configuration creating a magnetic field of 6 T, which confines the ions in the radial plane and additional electric fields trap them in the axial plane. Visible access to the trap region is gained via a segmented trap center of the EBIT (Fig. 2). This allows to radially install seven high resolution Si(Li) X-ray detectors with a total solid angle of 2.1%. For the b-detection a silicon surface barrier (SSB) detector will be installed on axis. Using this detector the number of ions in the trap will be monitored by detecting b-decay electrons of stored nuclei. The strong magnetic field will guide these electrons from their place of origin to the SSB detector at the end of the trap (Fig. 3). A soft anti-coincidence between beta decay electrons and X-rays allows a further increase in the accuracy of the EC branching ratio determination by reducing the background. So far, the performance of a SSB detector, mounted on a ceramic disc, with a thickness of 523 lm has been tested in the TITAN beamline with 9Li. Due to this special mounting technique no UHV contamination could be observed and a pressure of less than 2 109 mbar was reached. The Q-value of 9Li lies with 12 MeV above the expected Q-values of Q 3 MeV in the BR measurements. Nevertheless a clear decay spectrum of the b-decay could be recorded (Fig. 4) and the life time of 178.3 ± 0.4 ms [14] can be reproduced within one r. Overall this SSB detector performs excellent for the detection of b-decay electrons under UHV conditions. Due to the operation of the system at 4 K, the background pressure is less than 1011 mbar, allowing long
2. Experiment 2.1. Setup TITAN [9] is a new facility of ion traps at the radioactive isotope facility ISAC at TRIUMF [10] where an on-
Fig. 1. Photograph of the EBIT on the TITAN platform.
T. Brunner et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4643–4646
4645
will be possible to increase the number up to 109 ions in the trap. This novel technique of observing X-ray emission following an EC of isotopes stored in the Penning trap allows a backing free determination of these very small branching ratios [7]. 2.2. Simulations
Fig. 2. Schematic of the trap center (left) and a cut through the center of the trap (right) with the position of one of the X-ray detectors.
Fig. 3. Schematic of trap center including magnet and vacuum housing.
180
Counts exp. Fit
160
Counts [a.u.]
140 120 100 80 60 40 20 0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Time [s]
Fig. 4. Beta decay spectrum of 9Li taken with the SSB detector.
storage times of several minutes. At the moment 105 to 106 ions can be stored in the trap. Simulations indicate, that with further improvements, like sideband cooling [15], it
In order to optimize the detection efficiency of electron capture branching ratio and b-decay electrons, simulations were performed. SIMION [16] simulations showed that electrons that are emitted by nuclei inside the trap with a vertical distance of more than 2.5 mm from the trap axis are lost due to collisions with the trap electrodes. This results in the necessity to cool the ions by sideband cooling inside the trap in order to decrease the radius of the ion cloud. Further loss of electrons arises from the geometrical arrangement of the magnetic coils. These are positioned in a Helmholtz-like configuration and produce two magnetic field maxima at both entrances of the trap. This geometrical arrangement produces a magnetic bottle. Electrons that are emitted with a parallel-to-perpendicular velocity ratio towards the beam axis of less than rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi vk Bmax ¼ 1 ð3Þ v? crit Bmin are trapped between the two magnetic field maxima Bmax and the field minimum Bmin in the center of the trap. For soft anti-coincidence of b detection with X-ray detection these are lost. In case of the TITAN EBIT the critical angle amounts to about 75° respective to the beam axis and results in an electron loss of about 16%. The necessary diameter of the SSB b detector is determined by its distance to the trap. Due to the curvature of the magnetic field-lines the electron trajectories diverge when the electrons leave the trap. Hence, the detector must be placed as close as possible to the last trap electrode. Therefore the silicon detector will be installed on axis in a vacuum chamber just after the exit of the trap. A detector diameter of 27 mm is chosen as the diameter of the simulated electron beam is 25 mm. A SSB detector provides very high efficiency, nearly 100% for electrons with several MeV kinetic energy. The Q-values for the b-branches of the nuclei that are subject to the EC branching ratio measurements are around 3 MeV (Fig. 5). For these energy ranges the detection efficiencies of multi channel plates, as well as channeltrons, are low even if the magnetic field strength at the position of the detector would allow their use [17]. Plastic scintillators and thick semi-conductor detectors are also not ideal because they outgas or need cooling to increase the signal to noise ratio. These drawbacks do not exist for a silicon surface barrier detector which makes it the right choice for this application. The SSB b detection threshold is about 33 keV, so only 0.2% of the electrons would not be
4646
T. Brunner et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4643–4646
Fig. 5. Electron capture and b-decay branches of
100
Tc.
detected. All electrons that originate from Auger processes, competing with X-ray emission, will be invisible. A thickness of 500 lm of the silicon surface barrier detector provides low noise but also restricts the energy resolution. Only electrons with less than 600 keV, about 15% of the b spectrum, are stopped in the detector. Faster electrons pass through the silicon and leave an energy loss signal. Hence, this detector is suitable for counting electrons but will not provide a full-energy spectrum. Traces of electrons, for optimizing the position and geometry of the detector are simulated with GEANT [18]. For measuring the electron capture processes high resolution Si(Li) detectors will be used to detect X-rays that are emitted after the capture of K-shell electrons. The expected X-ray energies vary from 17.5 keV in the case of 100Tc up to 27.5 keV in case of 128I. The only exception is 76As with X-ray energies of 9 keV. Simulations with GEANT showed that a detector thickness of 2 mm is sufficient to detect all emitted X-ray radiation in this energy range. The X-rays exit the trap through openings of the segmented trap. To reduce the heat load the whole trap including the superconducting magnet is inside a thermal shield. Beryllium windows of 25 lm will be installed in the outer thermal shield and provide as little radiation attenuation as possible. Another Be-window of 500 lm welded on a flange will be used to close the vacuum chamber of the trap. These two windows lead to a calculated attenuation of less than 4% for 17.5 keV X-rays. 3. Outlook During the next months more detailed GEANT simulations will be performed to investigate in EC branching ratios and possible background. Energy deposition in the SSB detector as well as the soft anti-coincidence is of primary interest. A new housing for the detector is under construction as the present housing is not big enough to hold the detector. A first SSB detector was bought and first tests with 207Bi as well as 8Li and 9Li were performed. So far the detector works as expected and further tests with b sources will be carried out to compare the experimental energy spectrum with the one simulated with GEANT. At the end of November 2007 the EBIT beam line will be commissioned and initial off-line tests with the EBIT
are planned. During the first quarter of 2008 an electron capture branching ratio measurement is planned. For this measurement 100Tc with a half life of (15.8 ± 0.1) s will be investigated (see Fig. 5). Therefore 10 ion bunches from the radio-frequency quadrupole are accumulated to trap 106 ions in the EBIT. A detection time of 15 s will yield in 50,000 b-decays but only 0.9 EC decays. The detection efficiency mentioned above results in 5.6 103 detected EC events in 15 s. For a 10% accuracy one needs 100 detected events that requires about 17,700 trap fills and an operation time of 74 h. The 100Tc EC determination allows a direct comparison between this novel and a conventional technique [19,20]. In the near future there are further EC–BR measurements planned with isotopes like 110 Ag, 114In, 150Nd and 76As that decay into isotopes investigated in bb-decay experiments such as Majorana, COBRA, CUORE, SNO+ and others. Acknowledgements One of the authors (T.B.) wishes to thank the ‘‘Evangelisches Studienwerk e.V. Villigst” for their financial support. The TITAN collaboration acknowledges the help of the TRIUMF support groups as well as the assistance of the Max–Planck–Institut for Nuclear Physics in Heidelberg concerning the EBIT. Especially J.R. Crespo Lo´pez–Urrutia, M. Froese, G. Sickler and J. Ullrich shall be thanked for their assistance and collaboration with the EBIT project. TITAN is supported by NSERC. References [1] J. Suhonen, Phys. Lett. B 607 (2005) 87. [2] S.M. Bilenky, A. Faessler, F. Sˇimkovic, Phys. Rev. D 70 (2004) 033003. [3] H.V. Klapdor-Kleingrothaus et al., Nucl. Phys. B 100 (2001) 309. [4] M. Doi, T. Kotani, E. Takasugi, Prog. Theor. Phys. Suppl. 83 (1985) 1. [5] S.R. Elliott, Int. J. Mod. Phys. A 18 (2003) 4097. [6] C.E. Aalseth et al., Phys. Rev. D 65 (2002) 092007. [7] J. Dilling, I. Tanihata, D. Frekers, Can. J. Phys. 85 (2007) 57. [8] V.A. Rodin et al., Phys. Rev. C 68 (2003) 044302. [9] J. Dilling et al., Nucl. Instr. Meth. B 204 (2003) 492. [10] M. Dombsky et al., Nucl. Phys. A 701 (2002) 486. [11] M. Smith et al., Hyperfine Interact. 173 (2006) 171. [12] J. Dilling et al., Int. J. Mass. Spec. 251 (2006) 198. [13] G. Sikler et al., Eur. Phys. J. A 25 (2005) 63. [14] D.E. Alburger, D.H. Wilkinson, Phys. Rev. C 13 (1976) 835. [15] F. Ames et al., Nucl. Instr. Meth. A 538 (2005) 17. [16] D. Dahl, Int. J. Mass Spec. 200 (2000) 3. [17] C.I. Coleman, Rev. Sci. Instr. 53 (1982) 6. [18] S. Agostinelli et al., Nucl. Instr. Meth. A 506 (2003) 250. [19] H. Akimune et al., Phys. Lett. B 394 (1997) 23. [20] A. Garcı´a et al., Phys. Rev. C47 (1993) 2910.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 4643–4646 www.elsevier.com/locate/nimb
Electron capture branching ratio measurements in an ion trap for double beta decay experiments at TITAN T. Brunner a,b,*, M. Brodeur a, C. Champagne a, D. Frekers c,a, R. Kru¨cken b, A. Lapierre a, P. Delheij a, R. Ringle a, V. Ryjkov a, M. Smith a, I. Tanihata a, J. Dilling a b
a TRIUMF, 4004 Wesbrook Mall, Vancouver, Canada Physik Department E12, Technische Universita¨t Mu¨nchen, James-Franck-Strasse, Garching, Germany c Universita¨t Mu¨nster, IKP, Wilhelm–Klemm–Strasse 9, Mu¨nster, Germany
Available online 7 June 2008
Abstract Double beta decay (bb) is a nuclear decay mode expected to appear in at least two varieties, the double-neutrino (2m) and the zeroneutrino (0m) mode. The 0mbb-decay is of particular interest as it requires the neutrino to be a Majorana particle. The search for such a decay is presently being carried out or planned in a number of experiments, such as EXO, MAJORANA, GERDA, CUORE, COBRA, NEMO-III and SNO+. The 0m-decay rate depends on the neutrino mass but, unfortunately, also on a rather complex nuclear matrix element, making the extraction of the mass heavily dependent on the underlying theoretical nuclear model. However, all theoretical models can readily be tested against the 2m mode, which, unlike its 0m counterpart, only involves simple Gamow–Teller nuclear matrix elements. These elements can be determined experimentally either through charge-exchange reactions or, for the ground-state transition, through the electron capture (EC) or single b-decay of the intermediate odd–odd nucleus. The present program is geared towards the measurement of the EC branching ratios (BR). In most cases, these ratios are poorly known or not known at all, because EC is usually suppressed by several orders of magnitude compared to the b-decay counterpart due to energy considerations. Traditional methods for measuring these ratios have so far suffered from overwhelming background generated by these high-energy electrons. Recently, a unique background-free method for measuring EC branching ratios was proposed using the TITAN ion trap at the TRIUMF ISAC (Isotope Separator and ACcelerator) radioactive beam facility. The measurements will make use of the EBIT (Electron Beam Ion Trap) operating in Penning mode where electrons from the b-decay will be confined by the magnetic field. K-shell X-rays from EC will be detected by seven X-ray detectors located around the trap, thus providing orders of magnitude background suppression and thus ideal low-BR measurement environment. Ó 2008 Elsevier B.V. All rights reserved. Keywords: TITAN; EBIT; Electron capture; Double beta decay; Branching ratio; Majorana neutrinos; Nuclear matrix element
1. Introduction Recent large-scale neutrino experiments like SNO and SuperK confirmed evidence for neutrino oscillations [1] that can only occur if the neutrino is a massive particle. It is found that the neutrino eigenstates ml are a superposition of the three neutrino mass eigenstates mi that are con*
Corresponding author. Address: TRIUMF, 4004 Wesbrook Mall, Vancouver, Canada. E-mail address: [email protected] (T. Brunner). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.05.105
nected by the elements Ui of the Pontecorvo–Maki–Sakata unitary neutrino matrix [2] j < mml > j ¼
3 X
U li mi ;
l ¼ e; l; s:
ð1Þ
i¼l
These m-oscillation experiments allow the determination of mixing angle hij as well as the squared mass difference of different neutrino flavors dm2 [2]; but by using this relative mass scale the absolute neutrino mass mm cannot be determined [1].
4644
T. Brunner et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4643–4646
Measurements of bb-decays such as the still to be confirmed Heidelberg–Moscow experiment [3] would provide the mass of neutrinos. Double beta decays occur in even– even nuclei where single beta decays are energetically not possible [4]. Within the Standard Model (SM), the 2mbb-decay is an allowed second order weak process with half lives of more than 1019 yr. In the case of 0mbb, two neutrons simultaneously decay into two protons and two electrons without the emission of neutrinos. This process violates lepton number conservation and is therefore forbidden in the SM [4]. Several collaborations, EXO, Majorana, NEMO-III and CUORE [5], are investigating this decay which, in the case 25 of 76Ge has a half life longer than T 0m 1=2 > 1:57 10 yr on a 90% C.L. [6]. If this decay is observed the neutrino character has to be that of a Majorana particle where the particle is its own anti-particle. In this decay the electron-neutrino mass can be determined directly from the 0mbb half life and the nuclear matrix element FN 1=2 j < mme > j ¼ ðF N T 0m eV: 1=2 Þ
ð2Þ
Either the shell-model or the proton–neutron quasi-particle random phase approximation (pn-QRPA, see overview in [1,4]) are used to calculate FN. The latter description uses an adjustable particle–particle parameter gPP that defines part of the many-body Hamiltonian for all single and double beta decay calculations [1,7]. 2mbbdecays seem to be rather sensitive to gPP whereas 0mbb-decays are rather insensitive. Hence, 2mbb-decays are used to determine this parameter by fitting calculated nuclear matrix elements to experimental half lives. Later, this gPP is used to determine the nuclear matrix elements needed to describe the 0mbb process [8]. Unfortunately, the gPP determined by this method does not accurately reproduce the nuclear matrix elements for single beta decays and electron capture transitions. In fact, there seems to be two compensating errors: too large EC nuclear matrix elements are compensated by too small b nuclear matrix elements so that, in general, gPP reproduces the 2mbb half life. In order to back up the pn-QRPA theory and to determine FN it is important to know the nuclear matrix elements of the involved nuclei. A detailed discussion is provided in [1]. One way to gain this information is by measuring the b and electron capture branching ratios. Currently, the electron capture branching ratio is not well known due to its small value of the order of 103 and less. Conventional techniques have reached a limit of sensitivity, so a new experiment using a novel approach is being set up at TITAN (TRIUMF Ion Trap for Atomic and Nuclear physics) to determine this BR.
line beam of nuclei is cooled and bunched by a gas-filled linear radio-frequency quadrupole [11] and then delivered to several traps. A variety of measurements can be performed, such as high-precision mass spectroscopy, laser spectroscopy, and X-ray spectroscopy [12]. The central component of the EC–BR measurements is the EBIT [13] that will be used in Penning mode, hence without electron beam (Fig. 1). The trap is generated by two super-conducting coils in a Helmholtz configuration creating a magnetic field of 6 T, which confines the ions in the radial plane and additional electric fields trap them in the axial plane. Visible access to the trap region is gained via a segmented trap center of the EBIT (Fig. 2). This allows to radially install seven high resolution Si(Li) X-ray detectors with a total solid angle of 2.1%. For the b-detection a silicon surface barrier (SSB) detector will be installed on axis. Using this detector the number of ions in the trap will be monitored by detecting b-decay electrons of stored nuclei. The strong magnetic field will guide these electrons from their place of origin to the SSB detector at the end of the trap (Fig. 3). A soft anti-coincidence between beta decay electrons and X-rays allows a further increase in the accuracy of the EC branching ratio determination by reducing the background. So far, the performance of a SSB detector, mounted on a ceramic disc, with a thickness of 523 lm has been tested in the TITAN beamline with 9Li. Due to this special mounting technique no UHV contamination could be observed and a pressure of less than 2 109 mbar was reached. The Q-value of 9Li lies with 12 MeV above the expected Q-values of Q 3 MeV in the BR measurements. Nevertheless a clear decay spectrum of the b-decay could be recorded (Fig. 4) and the life time of 178.3 ± 0.4 ms [14] can be reproduced within one r. Overall this SSB detector performs excellent for the detection of b-decay electrons under UHV conditions. Due to the operation of the system at 4 K, the background pressure is less than 1011 mbar, allowing long
2. Experiment 2.1. Setup TITAN [9] is a new facility of ion traps at the radioactive isotope facility ISAC at TRIUMF [10] where an on-
Fig. 1. Photograph of the EBIT on the TITAN platform.
T. Brunner et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4643–4646
4645
will be possible to increase the number up to 109 ions in the trap. This novel technique of observing X-ray emission following an EC of isotopes stored in the Penning trap allows a backing free determination of these very small branching ratios [7]. 2.2. Simulations
Fig. 2. Schematic of the trap center (left) and a cut through the center of the trap (right) with the position of one of the X-ray detectors.
Fig. 3. Schematic of trap center including magnet and vacuum housing.
180
Counts exp. Fit
160
Counts [a.u.]
140 120 100 80 60 40 20 0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Time [s]
Fig. 4. Beta decay spectrum of 9Li taken with the SSB detector.
storage times of several minutes. At the moment 105 to 106 ions can be stored in the trap. Simulations indicate, that with further improvements, like sideband cooling [15], it
In order to optimize the detection efficiency of electron capture branching ratio and b-decay electrons, simulations were performed. SIMION [16] simulations showed that electrons that are emitted by nuclei inside the trap with a vertical distance of more than 2.5 mm from the trap axis are lost due to collisions with the trap electrodes. This results in the necessity to cool the ions by sideband cooling inside the trap in order to decrease the radius of the ion cloud. Further loss of electrons arises from the geometrical arrangement of the magnetic coils. These are positioned in a Helmholtz-like configuration and produce two magnetic field maxima at both entrances of the trap. This geometrical arrangement produces a magnetic bottle. Electrons that are emitted with a parallel-to-perpendicular velocity ratio towards the beam axis of less than rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi vk Bmax ¼ 1 ð3Þ v? crit Bmin are trapped between the two magnetic field maxima Bmax and the field minimum Bmin in the center of the trap. For soft anti-coincidence of b detection with X-ray detection these are lost. In case of the TITAN EBIT the critical angle amounts to about 75° respective to the beam axis and results in an electron loss of about 16%. The necessary diameter of the SSB b detector is determined by its distance to the trap. Due to the curvature of the magnetic field-lines the electron trajectories diverge when the electrons leave the trap. Hence, the detector must be placed as close as possible to the last trap electrode. Therefore the silicon detector will be installed on axis in a vacuum chamber just after the exit of the trap. A detector diameter of 27 mm is chosen as the diameter of the simulated electron beam is 25 mm. A SSB detector provides very high efficiency, nearly 100% for electrons with several MeV kinetic energy. The Q-values for the b-branches of the nuclei that are subject to the EC branching ratio measurements are around 3 MeV (Fig. 5). For these energy ranges the detection efficiencies of multi channel plates, as well as channeltrons, are low even if the magnetic field strength at the position of the detector would allow their use [17]. Plastic scintillators and thick semi-conductor detectors are also not ideal because they outgas or need cooling to increase the signal to noise ratio. These drawbacks do not exist for a silicon surface barrier detector which makes it the right choice for this application. The SSB b detection threshold is about 33 keV, so only 0.2% of the electrons would not be
4646
T. Brunner et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4643–4646
Fig. 5. Electron capture and b-decay branches of
100
Tc.
detected. All electrons that originate from Auger processes, competing with X-ray emission, will be invisible. A thickness of 500 lm of the silicon surface barrier detector provides low noise but also restricts the energy resolution. Only electrons with less than 600 keV, about 15% of the b spectrum, are stopped in the detector. Faster electrons pass through the silicon and leave an energy loss signal. Hence, this detector is suitable for counting electrons but will not provide a full-energy spectrum. Traces of electrons, for optimizing the position and geometry of the detector are simulated with GEANT [18]. For measuring the electron capture processes high resolution Si(Li) detectors will be used to detect X-rays that are emitted after the capture of K-shell electrons. The expected X-ray energies vary from 17.5 keV in the case of 100Tc up to 27.5 keV in case of 128I. The only exception is 76As with X-ray energies of 9 keV. Simulations with GEANT showed that a detector thickness of 2 mm is sufficient to detect all emitted X-ray radiation in this energy range. The X-rays exit the trap through openings of the segmented trap. To reduce the heat load the whole trap including the superconducting magnet is inside a thermal shield. Beryllium windows of 25 lm will be installed in the outer thermal shield and provide as little radiation attenuation as possible. Another Be-window of 500 lm welded on a flange will be used to close the vacuum chamber of the trap. These two windows lead to a calculated attenuation of less than 4% for 17.5 keV X-rays. 3. Outlook During the next months more detailed GEANT simulations will be performed to investigate in EC branching ratios and possible background. Energy deposition in the SSB detector as well as the soft anti-coincidence is of primary interest. A new housing for the detector is under construction as the present housing is not big enough to hold the detector. A first SSB detector was bought and first tests with 207Bi as well as 8Li and 9Li were performed. So far the detector works as expected and further tests with b sources will be carried out to compare the experimental energy spectrum with the one simulated with GEANT. At the end of November 2007 the EBIT beam line will be commissioned and initial off-line tests with the EBIT
are planned. During the first quarter of 2008 an electron capture branching ratio measurement is planned. For this measurement 100Tc with a half life of (15.8 ± 0.1) s will be investigated (see Fig. 5). Therefore 10 ion bunches from the radio-frequency quadrupole are accumulated to trap 106 ions in the EBIT. A detection time of 15 s will yield in 50,000 b-decays but only 0.9 EC decays. The detection efficiency mentioned above results in 5.6 103 detected EC events in 15 s. For a 10% accuracy one needs 100 detected events that requires about 17,700 trap fills and an operation time of 74 h. The 100Tc EC determination allows a direct comparison between this novel and a conventional technique [19,20]. In the near future there are further EC–BR measurements planned with isotopes like 110 Ag, 114In, 150Nd and 76As that decay into isotopes investigated in bb-decay experiments such as Majorana, COBRA, CUORE, SNO+ and others. Acknowledgements One of the authors (T.B.) wishes to thank the ‘‘Evangelisches Studienwerk e.V. Villigst” for their financial support. The TITAN collaboration acknowledges the help of the TRIUMF support groups as well as the assistance of the Max–Planck–Institut for Nuclear Physics in Heidelberg concerning the EBIT. Especially J.R. Crespo Lo´pez–Urrutia, M. Froese, G. Sickler and J. Ullrich shall be thanked for their assistance and collaboration with the EBIT project. TITAN is supported by NSERC. References [1] J. Suhonen, Phys. Lett. B 607 (2005) 87. [2] S.M. Bilenky, A. Faessler, F. Sˇimkovic, Phys. Rev. D 70 (2004) 033003. [3] H.V. Klapdor-Kleingrothaus et al., Nucl. Phys. B 100 (2001) 309. [4] M. Doi, T. Kotani, E. Takasugi, Prog. Theor. Phys. Suppl. 83 (1985) 1. [5] S.R. Elliott, Int. J. Mod. Phys. A 18 (2003) 4097. [6] C.E. Aalseth et al., Phys. Rev. D 65 (2002) 092007. [7] J. Dilling, I. Tanihata, D. Frekers, Can. J. Phys. 85 (2007) 57. [8] V.A. Rodin et al., Phys. Rev. C 68 (2003) 044302. [9] J. Dilling et al., Nucl. Instr. Meth. B 204 (2003) 492. [10] M. Dombsky et al., Nucl. Phys. A 701 (2002) 486. [11] M. Smith et al., Hyperfine Interact. 173 (2006) 171. [12] J. Dilling et al., Int. J. Mass. Spec. 251 (2006) 198. [13] G. Sikler et al., Eur. Phys. J. A 25 (2005) 63. [14] D.E. Alburger, D.H. Wilkinson, Phys. Rev. C 13 (1976) 835. [15] F. Ames et al., Nucl. Instr. Meth. A 538 (2005) 17. [16] D. Dahl, Int. J. Mass Spec. 200 (2000) 3. [17] C.I. Coleman, Rev. Sci. Instr. 53 (1982) 6. [18] S. Agostinelli et al., Nucl. Instr. Meth. A 506 (2003) 250. [19] H. Akimune et al., Phys. Lett. B 394 (1997) 23. [20] A. Garcı´a et al., Phys. Rev. C47 (1993) 2910.