- Email: [email protected]
The MIT-Bates large acceptance spectrometer toroid
Nuclear Physics A721 (2003) 1075-1078~
ELSEVIER
www.elsevier.com/locate/npe
The MIT-Bates
Large Acceptance Spectrometer Toroid
R. Alarcon” ‘Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA The Bates Large Acceptance Spectrometer Toroid (BLAST) is a detector designed to study the spin-dependent electromagnetic response of few-body nuclei at momentum transfers up to 1 (GeV/c)’ at the MIT-Bates Linear Accelerator Center’s South Hall Ring (SHR). The BLAST detector consists of an eight-sector copper coil array producing a toroidal magnetic field, instrumented with two opposing wedge-shaped sectors of wire chambers, scintillation detectors, Cerenkov counters, neutron detectors, a lead-glass forward calorimeter, and recoil detectors. A status of the project is presented as well as highlights of the scientific program. 1. Introduction
The Bates Large Acceptance Spectrometer Toroid (BLAST) is a detector designed to study in a comprehensive and precise way the spin dependent electromagnetic response of few-body nuclei. The BLAST scientific program is focussed on the study of these systems in terms of nucleon structure, the ground state few body structure built from the nucleon-nucleon interaction and the nature of the interaction of the virtual photon for Q2 5 1 (GeV/c)2. T o accomplish its scientific goals, BLAST utilizes the latest technology available in the form of polarized electron scattering from pure, polarized internal gas targets. The ability with BLAST to carry out multiparticle detection over a large solid angle from polarized internal targets will provide an unprecedented and unique opportunity to study simultaneously the spin structure of the few-body nuclear ground states, the reaction mechanism, and nucleon form factors. 2. The
BLAST
Project
The BLAST detector consists of an eight-sector copper coil array producing a toroidal magnetic field, instrumented with two opposing wedge-shaped sectors of wire chambers, scintillation detectors, Cerenkov counters, neutron detectors, a lead glass calorimeter and recoil detectors [l]. The open geometry maximizes acceptance while allowing good momentum and angular resolution, and with a luminosity capability that is matched to the densities of the polarized internal targets. Clear upgrade possibilities exist so that the detector can evolve to match developing physics priorities. Fig. 1 shows a top view of the BLAST spectrometer showing the magnetic coils, the two sectors to be initially instrumented and the different detector elements. The tracking drift 03759474/03/$ - see front matter 0 2003 Elsevier Science B.V doi:10.1016/S0375-9474(03)01288-O
All rights reserved.
1076~
R. Alarcon/Nuclear PhysicsA721 (2003) 1075-I 078~
Figure 1. Top view of the BLAST spectrometer showing the magnetic coils, the two sectors to be initially instrumented and the different detector elements.
chambers are located between the coils followed by the Cerenkov detectors, the timing scintillators and the neutron detector array. Updated information can be found at the Bates web site (http://blast.lns.mit.edu/). Construction of BLAST, by an international collaboration of physicists from 11 institutions, was finalized during the fall of 2001. This was followed by an installation phase of the complete system at the Bates’s SHR. Commissioning of the BLAST detector with polarized beam and a polarized hydrogen/deuterium target started in June 2002. The scientific program is scheduled to start data taking during the spring of 2003. 3. Scientific
Program:
Highlights
BLAST will provide precise information on nucleon form factors for momentum transfers up to about 1 (GeV/c)*. In particular, BLAST will provide data on the neutron magnetic and electric form factors with both deuteron and 3He targets. These are fundamental quantities that are essential to any description of electromagnetic scattering from nuclei, and crucial input to the extraction of strange form factors from parity-violation experiments [2]. Also, because of the significantly larger binding energy of the neutron in 3He compared to that in deuterium, it is possible that the neutron charge distribution in 3He may be modified from its free value. BLAST has the necessary precision to probe for such an effect. Polarization data on the charge form factor of the neutron GEM [3-61 are summarized in Fig. 2 together with the projected uncertainties from BLAST. At Bates BLAST will measure Gsn(Q2) from 0.1 to 0.8 (GeV/c)2 with high precision both on deuterium and 3He within the same apparatus. With overall uncertainties < 5 % it should be possible to carry out a high precision comparison of GEM in the deuteron and 3He. In addition,
R. Alarcon/Nuclear PhysicsA721 (2003) 1075c-I 078~
1077c
BLAST will address the reaction mechanism effects by the simultaneous measurement of several reaction channels over a broad kinematics range and for various orientations of the target polarization direction. BLAST will use its symmetric capability to perform a simultaneous determination of G M”(&*) at identical kinematics to GEM, both in the case of deuterium and 3He.
0 0 / 0
/
““1 0.2
I
BLAST BLAST
‘H (projected) --He (projected)
I
I
I
0.4 d
[
(Ge
” 0.6
I
” 0.8
Wcfl
Figure 2. Values for GEn as extracted from spin-dependent electron scattering. Also shown are the BLAST projections from polarized deuterium and 3He. The curves correspond to various theoretical projections.
BLAST will carry out a precise measurement of the spin-dependent momentum distribution in few-body nuclei in order to understand the spin structure of few-body systems in terms of the successful theoretical framework which has been developed primarily for unpolarized scattering. Effects such as final-state interactions, meson exchange currents, and the off-shell nature of the bound nucleon can be studied over the broad kinematics range provided by the BLAST detector. In addition, the spin-dependent momentum distributions are used as input for calculations of spin-dependent scattering in the deepinelastic region where polarized deuterium and 3He are used to determine the neutron spin structure at the quark level. With a tensor polarized deuterium target BLAST will provide precise data from elastic e-d scattering (T2a), particularly in the region of the first minimum of the charge form factor of the deuteron. In addition, data will be obtained in the same experiment for the exclusive scattering channels. BLAST will carry out measurements of spin-dependent charged-pion electroproduction on few-body systems from threshold to beyond the A-resonance. Such studies are important for understanding the role of the nucleon resonance in few-body systems. BLAST allows reconstruction of the resonance from its r-nucleon decay channel. In this way, for example, the presence of pre-existing A-components in the 3He ground state can be
1078c
R. Alarcon/Nuclear Physics A721 (2003) 1075c-1078~
studied. With a polarized proton target and a polarized electron beam, BLAST will study the N+a transition to isolate components beyond the dominant Ml transition and will provide additional precise data on the proton form factors. In addition, at lower energies, an experiment is planned to extract the charge radius of the proton [7]. There is also a rich program of physics to be pursued with unpolarized targets and light nuclei. Examples are the study of multinucleon processes which are well suited for a large acceptance detector like BLAST, measurements of the neutron momentum distributions in nuclei, and the detection of heavy recoils in processes of astrophysical significance. 4. Summary
The BLAST detector, at the MIT-Bates Linear Accelerator Center, offers a new and unique capability for intermediate nuclear physics. This capability consists of the combination of highly polarized electron beams, polarized internal gas targets of hydrogen, deuterium and 3He, and large acceptance magnetic detector. Clear upgrade possibilities exist so that the BLAST detector can evolve to match developing physics priorities. Construction of the BLAST detector started at the end of 1997 and it was completed by the fall of 2001. Following an installation phase, commissioning of the detector with polarized beam and a polarized hydrogen/deuterium target began during June of 2002. The initiation of the scientific program is scheduled to start in the spring of 2003. The BLAST scientific program has the following main thrusts: precise measurements of the form factors of the proton, neutron, and deuteron for momentum transfers up to 1.0 (GeV/c)‘; a comprehensive program of spin-dependent electron scattering from fewbody nuclei; a program of multi-nucleon physics; and measurements of cross sections with astrophysical significance [8]. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
BLAST Technical Design Report, Bates Linear Act. Center, (1997). K. Aniol et al., Phys. Rev. Lett. 82, 1096 (1999). M. Meyerhoff et al., Phys. Lett B327, 201 (1994). M. Ostrick et al, Phys. Rev. Lett. 83, 276 (1999). I. Passchier et al., Phys. Rev. Lett. 82, 4988 (1999). D. Rohe et al., Phys. Rev. Lett. 83, 4257 (1999). Bates Proposal 00-02, H. Gao and J. Calarco, spokespersons. Bates proposal 00-01, G. Tsentalovich, spokesperson.
ELSEVIER
www.elsevier.com/locate/npe
The MIT-Bates
Large Acceptance Spectrometer Toroid
R. Alarcon” ‘Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA The Bates Large Acceptance Spectrometer Toroid (BLAST) is a detector designed to study the spin-dependent electromagnetic response of few-body nuclei at momentum transfers up to 1 (GeV/c)’ at the MIT-Bates Linear Accelerator Center’s South Hall Ring (SHR). The BLAST detector consists of an eight-sector copper coil array producing a toroidal magnetic field, instrumented with two opposing wedge-shaped sectors of wire chambers, scintillation detectors, Cerenkov counters, neutron detectors, a lead-glass forward calorimeter, and recoil detectors. A status of the project is presented as well as highlights of the scientific program. 1. Introduction
The Bates Large Acceptance Spectrometer Toroid (BLAST) is a detector designed to study in a comprehensive and precise way the spin dependent electromagnetic response of few-body nuclei. The BLAST scientific program is focussed on the study of these systems in terms of nucleon structure, the ground state few body structure built from the nucleon-nucleon interaction and the nature of the interaction of the virtual photon for Q2 5 1 (GeV/c)2. T o accomplish its scientific goals, BLAST utilizes the latest technology available in the form of polarized electron scattering from pure, polarized internal gas targets. The ability with BLAST to carry out multiparticle detection over a large solid angle from polarized internal targets will provide an unprecedented and unique opportunity to study simultaneously the spin structure of the few-body nuclear ground states, the reaction mechanism, and nucleon form factors. 2. The
BLAST
Project
The BLAST detector consists of an eight-sector copper coil array producing a toroidal magnetic field, instrumented with two opposing wedge-shaped sectors of wire chambers, scintillation detectors, Cerenkov counters, neutron detectors, a lead glass calorimeter and recoil detectors [l]. The open geometry maximizes acceptance while allowing good momentum and angular resolution, and with a luminosity capability that is matched to the densities of the polarized internal targets. Clear upgrade possibilities exist so that the detector can evolve to match developing physics priorities. Fig. 1 shows a top view of the BLAST spectrometer showing the magnetic coils, the two sectors to be initially instrumented and the different detector elements. The tracking drift 03759474/03/$ - see front matter 0 2003 Elsevier Science B.V doi:10.1016/S0375-9474(03)01288-O
All rights reserved.
1076~
R. Alarcon/Nuclear PhysicsA721 (2003) 1075-I 078~
Figure 1. Top view of the BLAST spectrometer showing the magnetic coils, the two sectors to be initially instrumented and the different detector elements.
chambers are located between the coils followed by the Cerenkov detectors, the timing scintillators and the neutron detector array. Updated information can be found at the Bates web site (http://blast.lns.mit.edu/). Construction of BLAST, by an international collaboration of physicists from 11 institutions, was finalized during the fall of 2001. This was followed by an installation phase of the complete system at the Bates’s SHR. Commissioning of the BLAST detector with polarized beam and a polarized hydrogen/deuterium target started in June 2002. The scientific program is scheduled to start data taking during the spring of 2003. 3. Scientific
Program:
Highlights
BLAST will provide precise information on nucleon form factors for momentum transfers up to about 1 (GeV/c)*. In particular, BLAST will provide data on the neutron magnetic and electric form factors with both deuteron and 3He targets. These are fundamental quantities that are essential to any description of electromagnetic scattering from nuclei, and crucial input to the extraction of strange form factors from parity-violation experiments [2]. Also, because of the significantly larger binding energy of the neutron in 3He compared to that in deuterium, it is possible that the neutron charge distribution in 3He may be modified from its free value. BLAST has the necessary precision to probe for such an effect. Polarization data on the charge form factor of the neutron GEM [3-61 are summarized in Fig. 2 together with the projected uncertainties from BLAST. At Bates BLAST will measure Gsn(Q2) from 0.1 to 0.8 (GeV/c)2 with high precision both on deuterium and 3He within the same apparatus. With overall uncertainties < 5 % it should be possible to carry out a high precision comparison of GEM in the deuteron and 3He. In addition,
R. Alarcon/Nuclear PhysicsA721 (2003) 1075c-I 078~
1077c
BLAST will address the reaction mechanism effects by the simultaneous measurement of several reaction channels over a broad kinematics range and for various orientations of the target polarization direction. BLAST will use its symmetric capability to perform a simultaneous determination of G M”(&*) at identical kinematics to GEM, both in the case of deuterium and 3He.
0 0 / 0
/
““1 0.2
I
BLAST BLAST
‘H (projected) --He (projected)
I
I
I
0.4 d
[
(Ge
” 0.6
I
” 0.8
Wcfl
Figure 2. Values for GEn as extracted from spin-dependent electron scattering. Also shown are the BLAST projections from polarized deuterium and 3He. The curves correspond to various theoretical projections.
BLAST will carry out a precise measurement of the spin-dependent momentum distribution in few-body nuclei in order to understand the spin structure of few-body systems in terms of the successful theoretical framework which has been developed primarily for unpolarized scattering. Effects such as final-state interactions, meson exchange currents, and the off-shell nature of the bound nucleon can be studied over the broad kinematics range provided by the BLAST detector. In addition, the spin-dependent momentum distributions are used as input for calculations of spin-dependent scattering in the deepinelastic region where polarized deuterium and 3He are used to determine the neutron spin structure at the quark level. With a tensor polarized deuterium target BLAST will provide precise data from elastic e-d scattering (T2a), particularly in the region of the first minimum of the charge form factor of the deuteron. In addition, data will be obtained in the same experiment for the exclusive scattering channels. BLAST will carry out measurements of spin-dependent charged-pion electroproduction on few-body systems from threshold to beyond the A-resonance. Such studies are important for understanding the role of the nucleon resonance in few-body systems. BLAST allows reconstruction of the resonance from its r-nucleon decay channel. In this way, for example, the presence of pre-existing A-components in the 3He ground state can be
1078c
R. Alarcon/Nuclear Physics A721 (2003) 1075c-1078~
studied. With a polarized proton target and a polarized electron beam, BLAST will study the N+a transition to isolate components beyond the dominant Ml transition and will provide additional precise data on the proton form factors. In addition, at lower energies, an experiment is planned to extract the charge radius of the proton [7]. There is also a rich program of physics to be pursued with unpolarized targets and light nuclei. Examples are the study of multinucleon processes which are well suited for a large acceptance detector like BLAST, measurements of the neutron momentum distributions in nuclei, and the detection of heavy recoils in processes of astrophysical significance. 4. Summary
The BLAST detector, at the MIT-Bates Linear Accelerator Center, offers a new and unique capability for intermediate nuclear physics. This capability consists of the combination of highly polarized electron beams, polarized internal gas targets of hydrogen, deuterium and 3He, and large acceptance magnetic detector. Clear upgrade possibilities exist so that the BLAST detector can evolve to match developing physics priorities. Construction of the BLAST detector started at the end of 1997 and it was completed by the fall of 2001. Following an installation phase, commissioning of the detector with polarized beam and a polarized hydrogen/deuterium target began during June of 2002. The initiation of the scientific program is scheduled to start in the spring of 2003. The BLAST scientific program has the following main thrusts: precise measurements of the form factors of the proton, neutron, and deuteron for momentum transfers up to 1.0 (GeV/c)‘; a comprehensive program of spin-dependent electron scattering from fewbody nuclei; a program of multi-nucleon physics; and measurements of cross sections with astrophysical significance [8]. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
BLAST Technical Design Report, Bates Linear Act. Center, (1997). K. Aniol et al., Phys. Rev. Lett. 82, 1096 (1999). M. Meyerhoff et al., Phys. Lett B327, 201 (1994). M. Ostrick et al, Phys. Rev. Lett. 83, 276 (1999). I. Passchier et al., Phys. Rev. Lett. 82, 4988 (1999). D. Rohe et al., Phys. Rev. Lett. 83, 4257 (1999). Bates Proposal 00-02, H. Gao and J. Calarco, spokespersons. Bates proposal 00-01, G. Tsentalovich, spokesperson.