The superconducting toroidal spectrometer for hypernuclear studies at KEK-PS

Nuclear Instruments and Methods in Physics Research A 416 (1998) 253—262

The superconducting toroidal spectrometer for hypernuclear studies at KEK-PS A. Kawachi!,*,1, H. Tamura!,2, J. Imazato", M. Aoki#, J. Chiba", Y. Fujita#, R.S. Hayano!, T. Ishikawa!, K. Kubota!, J.M. Lee$,3, T. Miyamoto!, H. Outa#, I.S. Park%, Y. Shimizu!,4, T. Yamazaki#,5 ! Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan " High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan # Institute for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188-8501, Japan6 $ Yonsei University, Seoul, South Korea % Korea University, Seoul, South Korea Received 25 May 1998; accepted 27 May 1998

Abstract A spectrometer using a toroidal magnet with 12 superconducting coils was constructed at the north experimental hall of the KEK 12 GeV proton synchrotron. The spectrometer comprised 12 individual sectors surrounding the target point and served to measure p~ momentum for the purpose of hypernuclear studies via the (stopped K~, p~) reaction. A large solid angle of more than 6%]4p str was achieved in a wide momentum range between 180 and 350 MeV/c at the maximum 1.8 T magnetic excitation, and in a range between 70 and 150 MeV/c at 0.65 T. A momentum resolution of 1.5 MeV/c FWHM was measured at 205 MeV/c, and a resolution around 3 MeV/c FWHM at 130—200 MeV/c was obtained including energy loss fluctuation in the experimental thick targets. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 29.30.Aj; 29.40.Gx Keywords: Magnetic spectrometer; Superconducting magnet; Tracking chambers; Hypernuclei

* Corresponding author. Tel.: #81 424 69 9597; fax: #81 424 62 3096; e-mail: [email protected]. 1 Present address: Institute for Cosmic Ray Research, Tokyo 188-8502, Japan. 2 Present address: Physics Department, Tohoku University, Miyagi 980-8578, Japan. 3 Present address: Korea Research Institute of Standards and Science, Yuseong-Gu, Taejon 305-346, Korea.

4 Present address: Physics Department, Osaka University, Osaka 560-0043, Japan. 5 Present address: Japan Society for the Promotion of Science, Tokyo 102-0083, Japan. 6 Present organization: High Energy Accelerator Research Organization.

0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 6 9 9 - 8

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1. Introduction A series of hypernuclear experiments using the (stopped K~, p~) reaction was performed at the K5 beam line at the 12 GeV proton synchrotron (PS) of High Energy Accelerator Research Organization (KEK), employing a magnetic spectrometer with a superconducting toroidal magnet. Hereafter the spectrometer is called the Toroidal Spectrometer. The Toroidal Spectrometer was constructed for spectroscopic studies of directly and indirectly produced R- and K-hypernuclei from stopped K~ absorption [1], by measuring momenta of pions emitted in formations and decays of hypernuclei. The requirements for the design were, (i) a large solid angle (&10%]4p str) to supplement the low production rate of hypernuclei, (ii) a wide momentum acceptance, and (iii) a momentum resolution better than a few MeV/c FWHM to discriminate different peaks corresponding to various hypernuclei. The spectrometer had a momentum coverage of 70—350 MeV/c, at the magnetic field of 0.65—1.8 T. The momentum range covered pions from K-hypernuclear formation (&260 MeV/c), from R-hypernuclear formation (&170 MeV/c), and weak decay of K-hypernuclei (&100 MeV/c). A wide acceptance between 100 and 250 MeV/c was suited for coincidental measurements of pions from hypernuclear formations and decays.

Fig. 1. Schematic drawings of the toroidal spectrometer magnet, a front view (left) and a cross-section (right). Twelve magnet gaps and iron poles (hatched) were arranged in a circular shape.

In recent experiments, we focused on the study of production mechanisms of K-hyperfragments [2—4]. Light K-hypernuclei were produced as fragments from K~ absorption at rest in the target nuclei, and these hyperfragments were identified by p~ momenta in their mesonic decay. Measurements of hypernuclear gamma-transition by additional NaI counters were also carried out. By selecting twopion events, we measured the spectrum of the pions emitted from the production of a hyperon (K~ N PKp~, Rp~) in coincidence with the pions emitted from weak decays of K-hyperfragments, and thus the intermediate state which contributes to the hyperfragment production was studied.

2. Setup overview The Toroidal Spectrometer was based on the toroidal magnet of 12 magnet gaps surrounding the reaction point [5]. A schematic view of the magnet is shown in Fig. 1. Each magnet gap was equipped with an individual tracking and particle-identification system. The cross-section of the experimental setup is shown in Fig. 2. With this circular configuration of 12 gaps, a large solid angle was obtained for isotropically emitted particles from the target, while the background by the beam particles moving in the forward direction can be avoided. As shown in Fig. 2, incoming kaons were defined by plastic scintillation counters and Lucite Cherenkov counters placed on the beam axis, and the kaon trajectories were measured with two MWPCs. After a graphite degrader, a typical K~ range in the experimental target was about 10 g/cm2 and the path length for outgoing pions was about 3 g/cm2. A plastic counter was set just downstream of the target to generate a veto signal for “stopped” K~. Momenta of charged particles emitted from the target were measured in any of the 12 magnet gaps with its set of 4 tracking chambers. The effect of multiple scattering was a dominant factor in the momentum resolution, therefore the number of tracking chambers and their thickness must be minimized. We placed helium-filled 25 lm Mylar bags between the chambers as well as in the pole gap. For a typical trajectory, the root-summation

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Table 1 Performances of K5 beam line Performance of K5 beam line (without the degrader) Beam momentum Momentum bite Yield at 550 MeV/c

K` K~

550 MeV/c at maximum $4% 50]103(1]1012 protons) 10]103(1]1012 protons)

Typical condition of the experiments (with the degrader) Yield at 550 MeV/c

K` K~

12 20]103(1]1012 protons) 3]103(1]1012 protons)

p/K ratio at the Cherenkov counter K` 4 K~ 20 Stopped K~ in the target 2]103(1]1012 protons) Note: The design values were taken from Ref. [6].

Fig. 2. A schematic cross-section of the experimental setup; the Toroidal Spectrometer and the beam-line counters. A typical p~ trajectory is shown.

square of the amount of material along the path was &6]10~4 in the unit of radiation length. The incident kaon track together with the reconstructed outgoing particle trajectory, determined the reaction point in the target needed for the correction of energy loss in the target. Just downstream of the tracking chambers, there were two plastic counters for each gap, which gave time-of-flight information as well as the trigger signals.

3. K5 beam line Proton beams in the 12 GeV-PS were extracted from the EP1 beam line and made to hit a platinum production target 40 mm thick. The typical proton intensity was 2.5]1012 per pulse (0.25 Hz, 1.5 s duration) on the K5 production target. Produced secondary particles were momentum analyzed by

a bending magnet, and kaons were separated with an electrostatic mass separator and a mass slit [6]. In the present experiments, the K~ beam momentum was chosen to be 550 MeV/c, which was the maximum momentum of the K5 beam line. The momentum was optimized considering the decay loss in the beam line and the reaction loss in the degrader. The p~/K~ ratio in the experiments was about 20, and the stopped K~ yield in the target was about 5000 per pulse for a typical primary beam intensity of 2.5]1012 per pulse. The characteristics of the K5 beam line are summarized in Table 1, together with the typical running conditions during the experiments.

4. Toroidal magnet The magnet had a circular configuration of 12 magnet gaps with superconducting coils and iron poles. Each magnet gap had a uniform gap size (20 cm) with a rectangular pole face (82]76 cm2, in the radial and beam-axial directions). The number of sectors (12) was determined by optimizing the solid angle and the field strength. To realize a flat and wide momentum acceptance, we employed an optics having a quasi-focal plane at the gap exit so that a wide momentum range can be covered with relatively small detectors. By shifting a source point

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A. Kawachi et al. /Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 253—262 Table 2 Parameters of the toroidal magnet Total weight Number of sectors Pole gap Pole face Conductor Maximum field Maximum field on wire Ampere-turns per coil Operation current Stored energy

38 tons 12 20 cm 82]76 cm2 NbTi-Cu 1.83 T 2.5 T 369 kA 1550 A 2.2 MJ

Note: The design values were taken from Ref. [5].

Fig. 3. Median field distribution calculated for typical field settings, magnet excitations of 0.65, 1.1 and 1.8 T.

downstream from the magnet center as indicated in Fig. 2, we obtained some vertical focusing due to the oblique entrance. The field was a superposition of the dipole field from the iron core and the toroidal field from the coils (Fig. 3). At the excitation of 1.8 T, the average field integral was &1.5 T ) m. The radial dependence of the field of an air core magnet would be disadvantageous in bending power for high-momenta particles, and this results in narrower momentum acceptance. The field map of the magnet was calculated by a three-dimensional field code TOSCA, on a mesh of 2]5]5 cm3 (finer mesh in the azimuthal coordinate). The main feature of calculated field map was checked by the field measurements using Hall probes. In the main field setting (0.65 T excitation) of the present experiments where the iron saturation effect is small, gap-to-gap difference of the field distribution was caused mainly by nonequality of gaps rather than coil misalignment. The gap-size differences were not more than p&70 lm, and thus the deviation of real field distribution from the calculated one was considered to be negligible. The absolute field strengths were monitored with an NMR probe at the beginning and the end of each running cycle (typically 10 days). Drifts of the field during one running cycle was less than 0.02%. The parameters of the magnet are listed in Table 2.

The windings of NbTi—Cu monolith wire were indirectly cooled by two-phase helium. The operation current was 1550 A at the maximum excitation of 1.8 T and the stored energy was 2.2 MJ [5].

5. Detectors 5.1. Beam-line detectors The arrangement of the beam-line counters is shown in the central part of Fig. 2. Sets of plastic counters, B1 (10 mm thick), B2 (20 mm thick), and B3 (3 mm thick) were located in the beam. They defined the beam for the trigger, and the most up-stream counter B1 gave the beam timing for TDC modules to start. The most down-stream B3 counter was used as the TOF-start counter in offline analysis. The intrinsic time resolution of the B3 counter was p&80 ps. Three Lucite Cherenkov counters of threshold type placed downstream of the degrader were used to reject pions. The total inefficiency of the Cherenkov counters for pions were measured to be about 1%, and the pion contamination at the target point after pion rejection by these counters was &20% of the kaons. In the off-line analysis, the number of p~-incident events was further reduced to be less than 1% of the K~-incident events. Between the two beam counters, B2 and B3, two MWPCs (BC1 and BC2) were placed in order to measure the K~ track and to reconstruct the reaction vertex together with the p~ track.The BC1 and BC2 were anode readout MWPCs with

A. Kawachi et al. /Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 253—262 Table 3 Design parameters of the tracking chambers

C1 C2 C3 C4

Readout X/½

Area (mm)

cathode/anode (charge division) anode (charge division)/drift time cathode (charge division)/drift time cathode (charge division)/drift time

154]220

Cell size Thickness (mm) (¸ ) R 7]10~4

400]140

37.5

1.7]10~4

985]304

16.0

2.2]10~4

1305]400

16.0

2.2]10~4

1.0 mm wire spacing and a half gap of 5.0 mm, each having two anode planes (X, ½). We required a single hit for each of the planes to determine the K~ track unambiguously. The single-hit efficiency of each anode plane was 83—90%. 5.2. Tracking chambers We constructed and installed 12 sets of 4 thin tracking chambers, named C1, C2 at the entrance and C3, C4 at the exit of each magnet gap. The multiple scattering effect was calculated to give a major contribution to the momentum resolution, when the position resolution of each chamber is less than 1 mm FWHM for the momentum-deciding coordinate. Therefore, we required about 1 mm FWHM resolution to all of the four chambers. Parameters and resolutions of these tracking chambers are summarized in Table 3. The C1 chamber was a hexagonal-prism MWPC consisting of six planar chambers. The C2, C3, and C4 chambers (one chamber for each gap, respectively) were two-dimensional readout drift chambers using a charge division method to obtain the hit position in the momentum-deciding (capital X) coordinate, and using drift time for the ½-coordinate. We employed different types of charge division methods for these four chambers (Table 3). This method reduced the total number of readout channels as compared with conventional readout method, hence increased the acceptable event rate in the data-acquisition system. Since the present spectrometer required sets of the tracking chambers, reducing cost of the readout electronics was an-

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other important point in design. In the charge division method, however, we must calibrate the relationship between the absolute hit positions and the corresponding “charge ratio”. For the calibration, we used a collimated 55Fe source (5.9 keV X-ray source) precisely located above the anode wire and measured the charge ratio moving the source along the anode wire. We also needed to have a good knowledge of the linearity and the total gain of the readout electronics. The drift chambers (C2, C3, and C4) were operated with a gas multiplication factor of more than 105 to obtain good signal-to-noise ratio. The gas mixture of Argon : Isobutane : Methylal"57 : 40 : 3 was used. For MWPCs (BC1, BC2, and C1), the gas mixture was Argon : Isobutane : Freon : Methylal"70.5 : 25 : 0.5 : 4. The gas was supplied at atmospheric pressure. The position resolutions of the X-coordinates of the C1, C2, and C3 chambers were checked in a test experiment with pion beams which were traced by cathode readout MWPCs with 0.2 mm FWHM resolution. The resolutions were measured to be 0.6, 0.8 and 1.3 mm (FWHM) for C1, C2, and C3, respectively. In the present experiments, the X-hit positions were obtained by off-line analyses with an accuracy of 0.7 mm FWHM for C1 and 1.5 mm FWHM for C2, C3, and C4, all over the effective areas. The position resolutions for the ½-coordinates were obtained to be 2.0 mm FWHM for C1 and 1.0 mm FWHM for C2, C3, and C4. 5.2.1. C1 Fig. 4 shows a schematic view of C1 and the structure of one MWPC plane. The ½-coordinate was obtained by charge division on anode charges. The anode wires of 2 mm spacing spanned in the X-direction were connected together with resistors, and the charges divided by the resistor chain (76 mm) were read out at both ends of the chain. The X-coordinate was obtained from the induced charge distribution on cathode pads (10 mm wide), which were made of copper foils printed on 50 lm thick Kapton films. 5.2.2. C2 Fig. 5 schematically shows the structure of the C2 chamber. In C2, the X-coordinates were

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Fig. 4. A schematic view of the C1 chamber consisted of six planar MWPCs. The structure of one plane is also drawn.

Fig. 5. A schematic view of the C2 chamber. ADC information of the anode readout signals were used to obtain the X-hit positions by dividing the charges from both the ends of the anode, and TDC information gave the ½-hit positions by drift time.

obtained by a conventional charge division method by reading out the charges from both ends of a resistive anode wire. The anode wire was 10 lm/ Ni—Cr and the total resistance of the wire length of 500 mm was 6 k). The time constant of the anode charge dispersion was 24 ns. Each signal was processed by a charge pre-amplifier, a shaping amplifier (the shaping time was 0.5 ls), and a peak-hold ADC. A typical counting rate was a few kHz per wire. The ½coordinates were obtained from drift time. The maximum drift length (half size of a cell) was 37.5 mm, and the typical drift velocity was 55 lm/ns. Field-shaping wires were used to obtain a uniform electric field.

5.2.3. C3 and C4 The C3 and C4 chambers were of identical design and were different only in size. A wide momentum acceptance required an effective length in X of more than one meter for the C3 and C4 chambers. The conventional charge division method was not suitable for these chambers; high-resistance anode wire was needed for the required resolution, while high-resistance long wires would be fragile and would not allow operations in our high counting rate condition. We thus adopted a new type of charge division, “cathode charge division” [7]. Cathode wires, with 2.5 mm spacing orthogonal to the anode wires (Fig. 6), were connected together at one end to resistance chains. Instead of reading out the charges from the ends of each anode wire, we read out the induced charge on the cathode common to all anode wires. We divided the resistor chain (12 k)/m) into a few partitions and put a readout tap at the dividing points. Each partition had a length of about 500 mm (the time constant of the charge dispersion was about 200 ns), and the signals from the readout taps were processed with an electronics system similar to the C2 anode readout system. For the operation under a counting rate of about 10 kHz per plane, the shaping time for C3 and C4 was set as 2 ls where some nonlinearity was introduced due to insufficient charge collection. This nonlinearity was corrected by further calibrations. The ½-coordinates were obtained from drift time. The half length of a drift cell was 16 mm, and

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corrections in off-line analysis. The intrinsic time resolution of the counters was measured in a test experiment to be p&80 ps.

6. Data acquisition 6.1. Trigger The trigger condition was “stopped K~” and “any charged particles at the exit of the spectrometer” with an additional fast-clear. The “stopped K~” was defined as; stopped K~"B1 * B2 * B3 *LC[ * V1

Fig. 6. Schematic views of the C3 chamber, which had 2 partitions with 3 readout taps. The X-hit positions in a partition were obtained by dividing the cathode charges from readout taps at both ends of the partition. The C4 chamber had 3 partitions with 4 readout taps.

there were 10 such cells in C3 and 13 in C4. The typical drift velocity was 50 lm/ns. Since field-shaping could not be applied because of the cathode readout, we required a cell smaller than in C2. At the location of C3, the magnetic fringing field was about 1 kG in the maximum 1.8 T excitation of the magnet. However, we found that the deflections relative to straight lines were 0.2 mm only even at the field setting of 1.8 T. 5.3. TOF-stop counters At the most downstream of the spectrometer, two plastic counters were located after the C4 chamber for each gap. The counter size was 1200 mm long in the radial coordinate, 200 mm wide, and 42 mm thick. The scintillator material was Bicron BC-408. Each scintillation counter was read out at both ends by PMTs (Hamamatsu H1949), and was used as Time-of-Flight stop counter. The readout signals were processed by leading-edge discriminators and charge ADCs for further time-walk

where LC[ denotes a logic signal summed for the three Lucite Cherenkov counters, and V1 denotes the signal of the veto counter located after the target. At least one hit on any TOF-stop counters was required to ensure that some charged particles were detected in the most downstream of the magnet gaps. The event trigger condition thus was 12 Trigger"stopped K~* + TOF-stop(i) i/1 In addition, we made the “gap hit” logic, which was the coincidence between any anode hit of the C2 chamber and of the C3 chamber and hits of the TOF-stop counters all belonging to the same gap; 12 “gap hit”" + C2(i) * C3 (i) * TOF-stop(i). i/1 Since the hit signals from the drift chambers were delayed to the “Trigger” logic, the “gap hit” condition, when it was not filled, worked as a fast-clear in the stage of the data transfer. 6.2. Data acquisition system The CAMAC Auxiliary Crate Controller (CES2180 “Starburst” module) which has DCJ11 15.0 MHz CPU with 2 MB Static RAM directly accessed to the CAMAC modules. Acquired data were transferred to a kVAX via a parallel bus crate controller (Kinetics K3922 with a K2922 card) which supports Direct Memory Access. The stored

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data transfer was done during a beam-off period of 2.5 s in every 4 s and written on an 8 mm video tape via an SCSI interface. Gates for the peak-hold ADC’s of the C2, C3, and C4 chambers were separately provided for each gap to reduce the data length. Drift time information was processed by 32 ch TKO TDC modules scanned by TKO-CH (Controller Head) module, and TKO-MP (Memory Partner) CAMAC interface. Hit-wire addresses of MWPCs (BC1 and BC2) were recorded in “MWPC memory modules” and stored in the “memory buffer” utilizing the KEK encoder—buffer system. The scaler counts of the beam-line counters and the chambers were recorded with CAMAC scalers and monitored during the data taking. The trigger rate was about 500 for 2.5]1012 protons on the production target of 40 mm thick platinum. A typical ratio of the number of the accepted events to that of the triggers was about 70%, and the average event size was about 600 bytes.

7. Performances 7.1. Solid angle The solid angle for typical field settings in the experiments are shown in Fig. 7. In the calculation, the realistic geometry of the spectrometer was used. The total acceptance of 12 gap was about 10% of 4p str at the maximum. For a solid angle more than 6%]4p str the spectrometer had a broad momentum coverage, for example, between 180 and 350 MeV/c at the maximum 1.8 T, 110 and 220 MeV/c at 1.1 T, and 70 and 150 MeV/c at 0.65 T. 7.2. Momentum resolution Momentum of a pion was obtained by the track reconstruction in the spectrometer and by eventby-event correction of the energy loss in the target. The energy loss was calculated by integrating dE/dx (the Bethe—Bloch equation) along the p~ path in the target. The path length was derived from the reaction point, i.e. the vertex of K~ and

Fig. 7. Solid angle (in a unit of 4p str) of the spectrometer as a function of the p~ momentum calculated for typical field settings, magnet excitations of 0.65, 1.1 and 1.8 T.

p~ tracks. The overall p~ momentum resolution in the final spectra depended on (i) the intrinsic momentum resolution of the track reconstruction, (ii) spatial resolution of the reaction vertex and (iii) energy loss fluctuation. Since we used thick targets (about 3 g/cm2 for pions), the overall momentum resolution was primarily governed by the latter two factors. The intrinsic spectrometer resolution was measured with a thin (0.3 g/cm2) plastic-counter target in which the contribution of the energy loss correction to the momentum resolution was small. The momentum resolution and the absolute momentum scale were checked by the monochromatic peaks from K` decays; K`Pp`p0 (205.1 MeV/c), l`m (235.5 MeV/c). The magnetic field was set at 1.1 T (positive polarity) for this calibration, so that the calibration peak of K was near the center of p2 the spectrometer acceptance. The spectrometer resolution was 1.5 MeV/c FWHM (the average of 12 gaps) at 205 MeV/c after subtracting a small energy loss effect for this thin target. The intrinsic resolution of 12 gaps distributed as 1.3—1.7 MeV/c FWHM. The experimental targets consisted of a stack of thin sheets in order to reduce the ambiguity of the energy loss correction. The energy loss in the target was corrected assuming the average density of all the sheets of the materials and the spaces between them. The closest distance between the K~ and p~ tracks was used as a measure of the spatial resolution of the vertex. The width at the

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Fig. 8. A raw momentum spectrum of positively charged particles taken by stopping K` in the 7Li target. Two peaks correspond to the two main decay modes, K decay and K decay of p2 l2 K`.

half maximum was for example, about 5.5 mm for the 7Li target. With this performance of the vertex reconstruction, it was calculated for p~ momentum of 70—200 MeV/c that the vertex resolution effect gave a larger contribution than the intrinsic momentum resolution of the spectrometer (1.5 MeV/c) and gave about the same contribution as the Landau fluctuation of the energy loss. The K resolution at 205 MeV/c for the experip2 mental targets was also measured. Fig. 8 shows a (stopped K`, p`/l`) spectrum for the 7Li target. The spectra of all the gaps were summed. From the known branching ratio, the K decay yield is triple l2 of that of the K decay, however, as shown in l2 Fig. 7, the spectrometer acceptance suppressed the K decay peak by about one third. The relative l2 yields of the two peaks confirmed the validity of the calculated relative acceptance curve in Fig. 7. The peak width of 2.5 MeV/c FWHM at 205 MeV/c was observed, and the error of the absolute momentum of the K decay peak distributed within 0.5 MeV/c p2 for all the gaps. The resolution around 100 MeV/c was directly checked by the two-body decay peak of 4KH (4KH Pp~4He (132.9 MeV/c)). The peak width was

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Fig. 9. Acceptance-corrected p~ momentum spectra from stopped K~ absorption on the 9Be target. A peak from the two-body decay of K4H is observed at 133 MeV/c. A bump around 100 MeV/c mainly consisted of the three-body decays of 4KH, 4KHe and K5He.

3.0 MeV/c FWHM for the 9Be target as shown in Fig. 9. The absolute momentum was calibrated with this peak position. 7.3. TOF resolution The mass spectrum of particles was obtained by time difference between hits of the TOF-stop counters and the B3 counter together with the flight path of the track. The mass resolution was measured to be 10% FWHM for pions (Fig. 10), being enough to reject muons. The time difference between K~ stopping and p~ emission gives the “reaction time” in the target, for example, the lifetime of produced hyperfragments. The K~ stopping time and the p~ emission time were obtained from signals of the B3 counter and the TOF-stop counters, respectively, assuming that the particles in the spectrometer were pions. The K~ stopping time in the target was calculated event-by-event from the K~ range in the target. The reaction-time resolution, which includes time resolutions of the B3 and TOF-stop counters (both were p&80 ps), flight-path resolution of pions from the tracking reconstruction (corresponding to p&100 ps), and accuracy of correction for K~ stopping time from B3 to the target (p&100 ps),

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target, 3 MeV/c FWHM at 133 MeV/c was observed. Acknowledgements

Fig. 10. A typical mass distribution of emitted particles obtained from the time of flight and the flight path length.

was p&200 ps. The lifetime of K4H was deduced as the reaction time by our TOF system [8].

8. Summary We constructed a spectrometer using a superconducting toroidal magnet of 12 magnet gaps at KEK 12 GeV PS. The spectrometer was utilized in the hypernuclear studies via (stopped K~, p~) reaction. A large solid angle of more than 6%]4p str was achieved in a momentum range between 200 and 350 MeV/c at the maximum 1.8 T magnetic excitation, and in a range between 70 and 150 MeV/c at 0.65 T. The momentum resolution of 1.5 MeV/c FWHM was obtained at 205 MeV/c. Including the energy loss fluctuation in the experimental thick

We are grateful for the continuous encouragements of Professors K. Nakai and S. Iwata through the construction and the experiments. We acknowledge the help provided by the technical and support staff of the KEK, especially Professor K.H. Tanaka of the KEK-PS beam channel group who designed the K5 beam line. Special thanks go to Professor M. Iwasaki and Dr. S.N. Nakamura for their advice. This work was supported in part by the Grant-in-Aid for Special Scientific Research on Meson Science of the Ministry of Education, Science, Sports and Culture of Japan. One of the authors, A.K., was partially supported for this work by the Grant-in-Aid for Research Fellow of the Japan Society for the Promotion of Science of the Ministry of Education, Science, Sports and Culture of Japan. References [1] T. Yamazaki et al., Nuovo Cimento A 102 (1989) 695. [2] H. Tamura et al., Proc. 23rd INS Int. Symp. on Nuclear and Particle Physics with Meson Beams in the 1 GeV/c Region, Universal Academy Press, Tokyo, Japan, 1995, p. 199. [3] A. Kawachi et al., Proc. 25th INS Int. Symp. on Nuclear and Particle Physics with High-Intensity Proton Accelerators, World Scientific, Singapore, 1998, p. 463. [4] A. Kawachi, Doctor Thesis, University of Tokyo, 1997. [5] J. Imazato et al., 11th Int. Conf. on Magnet Technology, Tsukuba, 1989, p. 366. [6] K.H. Tanaka, Nucl. Instr. and Meth. A 363 (1995) 114. [7] A. Kawachi, H. Tamura, R.S. Hayano, Nucl. Instr. and Meth., to be published. [8] K. Kubota, Master Thesis, University of Tokyo, 1997.