- Email: [email protected]
WITCH: a recoil spectrometer for β-decay
Nuclear Instruments and Methods in Physics Research B 204 (2003) 521–525 www.elsevier.com/locate/nimb
WITCH: a recoil spectrometer for b-decay M. Beck a,*, F. Ames b, D. Beck e, G. Bollen d, B. Delaure a, J. Deutsch c, V.V. Golovko a, V.Yu. Kozlov a, I.S. Kraev a, A. Lindroth a, T. Phalet a, W. Quint e, K. Reisinger b, P. Schuurmans a, N. Severijns a, B. Vereecke a, S. Versyck a, The EUROTRAPS Collaboration a
Instituut voor Kern- en Stralingsfysica, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium b Ludwig-Maximilians-Universit€at, Sektion Physik, Schellingstraße 4/IV, D-80799 M€unchen, Germany c Institut de Physique Nucl eaire, UCL, Chemin du Cyclotron 2, B-1348 Louvain-la-Neuve, Belgium d Michigan State University, 103W Cyclotron, East Lansing, MI 48824, USA e GSI-Darmstadt, Planckstr. 1, D-64291 Darmstadt, Germany
Abstract The WITCH experiment will measure the recoil energy spectrum of the daughter ions in b-decay. The main parts of the experiment are two Penning traps and a subsequent retardation spectrometer. The b-decays take place in the ion cloud in the decay trap. Since the ion cloud is in vacuum and due to the cylindrical structure of the trap, the recoiling daughter ions can leave the cloud and the trap without any significant energy loss and can be energy analyzed in the retardation spectrometer. The WITCH experiment is set up foremost to study the electroweak interaction by measuring the b–m angular correlation in nuclear b-decay which can be inferred from the shape of the energy spectrum of the recoil ions. In the beginning the experiment will focus on pure Fermi decays which will allow to search for additional scalar coupling in the weak interaction. Since Penning traps have no restrictions regarding the element to be trapped the most suitable isotope can be picked for this purpose. The WITCH experiment is presently being set up at ISOLDE. Status and perspectives of the experiment will be presented in the following. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 23.40.B; 29.30; 39.10 Keywords: Weak interaction; b-decay; Scalar interaction; Ion trap; Penning trap; Retardation spectrometer
1. Motivation It has been found experimentally that of the five interaction types allowed by Lorentz invariance of *
Corresponding author. Tel.: +32-16-327265; fax: +32-16327985. E-mail address: [email protected] (M. Beck).
the Hamiltonian-vector (V), axialvector (A), scalar (S), tensor (T) and pseudoscalar interaction (PS) – only V- and A-interaction are realized in weak interaction. This is implemented in the very successful standard electroweak model. However, the experimental limits for S- and T-interaction are rather weak, i.e. only of the order 10% (or mboson 300 GeV).
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(02)02125-0
522
M. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 521–525
In order to improve the limits on S- and T-interaction, the WITCH experiment intends to determine the electron-neutrino angular correlation in nuclear b-decay with high precision [1]: xð#bm Þ 1 þ av=c cosð#bm Þð1 Cm=EbÞ
ð1Þ
with a the b–m angular correlation coefficient, #bm the angle between the electron and the neutrino, v=c the velocity, m the restmass and E the total energy of the electron, C ¼ ð1 ða ZÞ2 Þ1=2 , a the fine structure constant and Z the nuclear charge. The Fierz interference term b has experimentally been shown to be small [2]. Using the properties of the c-matrices and the general Hamiltonian [3] for the weak interaction it can be shown that the b-particle and the neutrino emitted in b-decay must have opposite helicities for V-interaction and same helicities for S-interaction. Therefore, in superallowed 0þ –0þ Fermi decays where the spins have to couple to zero the b-particle and the neutrino will be emitted preferentially into the same direction in the case of Vinteraction and into opposite directions in the case of S-interaction. On average this will lead to a large recoil energy for V-interaction and a small recoil energy for S-interaction. For pure Fermi decays the dependence of a on the interaction type is e S j2 j C e 0 j2 þ 2Zam=pImð C eS þ C e0 Þ 2 jC S S a¼ ð2Þ e 0 j2 e S j2 þ j C 2 þ jC S 0
e S ¼ CS and C e 0 ¼ CS where CS and CV are the with C S CV CV coupling constants for S- and V-interaction [3]. In the standard model only V-interaction contributes to the decay rate of superallowed Fermi transitions (0þ –0þ transitions) and therefore a ¼ 1. Any admixture of S to V in a Fermi transition would show itself as a < 1. Thus, by determining the b–m angular correlation coefficient a, it can be determined which interactions contribute. Since the neutrino cannot be measured directly in such an experiment the electron–neutrino angular correlation has to be inferred from other observables such as the shape of the energy spectrum of the electrons or the recoil ions. This is very difficult since the b-emitter usually is embedded in a solid matrix, e.g. an ion catcher, which will lead
to changes in the spectra due to energy losses. The b-particles will be scattered and the recoiling daughter nuclei most times will be stopped already in the source due to their very low energy of usually 1 keV. Consequently, the b–m angular correlation could only be measured in a few exceptional cases [4–6]. In order to avoid that the recoils are stopped in the source the WITCH experiment will utilize a Penning trap to store the radioactive ions. Here the ions will be stored in an ion cloud with only vacuum surrounding them. An open electrode structure of the trap (see Section 2) allows them to also leave the trap without scattering. The experiment will be located at ISOLDE [7] at CERN where a huge variety of nuclides are available with high intensities. The combination of this with a Penning trap will enable this experiment to select the most suited b-emitter that decays in a pure Fermi mode to search for scalar interaction. Additionally, it opens a general way for recoil spectroscopy in b-decay [8–15].
2. Experimental set-up 2.1. Beam transport The ions are produced by ISOLDE. In a first step they get trapped and cooled by REXTRAP [16]. From there they are transmitted in bunches through a horizontal beamline and a 90° bender into a vertical beamline. For test purposes and tuning there also will be an ion source available in the horizontal beamline. Up to the start of the vertical beamline the ions have the full ISOLDE energy of 60 keV. After the 90° bender the ions are deccelerated in several steps. They enter the first Penning trap with an energy of 50 eV. 2.2. Source The ions get trapped in the first Penning trap, the cooler trap (Fig. 1, left hand side). Here they are mass-selectively cooled and centered by buffer gas assisted cooling [17] to approximately room temperature. Then they are ejected through an opening in a differential pumping barrier into the
M. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 521–525
523
Fig. 1. The Penning traps. The cooler trap is to the left, the decay trap to the right. The diameter of the traps is 4 cm and the total length of the trap structure 40 cm. The differential pumping barrier between the traps is not shown.
second Penning trap, the decay trap (Fig. 1, right hand side). Both Penning traps have a cylindrical geometry. They are modified versions of the cooler trap of the ISOLTRAP high-precision mass measurement experiment [18]. The ions are kept in the decay trap for a couple of half-lives, i.e. between 1 and 10 s. During this time most of the ions will decay. The ion cloud in the decay trap constitutes the source for the experiment. The trap itself is at the beginning of the retardation spectrometer. 2.3. Spectrometer The retardation spectrometer is illustrated in Fig. 2. The recoiling daughter ions from b-decay in the measurement trap are emitted isotropically. The ions emitted backward in the direction of the cooler trap are lost. Those emitted forward into the spectrometer spiral from the high field (Bmax 9 T) to the low field (Bmin 0:1 T) region. A retardation potential is applied between these two regions. It probes the component of the ion energy parallel to the axis of the set-up. Due to careful shaping of the magnetic and electric fields and the principle of adiabatic invariance of the magnetic flux contained in the ion motion [19], a fraction of 1 Bmin =Bmax 98:9%
Fig. 2. Experimental set-up. The recoil ions from the b-decays in the trap will spiral into the spectrometer. There most of their radial energy is converted into axial energy and probed by the electrostatic retardation potential. The ions that pass get postaccelerated and are finally focused onto the detector.
ð3Þ
of the radial energy of the ions is converted into axial energy while the ions spiral from the high field to the low field region. Thus 98.9–100% of the ion energy, depending on the emission direction after the b-decay, is in the axial ion motion when they reach the low field region. Only ions with an axial energy in the low field region which exceeds the retardation potential can pass and will be
counted. This is the same fundamental principle that is used in the experiments to determine the mass of the electron antineutrino in Mainz [20] and Troitsk [21]. The ions that pass the retardation potential will immediately be accelerated to 10 keV by a potential applied to the acceleration electrodes. This is necessary in order that the ions do not continue
524
M. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 521–525
to follow the magnetic field lines but get off them nonadiabatically. Finally the ions are focused with an Einzel lens onto a segmented large area microchannel plate detector. By varying the retardation potential the energy spectrum of the recoil ions can then be determined. 3. Recoil spectrum and physics observables Due to the retardation principle, the recoil energy spectrum that WITCH measures is an integral spectrum. For better understanding, Fig. 3 shows a calculated differential spectrum. It consists of the continuous spectra from bþ -decay and the peaks from electron capture (EC)-decay. For both bþ and EC contributions several ionization states will be present in the spectrum as illustrated in the figure. The physics objectives that can be extracted follow from the features visible in the energy spectrum. From the shape of the bþ spectrum in pure Fermi-decays a search for S-interaction can be performed. Fig. 4 shows the results of a simulation concerning the sensitivity of the experiment in this case. Under reasonable assumptions, 107 events in the differential recoil energy spectrum are achievable. It follows directly that the WITCH experiment has a sensitivity of Da=a ¼ 0:5%ð1rÞ or better. This is at the forefront of what is possible at present.
Fig. 3. The expected spectrum. This is a calculation for 26 Alm with assumed, reasonable branching ratios for EC- to bþ -decay and charge state distributions.
Fig. 4. Precision on a (1r). This is the result of a simulation for 26 Alm . Three curves are shown for different analysis intervals. The x-axis are the events in the differential recoil energy spectrum. Typically, to get one differential event 104 ions need to be trapped.
bþ -decay is always accompanied by EC decay, albeit usually with small branching ratios. Since EC-decay results in two particles in the final state the recoiling daughter ion is monoenergetic. This results in peaks in the differential recoil spectrum. From the position of these EC-peaks the Q-value of the b-decay can be inferred. Under conservative assumptions, a precision of one to several keV should be achievable. This would be nearly as good as the results from ISOLTRAP. Despite the usually small branching ratios of EC-decay the WITCH experiment will be able to detect this decay mode down to branching ratios as small as 103 –104 . The charge state distribution after b-decay can also be extracted from the measurements. Further, it may be possible to determine Fermi to Gamow– Teller mixing ratios in mixed b-decays, b-decay branching ratios to different levels and level energies, while also the measurement of half-lives may be possible. Whether the WITCH experiment will be competitive in any of these areas needs to be seen. Two other interesting topics that need to be explored for the future are whether it will be possible to have polarized ions in the trap or to do intrap b-spectroscopy.
M. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 521–525
4. Status and conclusion The experiment has been approved as ISOLDE test experiment P111 with nine shifts of stable beam and nine shifts of radioactive beam. The experimental set-up is currently under construction at ISOLDE/CERN. As of May 2002 the horizontal beamline is mostly set up. The vertical beamline is under construction in Leuven and the support structures for the vertical beamline and the cryostat with the magnets is in place. The trap electrodes are under construction in Leuven as well. The cryostat with the magnets is expected to be delivered by Oxford Instruments in spring 2002. The remainder of 2002 will see the final assembly of the whole experiment. Already during this final assembly, beam tuning to the traps can be performed. It is planned to have the first experiment with radioactive beam, i.e. to measure the first recoil energy spectrum with this experiment, in 2003. For updates on the status of the experiment check [22]. In conclusion, by using Penning traps and a retardation spectrometer it will be possible to measure the recoil energy spectrum of the daughter ions after b-decay. For the first time it will be possible to perform recoil spectrometry for a wide variety of b-emitters.
References [1] J.D. Jackson, S.B. Treiman, H.W. Wyld, Nucl. Phys. 4 (1957) 206. [2] J.C. Hardy et al., Nucl. Phys. A 509 (1990) 429; A.S. Carnoy et al., Nucl. Phys. A 568 (1994) 265; I.S. Towner, J.C. Hardy, in: P. Herczeg, C.M. Hoffman, H.V. Klapdor-Kleingrothaus (Eds.), Proceedings of the Fifth International WEIN Symposium, Santa Fe, NM, USA, June 1998, World Scientific, Singapore, 1999, p. 338. [3] C.S. Wu, S.A. Moszkowski, Beta Decay, Wiley, 1966.
525
[4] C.H. Johnson et al., Phys. Rev. 132 (1963) 1149. [5] E.G. Adelberger et al., Phys. Rev. Lett. 83 (1999) 1299; A. Garcia, E.G. Adelberger, C. Ortiz, H.E. Swanson, M. Beck, O. Tengblad, M.J.G. Borge, I. Martel, H. Bichsel, ISOLDE Collaboration, Hyp. Int. 129 (2000) 237. [6] J.S. Allen et al., Phys. Rev. 116 (1959) 134. [7] E. Kugler et al., Nucl. Instr. and Meth. B 70 (1992) 41; E. Kugler, Hyp. Int. 129 (2000) 23; B. Jonson, A. Richter, Hyp. Int. 129 (2000) 1. [8] N. Severijns et al., Hyp. Int. 129 (2000) 223. [9] D. Beck et al., Proposal to the ISOLDE scientific committee, CERN/ISC 99-13, ISC/P111, April 1999. [10] D. Beck et al., Letter of Intent to the ISOLDE committee, CERN/ISC 98-21, ISC/I 30, November 1998. [11] M. Beck et al., in: Z. Parsa, W.J. Marciano (Eds.), Proceedings of the 7th Conference on the Intersection of Particle and Nuclear Physics, Quebec City, Canada, AIP Conference Proceedings 549 (2000) 934. [12] D. Beck et al., Nucl. Phys. A 701 (2002) 369c. [13] M. Beck et al., in: K. Cheung, J.F. Gunion, S. Mrenna (Eds.), Proceedings of the 7th International Symposium on Particles, Strings and Cosmology, Granlibakken, Lake Tahoe, CA, December 1999, World Scientific, Singapore, 2000, p. 544. [14] D. Beck et al., Proceedings of the International Workshop on Trapped Particles and Fundamental Physics, Monterey, CA, USA, 31 August–4 September 1998, AIP Conference Proceedings 457 (1999) 172. [15] D. Beck et al., Second Euroconference on Atomic Physics at Accelerators (APAC2000), Cargese, Corsica, France, October 2000. [16] F. Ames et al., Proceedings of the Exotic Nuclei and Atomic Masses, Bellaire, USA, 1998, AIP Conference Proceedings 445 (1998) 927. [17] G. Savard et al., Phys. Lett. A 158 (1991) 147. [18] G. Bollen et al., Nucl. Instr. and Meth. A 368 (1996) 675; D. Beck et al., Nucl. Instr. and Meth. B 126 (1997) 378; H. Reimbault-Hartman et al., Nucl. Instr. and Meth. B 126 (1997) 378; D. Lunney, G. Bollen, Hyp. Int. 129 (2000) 249. [19] J.D. Jackson, Classical Electrodynamics, Wiley, 1962, 419. [20] A. Picard et al., Nucl. Instr. and Meth. B 63 (1992) 345. [21] V.M. Lobashev, P.E. Spivak, Nucl. Instr. and Meth. A 240 (1985) 305. [22] http://www.fys.kuleuven.ac.be/iks/ko/research/witch/default. htm.
WITCH: a recoil spectrometer for b-decay M. Beck a,*, F. Ames b, D. Beck e, G. Bollen d, B. Delaure a, J. Deutsch c, V.V. Golovko a, V.Yu. Kozlov a, I.S. Kraev a, A. Lindroth a, T. Phalet a, W. Quint e, K. Reisinger b, P. Schuurmans a, N. Severijns a, B. Vereecke a, S. Versyck a, The EUROTRAPS Collaboration a
Instituut voor Kern- en Stralingsfysica, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium b Ludwig-Maximilians-Universit€at, Sektion Physik, Schellingstraße 4/IV, D-80799 M€unchen, Germany c Institut de Physique Nucl eaire, UCL, Chemin du Cyclotron 2, B-1348 Louvain-la-Neuve, Belgium d Michigan State University, 103W Cyclotron, East Lansing, MI 48824, USA e GSI-Darmstadt, Planckstr. 1, D-64291 Darmstadt, Germany
Abstract The WITCH experiment will measure the recoil energy spectrum of the daughter ions in b-decay. The main parts of the experiment are two Penning traps and a subsequent retardation spectrometer. The b-decays take place in the ion cloud in the decay trap. Since the ion cloud is in vacuum and due to the cylindrical structure of the trap, the recoiling daughter ions can leave the cloud and the trap without any significant energy loss and can be energy analyzed in the retardation spectrometer. The WITCH experiment is set up foremost to study the electroweak interaction by measuring the b–m angular correlation in nuclear b-decay which can be inferred from the shape of the energy spectrum of the recoil ions. In the beginning the experiment will focus on pure Fermi decays which will allow to search for additional scalar coupling in the weak interaction. Since Penning traps have no restrictions regarding the element to be trapped the most suitable isotope can be picked for this purpose. The WITCH experiment is presently being set up at ISOLDE. Status and perspectives of the experiment will be presented in the following. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 23.40.B; 29.30; 39.10 Keywords: Weak interaction; b-decay; Scalar interaction; Ion trap; Penning trap; Retardation spectrometer
1. Motivation It has been found experimentally that of the five interaction types allowed by Lorentz invariance of *
Corresponding author. Tel.: +32-16-327265; fax: +32-16327985. E-mail address: [email protected] (M. Beck).
the Hamiltonian-vector (V), axialvector (A), scalar (S), tensor (T) and pseudoscalar interaction (PS) – only V- and A-interaction are realized in weak interaction. This is implemented in the very successful standard electroweak model. However, the experimental limits for S- and T-interaction are rather weak, i.e. only of the order 10% (or mboson 300 GeV).
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(02)02125-0
522
M. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 521–525
In order to improve the limits on S- and T-interaction, the WITCH experiment intends to determine the electron-neutrino angular correlation in nuclear b-decay with high precision [1]: xð#bm Þ 1 þ av=c cosð#bm Þð1 Cm=EbÞ
ð1Þ
with a the b–m angular correlation coefficient, #bm the angle between the electron and the neutrino, v=c the velocity, m the restmass and E the total energy of the electron, C ¼ ð1 ða ZÞ2 Þ1=2 , a the fine structure constant and Z the nuclear charge. The Fierz interference term b has experimentally been shown to be small [2]. Using the properties of the c-matrices and the general Hamiltonian [3] for the weak interaction it can be shown that the b-particle and the neutrino emitted in b-decay must have opposite helicities for V-interaction and same helicities for S-interaction. Therefore, in superallowed 0þ –0þ Fermi decays where the spins have to couple to zero the b-particle and the neutrino will be emitted preferentially into the same direction in the case of Vinteraction and into opposite directions in the case of S-interaction. On average this will lead to a large recoil energy for V-interaction and a small recoil energy for S-interaction. For pure Fermi decays the dependence of a on the interaction type is e S j2 j C e 0 j2 þ 2Zam=pImð C eS þ C e0 Þ 2 jC S S a¼ ð2Þ e 0 j2 e S j2 þ j C 2 þ jC S 0
e S ¼ CS and C e 0 ¼ CS where CS and CV are the with C S CV CV coupling constants for S- and V-interaction [3]. In the standard model only V-interaction contributes to the decay rate of superallowed Fermi transitions (0þ –0þ transitions) and therefore a ¼ 1. Any admixture of S to V in a Fermi transition would show itself as a < 1. Thus, by determining the b–m angular correlation coefficient a, it can be determined which interactions contribute. Since the neutrino cannot be measured directly in such an experiment the electron–neutrino angular correlation has to be inferred from other observables such as the shape of the energy spectrum of the electrons or the recoil ions. This is very difficult since the b-emitter usually is embedded in a solid matrix, e.g. an ion catcher, which will lead
to changes in the spectra due to energy losses. The b-particles will be scattered and the recoiling daughter nuclei most times will be stopped already in the source due to their very low energy of usually 1 keV. Consequently, the b–m angular correlation could only be measured in a few exceptional cases [4–6]. In order to avoid that the recoils are stopped in the source the WITCH experiment will utilize a Penning trap to store the radioactive ions. Here the ions will be stored in an ion cloud with only vacuum surrounding them. An open electrode structure of the trap (see Section 2) allows them to also leave the trap without scattering. The experiment will be located at ISOLDE [7] at CERN where a huge variety of nuclides are available with high intensities. The combination of this with a Penning trap will enable this experiment to select the most suited b-emitter that decays in a pure Fermi mode to search for scalar interaction. Additionally, it opens a general way for recoil spectroscopy in b-decay [8–15].
2. Experimental set-up 2.1. Beam transport The ions are produced by ISOLDE. In a first step they get trapped and cooled by REXTRAP [16]. From there they are transmitted in bunches through a horizontal beamline and a 90° bender into a vertical beamline. For test purposes and tuning there also will be an ion source available in the horizontal beamline. Up to the start of the vertical beamline the ions have the full ISOLDE energy of 60 keV. After the 90° bender the ions are deccelerated in several steps. They enter the first Penning trap with an energy of 50 eV. 2.2. Source The ions get trapped in the first Penning trap, the cooler trap (Fig. 1, left hand side). Here they are mass-selectively cooled and centered by buffer gas assisted cooling [17] to approximately room temperature. Then they are ejected through an opening in a differential pumping barrier into the
M. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 521–525
523
Fig. 1. The Penning traps. The cooler trap is to the left, the decay trap to the right. The diameter of the traps is 4 cm and the total length of the trap structure 40 cm. The differential pumping barrier between the traps is not shown.
second Penning trap, the decay trap (Fig. 1, right hand side). Both Penning traps have a cylindrical geometry. They are modified versions of the cooler trap of the ISOLTRAP high-precision mass measurement experiment [18]. The ions are kept in the decay trap for a couple of half-lives, i.e. between 1 and 10 s. During this time most of the ions will decay. The ion cloud in the decay trap constitutes the source for the experiment. The trap itself is at the beginning of the retardation spectrometer. 2.3. Spectrometer The retardation spectrometer is illustrated in Fig. 2. The recoiling daughter ions from b-decay in the measurement trap are emitted isotropically. The ions emitted backward in the direction of the cooler trap are lost. Those emitted forward into the spectrometer spiral from the high field (Bmax 9 T) to the low field (Bmin 0:1 T) region. A retardation potential is applied between these two regions. It probes the component of the ion energy parallel to the axis of the set-up. Due to careful shaping of the magnetic and electric fields and the principle of adiabatic invariance of the magnetic flux contained in the ion motion [19], a fraction of 1 Bmin =Bmax 98:9%
Fig. 2. Experimental set-up. The recoil ions from the b-decays in the trap will spiral into the spectrometer. There most of their radial energy is converted into axial energy and probed by the electrostatic retardation potential. The ions that pass get postaccelerated and are finally focused onto the detector.
ð3Þ
of the radial energy of the ions is converted into axial energy while the ions spiral from the high field to the low field region. Thus 98.9–100% of the ion energy, depending on the emission direction after the b-decay, is in the axial ion motion when they reach the low field region. Only ions with an axial energy in the low field region which exceeds the retardation potential can pass and will be
counted. This is the same fundamental principle that is used in the experiments to determine the mass of the electron antineutrino in Mainz [20] and Troitsk [21]. The ions that pass the retardation potential will immediately be accelerated to 10 keV by a potential applied to the acceleration electrodes. This is necessary in order that the ions do not continue
524
M. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 521–525
to follow the magnetic field lines but get off them nonadiabatically. Finally the ions are focused with an Einzel lens onto a segmented large area microchannel plate detector. By varying the retardation potential the energy spectrum of the recoil ions can then be determined. 3. Recoil spectrum and physics observables Due to the retardation principle, the recoil energy spectrum that WITCH measures is an integral spectrum. For better understanding, Fig. 3 shows a calculated differential spectrum. It consists of the continuous spectra from bþ -decay and the peaks from electron capture (EC)-decay. For both bþ and EC contributions several ionization states will be present in the spectrum as illustrated in the figure. The physics objectives that can be extracted follow from the features visible in the energy spectrum. From the shape of the bþ spectrum in pure Fermi-decays a search for S-interaction can be performed. Fig. 4 shows the results of a simulation concerning the sensitivity of the experiment in this case. Under reasonable assumptions, 107 events in the differential recoil energy spectrum are achievable. It follows directly that the WITCH experiment has a sensitivity of Da=a ¼ 0:5%ð1rÞ or better. This is at the forefront of what is possible at present.
Fig. 3. The expected spectrum. This is a calculation for 26 Alm with assumed, reasonable branching ratios for EC- to bþ -decay and charge state distributions.
Fig. 4. Precision on a (1r). This is the result of a simulation for 26 Alm . Three curves are shown for different analysis intervals. The x-axis are the events in the differential recoil energy spectrum. Typically, to get one differential event 104 ions need to be trapped.
bþ -decay is always accompanied by EC decay, albeit usually with small branching ratios. Since EC-decay results in two particles in the final state the recoiling daughter ion is monoenergetic. This results in peaks in the differential recoil spectrum. From the position of these EC-peaks the Q-value of the b-decay can be inferred. Under conservative assumptions, a precision of one to several keV should be achievable. This would be nearly as good as the results from ISOLTRAP. Despite the usually small branching ratios of EC-decay the WITCH experiment will be able to detect this decay mode down to branching ratios as small as 103 –104 . The charge state distribution after b-decay can also be extracted from the measurements. Further, it may be possible to determine Fermi to Gamow– Teller mixing ratios in mixed b-decays, b-decay branching ratios to different levels and level energies, while also the measurement of half-lives may be possible. Whether the WITCH experiment will be competitive in any of these areas needs to be seen. Two other interesting topics that need to be explored for the future are whether it will be possible to have polarized ions in the trap or to do intrap b-spectroscopy.
M. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 521–525
4. Status and conclusion The experiment has been approved as ISOLDE test experiment P111 with nine shifts of stable beam and nine shifts of radioactive beam. The experimental set-up is currently under construction at ISOLDE/CERN. As of May 2002 the horizontal beamline is mostly set up. The vertical beamline is under construction in Leuven and the support structures for the vertical beamline and the cryostat with the magnets is in place. The trap electrodes are under construction in Leuven as well. The cryostat with the magnets is expected to be delivered by Oxford Instruments in spring 2002. The remainder of 2002 will see the final assembly of the whole experiment. Already during this final assembly, beam tuning to the traps can be performed. It is planned to have the first experiment with radioactive beam, i.e. to measure the first recoil energy spectrum with this experiment, in 2003. For updates on the status of the experiment check [22]. In conclusion, by using Penning traps and a retardation spectrometer it will be possible to measure the recoil energy spectrum of the daughter ions after b-decay. For the first time it will be possible to perform recoil spectrometry for a wide variety of b-emitters.
References [1] J.D. Jackson, S.B. Treiman, H.W. Wyld, Nucl. Phys. 4 (1957) 206. [2] J.C. Hardy et al., Nucl. Phys. A 509 (1990) 429; A.S. Carnoy et al., Nucl. Phys. A 568 (1994) 265; I.S. Towner, J.C. Hardy, in: P. Herczeg, C.M. Hoffman, H.V. Klapdor-Kleingrothaus (Eds.), Proceedings of the Fifth International WEIN Symposium, Santa Fe, NM, USA, June 1998, World Scientific, Singapore, 1999, p. 338. [3] C.S. Wu, S.A. Moszkowski, Beta Decay, Wiley, 1966.
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