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A trigger system for measurements of proton-induced rare hadronic reactions around Tp=400MeV
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 584 (2008) 174–185 www.elsevier.com/locate/nima
A trigger system for measurements of proton-induced rare hadronic reactions around T p ¼ 400 MeV S. Minamia, T. Itabashib, S. Ajimurac, T. Fukudad, H. Hayakawab, T. Hayakawab, W. Imotod, T. Kanieb, F. Khanamb, T. Kishimotob, H. Kohric, K. Matsuokab, Y. Mitomab, Y.S. Miyakeb, T. Morib, K. Morikubob, R. Murayamab, T. Nagaob, H. Noumie, T. Numatab, T. Ogaitof, P.K. Sahag, A. Sakaguchib,, M. Sekimotoe, Y. Shimizub, K. Sugitab, M. Sumihamac, K. Tamuraf, K. Teraib, K. Wakaeb a
Institute fu¨r Kernphysik, Johannes-Gutenberg Universita¨t Mainz, D-55099 Mainz, Germany b Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan c Research Center for Nuclear Physics (RCNP), Osaka University, Ibaraki, Osaka 567-0047, Japan d Laboratory of Physics, Osaka Electro-Communication University, Neyagawa, Osaka 572-8530, Japan e High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan f Department of Physics, Fukui University, Matsuoka, Fukui 910-1193, Japan g Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki 319-1195, Japan Received 8 September 2007; accepted 12 October 2007 Available online 17 October 2007
Abstract We developed a trigger system for the measurement of proton-induced rare hadronic reactions around the beam kinetic energy T p ¼ 400 MeV based on highly segmented trigger scintillation detectors and programmable logic modules. The trigger system was designed to enhance events with the negative-pion production by the difference of the curvatures of the particle tracks in a magnetic field. Since the production cross-section of the negative-pion by the proton-induced reactions was smaller by about 3 orders of magnitude than the total cross-section around the beam energy, we expected large reduction of the trigger rate by the negative-pion selection. The construction of the trigger system was not trivial due to a large detector acceptance that was inevitable to measure the rare reactions. The performance of the trigger system was evaluated by using T p ¼ 430 MeV proton beams. An excellent reduction, more than 2 orders of magnitude reduction, of the background events was achieved. r 2007 Elsevier B.V. All rights reserved. PACS: 29.30.h; 29.30.Aj; 29.30.Ep; 29.40.Mc Keywords: Intelligent trigger system; Negative-pion trigger
1. Introduction We are investigating several proton-induced rare hadronic reactions around a beam kinetic energy T p ¼ 400 MeV that close to and above the single pion production threshold by the p þ N collisions. One of the rare reactions is a subthreshold production of pþ p pairs by the proton-induced reactions off nuclear targets [1], and a Corresponding author.
E-mail address: [email protected] (A. Sakaguchi). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.10.018
typical production cross-section is estimated to be order of 10 nb/nucleon. Another rare hadronic reaction is a weak production of the strangeness degree of freedom, the nðp; LÞp reaction [2,3], which has a faint production crosssection, about 1 fb/nucleon [4,5]. Beam intensities of ordinary proton accelerators are high enough to produce particles from the rare hadronic reactions within a reasonable period of time, but practically a dedicated detector system is necessary to measure such rare hadronic reactions under huge backgrounds by the proton-induced reactions which have a cross-section of about 30 mb/nucleon in total.
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One of key issues of such measurements is the construction of a trigger system that can reduce backgrounds efficiently because the data acquisition system accepts only a part of the triggered events and it limits the amount of data we can obtain. The rare hadronic reactions we are investigating have pions in the final state, pþ p pairs and p from the decay of L hyperons. So, we believe the identification of pions in the final state can be an efficient trigger in the measurements of the rare hadronic reactions. Further, only p is the negative hadron to be produced around the beam energy T p ¼ 400 MeV, and the difference of the charge can be identified in the trigger level easily. Although the single p production reaction, the nðp; p Þpp reaction, becomes a background of the negative-pion identification trigger, the production cross-section of the single p , about 70 mb=nucleon at the proton beam energy [6,7], is considerably smaller than that of the proton elastic and inelastic collisions. So, we expect 2 or 3 orders of magnitude improvement of the trigger rate by the selection of a negative-pion. In this paper, we report the design and the performance of the trigger system for the identification of the negative-pions in detail. Other triggers based on the event topology and the difference of the kinematics between signal and background events are also very efficient to reduce the background events. We also discuss those triggers in this paper. A magnetic spectrometer system was built for the measurements of the proton-induced rare hadronic reactions. We describe the design of the spectrometer system in Section 2 and describe the trigger system in detail in Section 3. The performances of the trigger system obtained in a beam experiment are discussed in Section 4 and are summarized in Section 5. 2. Spectrometer system for measurements of rare hadronic reactions A magnetic spectrometer system was built at the ringcyclotron experimental facility of Research Center for Nuclear Physics (RCNP) for the measurements of the proton-induced rare hadronic reactions. Fig. 1 shows a schematic side view of the spectrometer system. The spectrometer system consists of a large volume solenoidal magnet, a cylindrical drift chamber (CDC), two sets of highly segmented plastic scintillator hodoscopes for the triggering and other detectors. We describe the design of the elements of the spectrometer system in the following. 2.1. Particle tracking devices The solenoidal magnet generates 0.3 T field in a volume of 1800 mm in length and 1422 mm in diameter. A CDC is installed in the magnetic field to measure trajectories and estimate momenta of charged particles emitted from the target. The length and diameter of the active volume of CDC are 1500 and 818 mm, respectively. The angular coverage of CDC is from 15 to 90 in the polar angle and
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2.06 m Return York Solenoid Coil
Inner Hodoscope (IH)
Outer Hodoscope (OH)
Vaccum Pipe
CDC Beam
CDC
Target
Outer Hodoscope (OH)
Flat Hodoscope (FH)
1.66 m
Fig. 1. Side view of the spectrometer system for the measurements of the proton-induced rare hadronic reactions constructed at the ring-cyclotron experimental facility of RCNP. See text for more details. Table 1 CDC wire configurations Layer
Cells/ layer
Wires/ cell
Wire position (stereo angle)
1st stereo 1 2 3 1st axial 2nd stereo
48 48 48 36 48
1 1 1 4 3
2nd axial
40
4
145 mm ð4:6 Þ 160 mm ð5:1 Þ 175 mm ð2:8 ) 243 mm, 251 mm, 259 mm, 267 mm 305 mm ð4:5 Þ, 313 mm ð4:6 Þ, 321 mm ð4:7 Þ 352 mm, 360 mm, 368 mm, 376 mm
Number of cells in a layer, number of sense wires in a cell and radius of each wire position in mm unit are listed. The wire angles are also listed for the stereo layers.
2p in the azimuthal angle. CDC has a 220 mm+ cylindrical hole in the center for the beam vacuum pipe and inner detectors. CDC has 2 axial and 2 stereo layers, and the wire configurations are listed in Table 1. The wires of the 1st stereo layer are 900 mm in length, and other wires have a 1500 mm length. Fig. 2 shows a part of the cell structures of the 1st stereo and the 1st axial layers. The 1st stereo layer has a simple box-type cell structure. The filled and open circles show the positions of the sense wires and the potential wires, respectively. The maximum drift length is about 12 mm. The filled boxes show the shield wires. The cell structure of the 1st axial layer is also shown in Fig. 2. The filled and open circles show the sense and the potential wires again, and the open boxes show the position
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particle track
1st axial
Fig. 3. Cross-sectional view of the IH layers. IH has two identical layers and each layer consists of 45 modules of plastic scintillation counters. One mm+ WLS-fibers are attached to both sides of the 10ðW Þ 400ðLÞ 1ðTÞ mm3 plastic scintillators.
1st stereo (3) 1st stereo (2) 1st stereo (1)
Window
Fig. 2. Schematic drawing of the cell structure of the 1st stereo and the 1st axial layers. The filled and open circles are the sense wires and the potential wires, respectively. The filled and open boxes show the shield wires and the guard wires, respectively.
of the guard wires which keep the field gradient in the drift space uniform. A typical drift length is 23 mm. The 2nd stereo and the 2nd axial layers have the similar cell structure as that of the 1st axial layer. Typical drift lengths are 21 and 24 mm for the 2nd stereo and the 2nd axial layers, respectively. The sense wires are Au-coated W wires with a 20 mm diameter, and the potential and the guard wires are Aucoated Al wires with a 80 mm diameter. The signals from 592 sense wires are read out by pre-amplifier cards with the Amplifier-Shaper-Discriminator (ASD) chips developed at KEK for the ATLAS experiment. The chamber gas is a mixture of Arð90%Þ þ CH4 ð10%Þ. The maximum electron drift time is about 200 ns for the 1st stereo layer and about 400 ns for other layers. Typical position resolutions of CDC are 300 mm and 3 mm (rms) in the r–f plane and the z-direction, respectively.
shown in Fig. 3. The IH layers surround the beam vacuum pipe at a radius of 90 mm. The scintillation light from an IH module is wavelength shifted and collected by 1 mm+ double-clad WLS-fibers (Kuraray Y-11(200)M) attached to both sides of the plastic scintillator. The collected light is read out by a multi-anode photomultiplier tube (Hamamatsu H6568-10). The IH modules are optically isolated from each other by Al-mylar and black vinyl sheets which are not shown in Fig. 3. OH consists of 52 modules of plastic scintillation counters, and each plastic scintillator has a 200ðW Þ 1500ðLÞ 3ðTÞ mm3 size. About a half of the OH modules are read out by fine-mesh dynode photomultiplier tubes (Hamamatsu H6614-01) attached to both ends of the plastic scintillators. Another half of the OH modules are read out by the WLS-fibers and the multi-anode photomultiplier tubes. The arrangement of OH is shown in Fig. 4. The OH modules are installed at the distance of 57 cm from the target and have a tilt angle of 36 which is optimized for the online triggering of negative-pions to be discussed in Section 3. The raw signal shapes from the multi-anode photomultiplier tubes for the IH modules and a half of the OH modules change largely event-by-event even for the same energy deposit. It comes from the relatively slow light emission of the WLS-fibers, the small number of photoelectrons and the fast rise-time of the photomultiplier tubes. To improve the event-by-event signal shape stability, the raw signals are fed into timing filter amplifiers, and the output signals are used for the discriminator and ADC inputs. The addition of the timing filter amplifiers improves the performance of the selection of the energy deposit in OH in the online trigger level to be discussed in Section 3.
2.2. Detectors for trigger purpose 2.3. Other detectors An inner hodoscope (IH) and an outer hodoscope (OH) are installed to produce the online trigger signals (see Fig. 1). IH has two identical layers, and a layer consists of 45 modules of plastic scintillation counters with a 10ðW Þ 400ðLÞ 1ðTÞ mm3 sensitive volume. The positions of the IH layers are shifted by a half width of the modules as
The flat hodoscope (FH) consists of 12 modules of plastic scintillation counters same as the OH modules in size (see Figs. 1 and 4). The FH modules are installed between CDC and the OH modules. The light signals from FH are detected by the Hamamatsu fine-mesh dynode
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Return York
FH modules
OH modules vacuum window
IH
CDC
Return York
Fig. 4. Cross-sectional beam view of the solenoidal magnet and the detector system. Charged particle tracks are also shown. The arrow with the solid curve and the arrow with the dashed curve correspond to a negatively and a positively charged particle tracks, respectively.
photomultiplier tubes attached to both ends of the FH scintillators. FH is used for the time-of-flight (TOF) measurement together with the RF signal from the accelerator. A typical TOF resolution is 500 ps (rms). 3. Trigger system for measurements of rare hadronic reactions 3.1. Kinematical parameters for trigger design Before the discussions on the trigger system, we wish to show kinematical conditions which affect to the design and the performance of the trigger system. Fig. 5 shows momentum distributions of negative-pions by simulation calculations. The absolute values of momenta (a) and the transverse momenta (b) of p from the subthreshold ðp; pþ p Þ reaction at T p ¼ 430 MeV, and those from the nðp; LÞp reaction at T p ¼ 400 MeV followed by the L hyperon decay, L ! pp , are shown in (c) and (d), respectively. The momenta of p range up to 200 MeV/c for both reactions, and the values of momenta are considerably smaller than the beam momentum, about 950 MeV/c, and the proton momenta from the inelastic scattering, about 500 MeV/c. At the lower ends of the momentum distributions, the stopping ranges of the negative-pions are comparable to the overall thickness of the trigger detectors, typically 1 g=cm2 . Due to the stopping of the negative-pions in the middle of the trigger detectors,
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we expect a large inefficiency of the online trigger in the region of the pion momentum smaller than 70 MeV/c which corresponds to the transverse momentum of about 40 MeV/c. The plots also tell that the transverse momenta of most of the negative-pions are smaller than 100 MeV/c. The bending angle of the low pT negative-pions is larger than 30 and the position displacement from the straight track is larger than 15 cm at the OH position in the 0.3 T solenoidal magnetic field. So, we can distinguish the tracks of negative-pions from the high-momentum proton tracks in the online trigger level. By taking into account these kinematical conditions, we discuss the design of the trigger system in the following. 3.2. Negatively charged particle trigger with OH In the proton-induced reactions off a nuclear target at the beam kinetic energy around T p ¼ 400 MeV, major ejectiles are protons and neutrons. Since we attempt to measure the pþ p pairs or the decay of L hyperon, L ! pp , the negative-pions can be distinguished from the background protons and neutrons easily by looking at the difference of particle charges, at least, in offline analyses. Our idea is that we apply the particle discrimination by the difference of the particle charges in the online trigger level. The production cross-section of the negative-pions at this energy region, roughly 70 mb=nucleon, is considerably smaller than the total reaction cross-section, about 30 mb/nucleon. So, selection of the negatively charged particles in the trigger level may reduce the experimental trigger rate drastically. Other sources of the negatively charged particles are electrons from the Dalitz-decay of p0 (p0 ! geþ e ) or the g ! eþ e conversion after the p0 ! gg decay. The amount of the electrons is expected to be smaller than the negative-pions, and the deterioration of the trigger quality by the electrons is small. The discrimination by particle charges can be achieved by the dedicated arrangement of the OH modules. The OH modules have tilt angles, and the number of hit OH modules by single charged particle depends on the curvature of the particle track. The tilt angle is set so that a negatively charged particle may hit three adjacent OH modules or more and a positive charged particle can hit only two OH modules or less. Typical particle trajectories are shown in Fig. 4 by the arrow with the solid curve (negatively charged) and the arrow with the dashed curve (positively charged). The track of negatively charged particle hits three adjacent OH modules and the track of positively charged particle goes through only one OH module in Fig. 4. The geometry of OH to satisfy the condition of the negatively charged particle trigger is determined by the following equation: sin aOH ¼
R2 ðW =2Þ2 tan 2yOH WR
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7000
9000 8000
6000
7000 5000 5000
Counts
Counts
6000
4000 3000
4000 3000 2000
2000 1000
1000
0
0 0
50 100 150 200 Pion Momentum (MeV/c)
250
0
50
150 100 Pion pT (MeV/c)
200
0
50
100 150 Pion pT (MeV/c)
200
18000 16000
6000
14000
5000
12000
4000
10000
Counts
Counts
7000
3000
8000 6000
2000
4000
1000
2000 0
0 0
50 100 150 200 Pion Momentum (MeV/c)
250
Fig. 5. Distributions of momenta of negative-pions by simulation calculations. The distributions of (a) the absolute value of momentum and (b) the transverse momentum from the ðp; pþ p Þ reaction at T p ¼ 430 MeV, and those from the nðp; LÞp reaction at T p ¼ 400 MeV followed by the L hyperon decay, L ! pp , are shown in (c) and (d), respectively.
where aOH and yOH are the OH tilt angle and the separation angle between adjacent OH modules, respectively, as shown in Fig. 6. R and W are the distance from the beam to the center of the OH module and the width of the OH modules, respectively. By using the values, R ¼ 57 cm, W ¼ 20 cm and yOH ¼ 6 , we obtain the OH tilt angle aOH ¼ 36 . The geometrical parameters of OH with the tilt angle are listed in Table 2. The OH geometry determines the trigger efficiency of negatively charged particles with the requirement of the three and more adjacent OH hits. Fig. 7 shows the calculated efficiency of the negatively charged particle trigger as a function of the transverse momentum of particles (solid curve). Low pT particles emitted from the target cannot reach to OH in the solenoidal magnetic field, so the trigger efficiency is zero for particles pT o22 MeV=c. Negatively charged particles with 22 MeV=copT o74 MeV=c always hit three and more OH modules, and the geometrical trigger efficiency is 100%. A part of negatively charged particles with the transverse momenta larger than 74 MeV/c
hit only two OH modules depending on the azimuthal angle of the emission from the target, so the trigger efficiency decreases slowly as the increase of the transverse momentum. The vertical dashed line in the figure shows a cut due to the particle stopping before reaching the last OH module for the low momentum negative-pions as discussed in Section 3.1. The trigger efficiency is better than 55% for the negatively charged particles with the transverse momentum pT o120 MeV=c which is close to the upper ends of the transverse momentum distributions of the negative-pions as shown in Fig. 5. Details of the trigger efficiency calculation are given in Appendix A. The rejection of positively charged particles by the trigger is perfect in an ideal situation, but in the practical setup there are several sources of the inefficiency of the rejection. One source of the inefficiency is the particle scattering in the OH modules. High-momentum protons emitted from the target run through one or two OH modules, and nuclear scatterings may occur in the OH
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OH modules 1 αOH 0.8 Trigger Efficiency
W
Rmax
0.6
Pion Stopping Range is Long Enough
0.4
θOH Rmin
R
0.2
0 0
Beam Fig. 6. Schematic beam view of the OH geometry for the design of the negatively charged particle trigger.
Table 2 Geometrical parameters of outer hodoscope (OH) OH geometrical parameters
Values
Angular separation between modules (yOH ) Tilt angle (aOH ) Radius of module center (R) Radius of scintillator inner edge (Rmin ) Radius of scintillator outer edge (Rmax )
6 36 57 cm 49.3 cm 65.4 cm
Definitions of the parameters are shown in Fig. 6.
modules. If the proton scattering angle is large enough, the scattered proton may hit several OH modules additionally, and more than two adjacent OH modules are fired in total. A typical thickness of the OH modules along the initial proton tracks is about 1 g=cm2 and the nuclear collision length for the plastic scintillator is about 60 g=cm2 . So, we expect a few % level of the inefficiency of the proton rejection due to the scattering in the OH modules. Other minor sources of the inefficiency are the accidental coincidence of the OH module hits, the proton nuclear scattering in IH and CDC, the proton knock out by energetic neutrons in OH, etc. The details of the inefficiency of the proton rejection are discussed in Section 4.3.
50
100 150 200 250 Transverse Momentum pT (MeV/c)
300
Fig. 7. Efficiency of the negatively charged particle trigger as a function of the transverse momentum pT (solid curve). The vertical dashed line shows a cut due to the particle stopping before reaching the last OH module for the low momentum negative-pions.
can be used for the selection of the negatively charged particles in the online trigger level. The idea of the particle selection is close to that of the trigger by the three adjacent OH hits described in the previous section, but this trigger by using the IH–OH correlation is quite complementary with the previous trigger method. Fig. 8 shows correlations between the azimuthal angle difference between IH and OH hits, fðIHÞ fðOHÞ, and the transverse momentum. The contour plot is obtained from experimental data. The solid and dashed contour lines correspond to distributions of negatively charged and positively charged particles, respectively. The distributions are consistent with the expected correlations by simple geometrical calculations for the negatively charged (solid curve) and positively charged particles (dashed curve). The figure shows that we can distinguish negatively charged particles from positively charged particles by setting appropriate cuts on the azimuthal angle difference. Since the azimuthal angle separation between OH modules is 6 and the separation between IH modules is 4 , we set a relatively loose cut on the azimuthal angle difference (vertical line). We expect the IH–OH correlation trigger rejects about a half of the background protons with high transverse momenta.
3.3. Trigger by IH and OH hit correlation
3.4. Two-track event trigger
The correlation of the IH hit and the OH hit positions also has information of the sign of the particle charge especially for particles with low transverse momenta, and
In the measurement of the ðp; pþ p Þ reaction, we require two tracks in the offline analysis, one positively charged and one negatively charged tracks. To improve the analysis
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180 300
225 200
250
Negative Pion
200
150
Rejected
Select
Counts
Transverse Momentum (MeV/c)
175
150
100
Positive Charge
125 100
Negative Charge
Proton 75 50
50
25 0 -50
-40
-30
-20
-10
0
10
20
30
40
50
φ (IH) - φ (OH) (degrees)
Fig. 8. Correlations between the azimuthal angle difference between IH and OH hits, fðIHÞ fðOHÞ, and the transverse momentum. The contour plot is obtained from experimental data. The solid and dashed curves are expected correlations obtained from simple geometrical calculations for negatively charged and positively charged particles, respectively.
efficiency, two-track event candidates are selected by the online trigger. The selection of events with two-track candidates is made by looking at the hit pattern or hit multiplicity of the IH modules. Since the IH has two layers made of 45 modules for each layer and a module has geometrical overlaps with adjacent IH modules in the different layer as shown in Fig. 3, a simple counting of hit multiplicity of the IH modules is not adequate. In the hit multiplicity calculation, a module ID difference larger than 1 is required to identify the two-track event candidates from the IH signals. 3.5. Trigger by energy loss in OH In the nðp; LÞp reaction followed by the L ! pp decay, the energy deposit of the decay p in OH is considerably smaller than that of background protons. So, a discrimination of the background protons may be possible by the selection of the energy deposit in OH. Fig. 9 shows typical distributions of the energy deposit in an OH module by a p (solid histogram) and a proton (dashed histogram) obtained from experimental data. The figure shows that the energy deposit of protons is about twice as large as that of negative-pions. We can reject about 50% of protons in keeping more than 95% of negative-pions with an energy threshold of 5 MeV. In the case of the combination of this energy loss trigger with the negatively charged particle trigger by OH, the energy loss upper limit, 5 MeV, is applied 3 times to the
0 0
2
4
8 10 6 Energy Deposit in OH (MeV)
12
14
Fig. 9. Typical distributions of the energy deposit in an OH module by a p (solid) and a proton (dashed) obtained from experimental data.
three adjacent OH modules. So, we expect the proton rejection inefficiency is about ð0:5Þ3 13% if the energy loss distribution of protons is mainly coming from eventby-event and module-by-module energy loss fluctuations. The proton rejection inefficiency is discussed in detail in Section 4.5. 4. Performance of trigger system 4.1. Experimental condition for trigger test We evaluated the performance of the trigger system by using T p ¼ 430 MeV proton beams at the WSS beam course of the ring-cyclotron experimental facility in RCNP of Osaka University. A copper target of 50 mm in thickness was used. The proton beam size on the target was less than 2 2 mm2 and the beam intensity was typically 0.1 nA. The IH and OH signals were processed by ordinary NIM front-end circuits. The processed IH and OH signals and CDC signals were sent to the DAQ system. High density FASTBUS ADC and TDC modules were used for the signal digitization. A dual buffer memory and an on-board CPU in a VME system were used for the data collection and the event building. A remote PC that was connected to the on-board CPU via network was used for the data recording on the disk. The logic signals from IH and OH discriminators were fed into programmable logic modules to generate trigger signals. Minimum bias data were also taken with the following simple trigger condition: ðminimum biasÞ ðIH ORÞ ðOH ORÞ.
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The minimum bias data were used as reference data to evaluate the performance of the additional trigger conditions. 4.2. Trigger circuits The programmable logic module, Universal Logic Module (ULM), had been developed at Osaka University to construct the trigger system with the large number of logic inputs. ULM has 16 4 channels of ECL inputs and 16 2 channels of ECL outputs. ULM also has auxiliary NIM inputs (two channels) and outputs (two channels) usually used for inter module communications. ULM has a programmable logic device (Altera APEX20KE) to handle input logic signals and generate required trigger signals. Fig. 10 shows the logic diagram of the trigger circuit made of ULMs. The trigger circuit consists of three blocks (A, B and C), and each trigger block made of several ULMs. The OH signals are discriminated with a low and a high thresholds. The OH discriminator output signals are fed into the trigger block A. The trigger block A takes a coincidence between the upstream and the downstream photomultiplier signals, takes triple-fold coincidences for all combinations of three adjacent OH modules, and then produces an OR signal of the coincidences as the negatively charged particle trigger by OH. The logic block A also produces the hit pattern of the triple-hold coincidences to be sent to the trigger block C. The block A can optionally generates the trigger signal with the requirement of the
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energy loss trigger by using the logic signal of the high threshold discriminators as veto signals. The discriminator signals of IH are fed into the trigger block B to generate the two-track event trigger. In the block B, the 90 IH logic input signals are reduced to 45 channels of hit pattern information to be sent to the trigger block C. The outputs from the trigger blocks A and B are combined in the trigger block C to generate the negatively charged particle trigger by the IH and OH hit correlation. Finally, a combination of the triggers is used as the main trigger for the DAQ start. 4.3. Performance of negatively charge particle trigger by OH A set of data was taken with the trigger to select the negatively charged particles by the three adjacent hits of the OH modules described in Section 3.2, and compared with the minimum bias data. Fig. 11 shows distributions of the transverse momentum per unit charge for all particles obtained with the two different trigger conditions, the minimum biased trigger (histogram) and the negatively charged particle trigger (data points). The distributions are normalized with the integrated beam current, and simple geometrical acceptances are corrected for. The positive values of the horizontal axis correspond to the yield of the positively charged particles, and most of the particles are protons in this experiment. The negative values correspond to the negatively charged particles and these are mainly negative-pions. Negative Charge
104
Positive Charge
Relative Yield (a.u.)
103
102
10
1 -400
-300
-200
-100
0
100
200
300
400
Transverse Momentum per Charge (MeV/c/e)
Fig. 10. Logic diagram of the trigger circuit made of programmable logic modules. The OH signals are discriminated with a low and a high thresholds. The IH and OH discriminator signals are fed into the trigger blocks A, B and C, to generate the trigger signals. See text for more details.
Fig. 11. Distributions of the transverse momentum per unit charge for all particles obtained with the two different trigger conditions, the minimum biased trigger (histogram) and the negatively charged particle trigger (data points).
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As one can see, the positively charged particles are reduced by about 2 orders of magnitude by the negatively charged particle trigger while the number of the negatively charged particles is changed little in the pT region from 50 to 150 MeV/c. So, the negatively charged particle trigger is working well as designed. The rate of the negatively charged particle trigger is listed in Table 3 together with the rate of the minimum bias trigger. The trigger reduction 1 factor from the minimum biased trigger is 30k=2:2M 73 , and it is consistent with the proton reduction factor of the pT distribution in Fig. 11 and consistent with the possible inefficiency of the proton rejection due to the proton large angle scattering in the OH modules (a few % level of inefficiency). As discussed in Section 3.2, the trigger efficiency of the negative-pions is a function of pT . To see the pT dependence, the ratio of the pT spectra with and without the negatively charged particle trigger is shown in Fig. 12. The points correspond to the experimental data, and the solid curve is the result of the geometrical trigger efficiency calculation (essentially same as the curve in Fig. 7). The general tendency of the pT dependence of the trigger efficiency is well reproduced by the simple calculation, but about 20% of inefficiency is observed for the experimental data even at the low pT region, pT 70 MeV=c. An apparent inefficiency of the trigger partly comes from the contamination of the background protons into the negatively charged particle region in Fig. 11. If a highmomentum proton is scattered in CDC, the CDC hit pattern may become similar with that of a negative-pion. The signature of such miss calculations of pT and particle charge can be seen in the pT =chargeo 300 MeV=c=e region in Fig. 11 because a negatively charged particle with such high transverse momentum is quite scarce kinematically. This proton contamination effect is relatively large at the high pT region, pT 300 MeV=c. A real inefficiency of the trigger may come from the discriminator threshold of OH modules. Since the PMT signals from the OH modules by the pion hits are relatively low, 5–10% level of the pion hits may have signals below Table 3 Online trigger rate, fraction of negative-pions and fraction of multi-track events for different triggers, (A) negatively charged particle trigger by OH, (B) trigger by IH–OH hit correlation, (C) two-track trigger, (D) trigger by energy loss in OH and combinations of them Trigger types
Trigger/ nC
p /all (%)
Multi-track fraction (%)
Minimum bias trigger Neg. charge trigger (A) IH–OH correlation (B) and (A) Two-track trigger (C)
2.2M 30k 15k
p0:21 8.2 16
11.0 14.4 –
1.3M
–
16.5
(A) and (C) (A), (B) and (C) Energy loss in OH (D) and (A)
– 12k 9.0k
– 15 30
19.3 19.2 –
1.2
1
0.8 Trigger Efficiency
182
0.6
0.4
0.2
0 0
50
100 150 200 Transverse Momentum (MeV/c)
250
300
Fig. 12. The trigger efficiency is extracted by dividing the pT spectrum with the negatively charged particle trigger by that from the minimum biased data (data points). The curve is essentially same as the curve in Fig. 7 that shows the trigger efficiency by the simple geometrical calculation.
the discriminator threshold. The minimum bias trigger requires only one OH signal above the discriminator threshold, and the negatively charged particle trigger by OH requires at least three OH signals above the threshold. It means the negatively charged particle trigger may have additional 10–20% inefficiency due to the low signal pulse height in comparison with the yield of the minimum bias trigger. This inefficiency may explain the trigger efficiency lower than the simple geometrical estimation in the low pT region. The fractions of p tracks in the acquired data are also listed in Table 3. The sign of inequality for the minimum bias trigger is coming from the possible proton contamination into the negative-pion events. The improvement of the fraction of p tracks from the minimum bias trigger to the negatively charged particle trigger is 8:2%=0:21% 39, and this value is roughly a half of the factor of the trigger rate change, about 73, mentioned previously. This difference can be explained by the trigger efficiency of the negative-pions averaged with the transverse momentum distribution. The averaged efficiency is about 60%. 4.4. Performance of trigger by IH and OH hit correlation Fig. 13 shows the distributions of the transverse momentum per unit charge. The histogram is the distribution obtained with the negatively charged particle trigger by OH. By requiring the additional trigger condition of the IH–OH hit correlation, the distribution with the data
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Positive Charge
1200
Relative Yield (a.u.)
1000
800
600
400
200
0 -200
-100 0 100 200 300 Transverse Momentum per Charge (MeV/c/e)
400
Fig. 13. Distribution of the transverse momentum per unit charge. The histogram shows the distribution obtained with the negatively charged particle trigger by OH. By requiring the additional trigger condition of the IH–OH hit correlation, the distribution by the data points is obtained.
points is obtained. The clear reduction of the yield of the positively charged particles, mainly protons, indicates a successful reduction of the background protons by the IH–OH correlation trigger. The reduction factor of the background protons is about a half and it is consistent with the cut criteria. The yield of the negatively charged particles is changed a little, the efficiency of the trigger is about 93%. The trigger rate and fraction of p tracks for the IH–OH correlation trigger is also listed in Table 3. The trigger rate reduction factor is 15k=30k ¼ 12, the improvement of the fraction of p tracks is 16%=8:2% 2:0 and these values are also consistent with the cut criteria. 4.5. Other triggers and overall trigger performance The triggers discussed above are based on the difference of the signs of the particle charges. We also tested other types of triggers dedicated for either of the rare hadronic reactions, the ðp; pþ p Þ reaction or the nðp; LÞp reaction followed by the L ! pp decay. For the ðp; pþ p Þ reaction, we evaluated the performance of the two-track trigger. The result is listed in Table 3. The fraction of the multi-track events is 11.0% for the minimum bias trigger data. The two-track trigger improves the fraction to 16.5%. The fraction is improved already in the negatively charged particle trigger by OH to 14.4%, and the combination of the two types of trigger results in the multi-track event fraction of 19.3%. The fraction is improved by a factor 19:3%=11:0% 1:8. For the ðp; pþ p Þ reaction measurement, we can use the
183
ðAÞ ðBÞ ðCÞ trigger condition in Table 3. The trigger condition reduces the trigger rate by the factor 1 and improves the fraction of p tracks 12k=2:2M 180 by a factor 15%=0:21% 71. The trigger rate 12k trigger/nC allows us to set the beam intensity to about 0.1 nA for the 50 mm Cu target if we assume the capability of the DAQ system is about 1k event/s. This beam intensity corresponds to the accessible cross-section of roughly 0.3 nb/nucleon in a day for the ðp; pþ p Þ reaction. In the estimation of the accessible crosssection, we used an effective nucleon number 5 and an overall efficiency of 0.03 due to the trigger efficiency and the detector acceptance for pþ p pairs. The accessible cross-section is large enough to measure the Cuðp; pþ p ÞX reaction within a few days of beamtime. For the nðp; LÞp reaction measurement, we evaluated the performance of the trigger by the energy loss in OH. The trigger rate of the energy loss trigger is listed in Table 3. The energy loss cut is made for all three adjacent OH modules by using the signals from the high threshold discriminators as vetos, and the cut rejects about a half of proton hits as shown in Fig. 9. If the distributions of the pulse heights of the three OH modules for the background protons mainly come from pulse height fluctuations, we expect a factor of 18 ¼ ð12 Þ3 background reduction by the threefold coincidence of the OH signals. On the other hand, if the pulse height of three OH modules have strong correlations, the reduction factor should be close to 12. The measured trigger rates are 9.0k and 30k and the fractions of p tracks are 30% and 8.2% with and without the trigger by the energy loss in OH, respectively. From the trigger rates and the fractions of p tracks, we can make a simple estimation of the background reduction factor r as follows: 30k ð1 0:082Þ r ¼ 9:0k ð1 0:30Þ
(1)
and we obtain the background reduction factor r 1=4:4. The trigger efficiency of the negative-pions can be estimated by the same manner as Eq. (1), and the efficiency is close to 100%. The obtained reduction factor r is considerably smaller than 12. The proton background reduction factor can be estimated more precisely from the ratio of the transverse momentum spectra with and without the OH energy loss trigger. Fig. 14 shows the ratio of the proton yields as a function of the transverse momentum. There is a strong pT dependence of the ratio. Since the averaged transverse momentum of protons is about 250 MeV/c, the typical ratio of the background proton distributions with and without the energy loss trigger is 0:14 17. The larger values of the ratio for the lower transverse momentum protons come from the contribution of the quasi-free Nðp; p pÞN reaction. Protons associated with the quasi-free negativepion production are also accumulated in the ratio calculation. It is an apparent inefficiency of the proton rejection and the actual proton rejection factor is believed to be close to 17. The large reduction factor means the
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performance of the trigger system was evaluated by using T p ¼ 430 MeV proton beams. An excellent reduction, more than 2 orders of magnitude reduction, of the background events was achieved, and the efficiency and the inefficiency of the trigger system were well understood by the simple considerations.
0.5 0.45
Reduction Factor of Proton Tracks
0.4 0.35
Acknowledgments
0.3
We acknowledge the outstanding work of the RCNP Accelerator group for delivering excellent quality of proton beams. We thank H. Toki, K. Hatanaka, K. Sato, S. Ninomiya, K. Nagayama and M. Uraki for the support of the construction of the beam line and the spectrometer system in the ring-cyclotron experimental facility of RCNP. We also thank M. Nomachi for the development of the programmable logic module (ULM) at Osaka University. This research was supported in part by the Ministry of Education, Science, Sports and Culture of Japan.
0.25 0.2 0.15 0.1 0.05 0 0
50
100
150
200
250
300
350
400
Transverse Momentum (MeV/c) Fig. 14. Ratio of the proton distributions with and without the OH energy loss trigger as a function of the transverse momentum.
distributions of the OH signal pulse heights from the background protons mainly come from event-by-event and module-by-module fluctuations. For the nðp; LÞp reaction measurement, we can use the trigger condition (A)(D) in Table 3. The trigger condition 1 reduces the trigger rate by a factor 9.0k/2.2M 240 , and the fraction of p tracks is improved by a factor 30%/0.21%140. 5. Summary We developed a trigger system for the measurement of the proton-induced rare hadronic reactions around the beam kinetic energy T p ¼ 400 MeV in which negativepions were created in the final states. The trigger system was based on the highly segmented trigger scintillation detectors and the programmable logic modules, and was designed to enhance events with the negative-pion production by the difference of the curvatures of the particle tracks in the magnetic field. Since the production crosssection of the negative-pion by the proton-induced reactions is expected to be smaller by about 3 orders of magnitude than the total cross-section at the beam energy, we expected large improvement by the trigger. The construction of the trigger system was not trivial due to a large detector acceptance that was inevitable to measure the rare reactions. The modules of the OH were arranged with a tilt angle so that the hit multiplicity of adjacent OH modules can identify the charge sign of the particles from the target. The hit position correlation between the IH and OH was also used to enhance the negatively charged particles. The
Appendix A. Efficiency calculation of trigger by outer hodoscope Most of the parameters of the negatively charged particle trigger by the outer hodoscope (OH) can be calculated analytically if only the geometrical cuts are considered. Low momentum charged particles cannot reach to OH if the radius of the curvature of the track projected on the transverse plane is smaller than Rmin =2. The condition introduces a threshold of the transverse momentum as follows: Rmin 2 where B is the strength of the solenoidal magnetic field. Since the condition just grants single hit of the OH module, the trigger condition of the three adjacent hits in OH introduces a slightly higher cut of the transverse momentum as follows: pT 40:3B
pT 40:3B
Rmin 2 cos yOH
where yOH is the angular separation between the adjacent OH modules defined previously in Fig. 6. The trigger efficiency reaches 100% as the increase of pT at pT ¼ 0:3B
Rmin . 2 cos 32 yOH
Since these pT values are very close to each other if yOH is small, the pT dependence of the trigger efficiency has a sharp rise at the low pT end. The trigger efficiency is kept 100% up to the transverse momentum: pT ¼ 0:3B
AB 2 sin b
(A.1)
where AB and b are the distance between points A and B and the angle ffAOB in Fig. A1, respectively. Above the
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trigger efficiency is written as follows: 1¼
B
b ðdB dA Þ yOH
dB ¼ sin1 A
Rmax
β Rmin
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Rmax ; 2r
dA ¼ sin1
Rmin 2r
where r is the radius of the curvature of the particle track projected on the transverse plane, r ¼ pT =0:3B. Rmax and Rmin are defined in Fig. 6. These calculations on the negatively charged particle trigger describe the pT dependence of the trigger efficiency shown in Fig. 7. References
O Fig. A1. Schematic beam view of the OH geometrical parameters for the efficiency calculation of the negatively charged particle trigger. The point O corresponds to the beam line. The points A and B are the inner and the outer edges of the OH modules, respectively.
transverse momentum given by Eq. (A.1), the trigger efficiency decreases gradually as the increase of pT , and the
[1] A. Sakaguchi, et al., RCNP proposal E251, 2004. [2] T. Kishimoto, et al., RCNP proposal E122, 1998. [3] T. Kishimoto, Nucl. Phys. A 629 (1998) 369; T. Kishimoto, et al., Nucl. Phys. A 663 (2000) 509; S. Ajimura, et al., Nucl. Phys. A 691 (2001) 312. [4] A. Parren˜o, A. Ramos, N.G. Kelkar, C. Bennhold, Phys. Rev. C 59 (1999) 2122. [5] T. Inoue, K. Sasaki, M. Oka, Nucl. Phys. A 684 (2001) 478. [6] B.J. VerWest, R.A. Arndt, Phys. Rev. C 25 (1979) 1979. [7] M. Daum, et al., Eur. Phys. J. C 23 (2002) 43.
Nuclear Instruments and Methods in Physics Research A 584 (2008) 174–185 www.elsevier.com/locate/nima
A trigger system for measurements of proton-induced rare hadronic reactions around T p ¼ 400 MeV S. Minamia, T. Itabashib, S. Ajimurac, T. Fukudad, H. Hayakawab, T. Hayakawab, W. Imotod, T. Kanieb, F. Khanamb, T. Kishimotob, H. Kohric, K. Matsuokab, Y. Mitomab, Y.S. Miyakeb, T. Morib, K. Morikubob, R. Murayamab, T. Nagaob, H. Noumie, T. Numatab, T. Ogaitof, P.K. Sahag, A. Sakaguchib,, M. Sekimotoe, Y. Shimizub, K. Sugitab, M. Sumihamac, K. Tamuraf, K. Teraib, K. Wakaeb a
Institute fu¨r Kernphysik, Johannes-Gutenberg Universita¨t Mainz, D-55099 Mainz, Germany b Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan c Research Center for Nuclear Physics (RCNP), Osaka University, Ibaraki, Osaka 567-0047, Japan d Laboratory of Physics, Osaka Electro-Communication University, Neyagawa, Osaka 572-8530, Japan e High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan f Department of Physics, Fukui University, Matsuoka, Fukui 910-1193, Japan g Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki 319-1195, Japan Received 8 September 2007; accepted 12 October 2007 Available online 17 October 2007
Abstract We developed a trigger system for the measurement of proton-induced rare hadronic reactions around the beam kinetic energy T p ¼ 400 MeV based on highly segmented trigger scintillation detectors and programmable logic modules. The trigger system was designed to enhance events with the negative-pion production by the difference of the curvatures of the particle tracks in a magnetic field. Since the production cross-section of the negative-pion by the proton-induced reactions was smaller by about 3 orders of magnitude than the total cross-section around the beam energy, we expected large reduction of the trigger rate by the negative-pion selection. The construction of the trigger system was not trivial due to a large detector acceptance that was inevitable to measure the rare reactions. The performance of the trigger system was evaluated by using T p ¼ 430 MeV proton beams. An excellent reduction, more than 2 orders of magnitude reduction, of the background events was achieved. r 2007 Elsevier B.V. All rights reserved. PACS: 29.30.h; 29.30.Aj; 29.30.Ep; 29.40.Mc Keywords: Intelligent trigger system; Negative-pion trigger
1. Introduction We are investigating several proton-induced rare hadronic reactions around a beam kinetic energy T p ¼ 400 MeV that close to and above the single pion production threshold by the p þ N collisions. One of the rare reactions is a subthreshold production of pþ p pairs by the proton-induced reactions off nuclear targets [1], and a Corresponding author.
E-mail address: [email protected] (A. Sakaguchi). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.10.018
typical production cross-section is estimated to be order of 10 nb/nucleon. Another rare hadronic reaction is a weak production of the strangeness degree of freedom, the nðp; LÞp reaction [2,3], which has a faint production crosssection, about 1 fb/nucleon [4,5]. Beam intensities of ordinary proton accelerators are high enough to produce particles from the rare hadronic reactions within a reasonable period of time, but practically a dedicated detector system is necessary to measure such rare hadronic reactions under huge backgrounds by the proton-induced reactions which have a cross-section of about 30 mb/nucleon in total.
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One of key issues of such measurements is the construction of a trigger system that can reduce backgrounds efficiently because the data acquisition system accepts only a part of the triggered events and it limits the amount of data we can obtain. The rare hadronic reactions we are investigating have pions in the final state, pþ p pairs and p from the decay of L hyperons. So, we believe the identification of pions in the final state can be an efficient trigger in the measurements of the rare hadronic reactions. Further, only p is the negative hadron to be produced around the beam energy T p ¼ 400 MeV, and the difference of the charge can be identified in the trigger level easily. Although the single p production reaction, the nðp; p Þpp reaction, becomes a background of the negative-pion identification trigger, the production cross-section of the single p , about 70 mb=nucleon at the proton beam energy [6,7], is considerably smaller than that of the proton elastic and inelastic collisions. So, we expect 2 or 3 orders of magnitude improvement of the trigger rate by the selection of a negative-pion. In this paper, we report the design and the performance of the trigger system for the identification of the negative-pions in detail. Other triggers based on the event topology and the difference of the kinematics between signal and background events are also very efficient to reduce the background events. We also discuss those triggers in this paper. A magnetic spectrometer system was built for the measurements of the proton-induced rare hadronic reactions. We describe the design of the spectrometer system in Section 2 and describe the trigger system in detail in Section 3. The performances of the trigger system obtained in a beam experiment are discussed in Section 4 and are summarized in Section 5. 2. Spectrometer system for measurements of rare hadronic reactions A magnetic spectrometer system was built at the ringcyclotron experimental facility of Research Center for Nuclear Physics (RCNP) for the measurements of the proton-induced rare hadronic reactions. Fig. 1 shows a schematic side view of the spectrometer system. The spectrometer system consists of a large volume solenoidal magnet, a cylindrical drift chamber (CDC), two sets of highly segmented plastic scintillator hodoscopes for the triggering and other detectors. We describe the design of the elements of the spectrometer system in the following. 2.1. Particle tracking devices The solenoidal magnet generates 0.3 T field in a volume of 1800 mm in length and 1422 mm in diameter. A CDC is installed in the magnetic field to measure trajectories and estimate momenta of charged particles emitted from the target. The length and diameter of the active volume of CDC are 1500 and 818 mm, respectively. The angular coverage of CDC is from 15 to 90 in the polar angle and
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2.06 m Return York Solenoid Coil
Inner Hodoscope (IH)
Outer Hodoscope (OH)
Vaccum Pipe
CDC Beam
CDC
Target
Outer Hodoscope (OH)
Flat Hodoscope (FH)
1.66 m
Fig. 1. Side view of the spectrometer system for the measurements of the proton-induced rare hadronic reactions constructed at the ring-cyclotron experimental facility of RCNP. See text for more details. Table 1 CDC wire configurations Layer
Cells/ layer
Wires/ cell
Wire position (stereo angle)
1st stereo 1 2 3 1st axial 2nd stereo
48 48 48 36 48
1 1 1 4 3
2nd axial
40
4
145 mm ð4:6 Þ 160 mm ð5:1 Þ 175 mm ð2:8 ) 243 mm, 251 mm, 259 mm, 267 mm 305 mm ð4:5 Þ, 313 mm ð4:6 Þ, 321 mm ð4:7 Þ 352 mm, 360 mm, 368 mm, 376 mm
Number of cells in a layer, number of sense wires in a cell and radius of each wire position in mm unit are listed. The wire angles are also listed for the stereo layers.
2p in the azimuthal angle. CDC has a 220 mm+ cylindrical hole in the center for the beam vacuum pipe and inner detectors. CDC has 2 axial and 2 stereo layers, and the wire configurations are listed in Table 1. The wires of the 1st stereo layer are 900 mm in length, and other wires have a 1500 mm length. Fig. 2 shows a part of the cell structures of the 1st stereo and the 1st axial layers. The 1st stereo layer has a simple box-type cell structure. The filled and open circles show the positions of the sense wires and the potential wires, respectively. The maximum drift length is about 12 mm. The filled boxes show the shield wires. The cell structure of the 1st axial layer is also shown in Fig. 2. The filled and open circles show the sense and the potential wires again, and the open boxes show the position
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particle track
1st axial
Fig. 3. Cross-sectional view of the IH layers. IH has two identical layers and each layer consists of 45 modules of plastic scintillation counters. One mm+ WLS-fibers are attached to both sides of the 10ðW Þ 400ðLÞ 1ðTÞ mm3 plastic scintillators.
1st stereo (3) 1st stereo (2) 1st stereo (1)
Window
Fig. 2. Schematic drawing of the cell structure of the 1st stereo and the 1st axial layers. The filled and open circles are the sense wires and the potential wires, respectively. The filled and open boxes show the shield wires and the guard wires, respectively.
of the guard wires which keep the field gradient in the drift space uniform. A typical drift length is 23 mm. The 2nd stereo and the 2nd axial layers have the similar cell structure as that of the 1st axial layer. Typical drift lengths are 21 and 24 mm for the 2nd stereo and the 2nd axial layers, respectively. The sense wires are Au-coated W wires with a 20 mm diameter, and the potential and the guard wires are Aucoated Al wires with a 80 mm diameter. The signals from 592 sense wires are read out by pre-amplifier cards with the Amplifier-Shaper-Discriminator (ASD) chips developed at KEK for the ATLAS experiment. The chamber gas is a mixture of Arð90%Þ þ CH4 ð10%Þ. The maximum electron drift time is about 200 ns for the 1st stereo layer and about 400 ns for other layers. Typical position resolutions of CDC are 300 mm and 3 mm (rms) in the r–f plane and the z-direction, respectively.
shown in Fig. 3. The IH layers surround the beam vacuum pipe at a radius of 90 mm. The scintillation light from an IH module is wavelength shifted and collected by 1 mm+ double-clad WLS-fibers (Kuraray Y-11(200)M) attached to both sides of the plastic scintillator. The collected light is read out by a multi-anode photomultiplier tube (Hamamatsu H6568-10). The IH modules are optically isolated from each other by Al-mylar and black vinyl sheets which are not shown in Fig. 3. OH consists of 52 modules of plastic scintillation counters, and each plastic scintillator has a 200ðW Þ 1500ðLÞ 3ðTÞ mm3 size. About a half of the OH modules are read out by fine-mesh dynode photomultiplier tubes (Hamamatsu H6614-01) attached to both ends of the plastic scintillators. Another half of the OH modules are read out by the WLS-fibers and the multi-anode photomultiplier tubes. The arrangement of OH is shown in Fig. 4. The OH modules are installed at the distance of 57 cm from the target and have a tilt angle of 36 which is optimized for the online triggering of negative-pions to be discussed in Section 3. The raw signal shapes from the multi-anode photomultiplier tubes for the IH modules and a half of the OH modules change largely event-by-event even for the same energy deposit. It comes from the relatively slow light emission of the WLS-fibers, the small number of photoelectrons and the fast rise-time of the photomultiplier tubes. To improve the event-by-event signal shape stability, the raw signals are fed into timing filter amplifiers, and the output signals are used for the discriminator and ADC inputs. The addition of the timing filter amplifiers improves the performance of the selection of the energy deposit in OH in the online trigger level to be discussed in Section 3.
2.2. Detectors for trigger purpose 2.3. Other detectors An inner hodoscope (IH) and an outer hodoscope (OH) are installed to produce the online trigger signals (see Fig. 1). IH has two identical layers, and a layer consists of 45 modules of plastic scintillation counters with a 10ðW Þ 400ðLÞ 1ðTÞ mm3 sensitive volume. The positions of the IH layers are shifted by a half width of the modules as
The flat hodoscope (FH) consists of 12 modules of plastic scintillation counters same as the OH modules in size (see Figs. 1 and 4). The FH modules are installed between CDC and the OH modules. The light signals from FH are detected by the Hamamatsu fine-mesh dynode
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Return York
FH modules
OH modules vacuum window
IH
CDC
Return York
Fig. 4. Cross-sectional beam view of the solenoidal magnet and the detector system. Charged particle tracks are also shown. The arrow with the solid curve and the arrow with the dashed curve correspond to a negatively and a positively charged particle tracks, respectively.
photomultiplier tubes attached to both ends of the FH scintillators. FH is used for the time-of-flight (TOF) measurement together with the RF signal from the accelerator. A typical TOF resolution is 500 ps (rms). 3. Trigger system for measurements of rare hadronic reactions 3.1. Kinematical parameters for trigger design Before the discussions on the trigger system, we wish to show kinematical conditions which affect to the design and the performance of the trigger system. Fig. 5 shows momentum distributions of negative-pions by simulation calculations. The absolute values of momenta (a) and the transverse momenta (b) of p from the subthreshold ðp; pþ p Þ reaction at T p ¼ 430 MeV, and those from the nðp; LÞp reaction at T p ¼ 400 MeV followed by the L hyperon decay, L ! pp , are shown in (c) and (d), respectively. The momenta of p range up to 200 MeV/c for both reactions, and the values of momenta are considerably smaller than the beam momentum, about 950 MeV/c, and the proton momenta from the inelastic scattering, about 500 MeV/c. At the lower ends of the momentum distributions, the stopping ranges of the negative-pions are comparable to the overall thickness of the trigger detectors, typically 1 g=cm2 . Due to the stopping of the negative-pions in the middle of the trigger detectors,
177
we expect a large inefficiency of the online trigger in the region of the pion momentum smaller than 70 MeV/c which corresponds to the transverse momentum of about 40 MeV/c. The plots also tell that the transverse momenta of most of the negative-pions are smaller than 100 MeV/c. The bending angle of the low pT negative-pions is larger than 30 and the position displacement from the straight track is larger than 15 cm at the OH position in the 0.3 T solenoidal magnetic field. So, we can distinguish the tracks of negative-pions from the high-momentum proton tracks in the online trigger level. By taking into account these kinematical conditions, we discuss the design of the trigger system in the following. 3.2. Negatively charged particle trigger with OH In the proton-induced reactions off a nuclear target at the beam kinetic energy around T p ¼ 400 MeV, major ejectiles are protons and neutrons. Since we attempt to measure the pþ p pairs or the decay of L hyperon, L ! pp , the negative-pions can be distinguished from the background protons and neutrons easily by looking at the difference of particle charges, at least, in offline analyses. Our idea is that we apply the particle discrimination by the difference of the particle charges in the online trigger level. The production cross-section of the negative-pions at this energy region, roughly 70 mb=nucleon, is considerably smaller than the total reaction cross-section, about 30 mb/nucleon. So, selection of the negatively charged particles in the trigger level may reduce the experimental trigger rate drastically. Other sources of the negatively charged particles are electrons from the Dalitz-decay of p0 (p0 ! geþ e ) or the g ! eþ e conversion after the p0 ! gg decay. The amount of the electrons is expected to be smaller than the negative-pions, and the deterioration of the trigger quality by the electrons is small. The discrimination by particle charges can be achieved by the dedicated arrangement of the OH modules. The OH modules have tilt angles, and the number of hit OH modules by single charged particle depends on the curvature of the particle track. The tilt angle is set so that a negatively charged particle may hit three adjacent OH modules or more and a positive charged particle can hit only two OH modules or less. Typical particle trajectories are shown in Fig. 4 by the arrow with the solid curve (negatively charged) and the arrow with the dashed curve (positively charged). The track of negatively charged particle hits three adjacent OH modules and the track of positively charged particle goes through only one OH module in Fig. 4. The geometry of OH to satisfy the condition of the negatively charged particle trigger is determined by the following equation: sin aOH ¼
R2 ðW =2Þ2 tan 2yOH WR
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7000
9000 8000
6000
7000 5000 5000
Counts
Counts
6000
4000 3000
4000 3000 2000
2000 1000
1000
0
0 0
50 100 150 200 Pion Momentum (MeV/c)
250
0
50
150 100 Pion pT (MeV/c)
200
0
50
100 150 Pion pT (MeV/c)
200
18000 16000
6000
14000
5000
12000
4000
10000
Counts
Counts
7000
3000
8000 6000
2000
4000
1000
2000 0
0 0
50 100 150 200 Pion Momentum (MeV/c)
250
Fig. 5. Distributions of momenta of negative-pions by simulation calculations. The distributions of (a) the absolute value of momentum and (b) the transverse momentum from the ðp; pþ p Þ reaction at T p ¼ 430 MeV, and those from the nðp; LÞp reaction at T p ¼ 400 MeV followed by the L hyperon decay, L ! pp , are shown in (c) and (d), respectively.
where aOH and yOH are the OH tilt angle and the separation angle between adjacent OH modules, respectively, as shown in Fig. 6. R and W are the distance from the beam to the center of the OH module and the width of the OH modules, respectively. By using the values, R ¼ 57 cm, W ¼ 20 cm and yOH ¼ 6 , we obtain the OH tilt angle aOH ¼ 36 . The geometrical parameters of OH with the tilt angle are listed in Table 2. The OH geometry determines the trigger efficiency of negatively charged particles with the requirement of the three and more adjacent OH hits. Fig. 7 shows the calculated efficiency of the negatively charged particle trigger as a function of the transverse momentum of particles (solid curve). Low pT particles emitted from the target cannot reach to OH in the solenoidal magnetic field, so the trigger efficiency is zero for particles pT o22 MeV=c. Negatively charged particles with 22 MeV=copT o74 MeV=c always hit three and more OH modules, and the geometrical trigger efficiency is 100%. A part of negatively charged particles with the transverse momenta larger than 74 MeV/c
hit only two OH modules depending on the azimuthal angle of the emission from the target, so the trigger efficiency decreases slowly as the increase of the transverse momentum. The vertical dashed line in the figure shows a cut due to the particle stopping before reaching the last OH module for the low momentum negative-pions as discussed in Section 3.1. The trigger efficiency is better than 55% for the negatively charged particles with the transverse momentum pT o120 MeV=c which is close to the upper ends of the transverse momentum distributions of the negative-pions as shown in Fig. 5. Details of the trigger efficiency calculation are given in Appendix A. The rejection of positively charged particles by the trigger is perfect in an ideal situation, but in the practical setup there are several sources of the inefficiency of the rejection. One source of the inefficiency is the particle scattering in the OH modules. High-momentum protons emitted from the target run through one or two OH modules, and nuclear scatterings may occur in the OH
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OH modules 1 αOH 0.8 Trigger Efficiency
W
Rmax
0.6
Pion Stopping Range is Long Enough
0.4
θOH Rmin
R
0.2
0 0
Beam Fig. 6. Schematic beam view of the OH geometry for the design of the negatively charged particle trigger.
Table 2 Geometrical parameters of outer hodoscope (OH) OH geometrical parameters
Values
Angular separation between modules (yOH ) Tilt angle (aOH ) Radius of module center (R) Radius of scintillator inner edge (Rmin ) Radius of scintillator outer edge (Rmax )
6 36 57 cm 49.3 cm 65.4 cm
Definitions of the parameters are shown in Fig. 6.
modules. If the proton scattering angle is large enough, the scattered proton may hit several OH modules additionally, and more than two adjacent OH modules are fired in total. A typical thickness of the OH modules along the initial proton tracks is about 1 g=cm2 and the nuclear collision length for the plastic scintillator is about 60 g=cm2 . So, we expect a few % level of the inefficiency of the proton rejection due to the scattering in the OH modules. Other minor sources of the inefficiency are the accidental coincidence of the OH module hits, the proton nuclear scattering in IH and CDC, the proton knock out by energetic neutrons in OH, etc. The details of the inefficiency of the proton rejection are discussed in Section 4.3.
50
100 150 200 250 Transverse Momentum pT (MeV/c)
300
Fig. 7. Efficiency of the negatively charged particle trigger as a function of the transverse momentum pT (solid curve). The vertical dashed line shows a cut due to the particle stopping before reaching the last OH module for the low momentum negative-pions.
can be used for the selection of the negatively charged particles in the online trigger level. The idea of the particle selection is close to that of the trigger by the three adjacent OH hits described in the previous section, but this trigger by using the IH–OH correlation is quite complementary with the previous trigger method. Fig. 8 shows correlations between the azimuthal angle difference between IH and OH hits, fðIHÞ fðOHÞ, and the transverse momentum. The contour plot is obtained from experimental data. The solid and dashed contour lines correspond to distributions of negatively charged and positively charged particles, respectively. The distributions are consistent with the expected correlations by simple geometrical calculations for the negatively charged (solid curve) and positively charged particles (dashed curve). The figure shows that we can distinguish negatively charged particles from positively charged particles by setting appropriate cuts on the azimuthal angle difference. Since the azimuthal angle separation between OH modules is 6 and the separation between IH modules is 4 , we set a relatively loose cut on the azimuthal angle difference (vertical line). We expect the IH–OH correlation trigger rejects about a half of the background protons with high transverse momenta.
3.3. Trigger by IH and OH hit correlation
3.4. Two-track event trigger
The correlation of the IH hit and the OH hit positions also has information of the sign of the particle charge especially for particles with low transverse momenta, and
In the measurement of the ðp; pþ p Þ reaction, we require two tracks in the offline analysis, one positively charged and one negatively charged tracks. To improve the analysis
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180 300
225 200
250
Negative Pion
200
150
Rejected
Select
Counts
Transverse Momentum (MeV/c)
175
150
100
Positive Charge
125 100
Negative Charge
Proton 75 50
50
25 0 -50
-40
-30
-20
-10
0
10
20
30
40
50
φ (IH) - φ (OH) (degrees)
Fig. 8. Correlations between the azimuthal angle difference between IH and OH hits, fðIHÞ fðOHÞ, and the transverse momentum. The contour plot is obtained from experimental data. The solid and dashed curves are expected correlations obtained from simple geometrical calculations for negatively charged and positively charged particles, respectively.
efficiency, two-track event candidates are selected by the online trigger. The selection of events with two-track candidates is made by looking at the hit pattern or hit multiplicity of the IH modules. Since the IH has two layers made of 45 modules for each layer and a module has geometrical overlaps with adjacent IH modules in the different layer as shown in Fig. 3, a simple counting of hit multiplicity of the IH modules is not adequate. In the hit multiplicity calculation, a module ID difference larger than 1 is required to identify the two-track event candidates from the IH signals. 3.5. Trigger by energy loss in OH In the nðp; LÞp reaction followed by the L ! pp decay, the energy deposit of the decay p in OH is considerably smaller than that of background protons. So, a discrimination of the background protons may be possible by the selection of the energy deposit in OH. Fig. 9 shows typical distributions of the energy deposit in an OH module by a p (solid histogram) and a proton (dashed histogram) obtained from experimental data. The figure shows that the energy deposit of protons is about twice as large as that of negative-pions. We can reject about 50% of protons in keeping more than 95% of negative-pions with an energy threshold of 5 MeV. In the case of the combination of this energy loss trigger with the negatively charged particle trigger by OH, the energy loss upper limit, 5 MeV, is applied 3 times to the
0 0
2
4
8 10 6 Energy Deposit in OH (MeV)
12
14
Fig. 9. Typical distributions of the energy deposit in an OH module by a p (solid) and a proton (dashed) obtained from experimental data.
three adjacent OH modules. So, we expect the proton rejection inefficiency is about ð0:5Þ3 13% if the energy loss distribution of protons is mainly coming from eventby-event and module-by-module energy loss fluctuations. The proton rejection inefficiency is discussed in detail in Section 4.5. 4. Performance of trigger system 4.1. Experimental condition for trigger test We evaluated the performance of the trigger system by using T p ¼ 430 MeV proton beams at the WSS beam course of the ring-cyclotron experimental facility in RCNP of Osaka University. A copper target of 50 mm in thickness was used. The proton beam size on the target was less than 2 2 mm2 and the beam intensity was typically 0.1 nA. The IH and OH signals were processed by ordinary NIM front-end circuits. The processed IH and OH signals and CDC signals were sent to the DAQ system. High density FASTBUS ADC and TDC modules were used for the signal digitization. A dual buffer memory and an on-board CPU in a VME system were used for the data collection and the event building. A remote PC that was connected to the on-board CPU via network was used for the data recording on the disk. The logic signals from IH and OH discriminators were fed into programmable logic modules to generate trigger signals. Minimum bias data were also taken with the following simple trigger condition: ðminimum biasÞ ðIH ORÞ ðOH ORÞ.
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The minimum bias data were used as reference data to evaluate the performance of the additional trigger conditions. 4.2. Trigger circuits The programmable logic module, Universal Logic Module (ULM), had been developed at Osaka University to construct the trigger system with the large number of logic inputs. ULM has 16 4 channels of ECL inputs and 16 2 channels of ECL outputs. ULM also has auxiliary NIM inputs (two channels) and outputs (two channels) usually used for inter module communications. ULM has a programmable logic device (Altera APEX20KE) to handle input logic signals and generate required trigger signals. Fig. 10 shows the logic diagram of the trigger circuit made of ULMs. The trigger circuit consists of three blocks (A, B and C), and each trigger block made of several ULMs. The OH signals are discriminated with a low and a high thresholds. The OH discriminator output signals are fed into the trigger block A. The trigger block A takes a coincidence between the upstream and the downstream photomultiplier signals, takes triple-fold coincidences for all combinations of three adjacent OH modules, and then produces an OR signal of the coincidences as the negatively charged particle trigger by OH. The logic block A also produces the hit pattern of the triple-hold coincidences to be sent to the trigger block C. The block A can optionally generates the trigger signal with the requirement of the
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energy loss trigger by using the logic signal of the high threshold discriminators as veto signals. The discriminator signals of IH are fed into the trigger block B to generate the two-track event trigger. In the block B, the 90 IH logic input signals are reduced to 45 channels of hit pattern information to be sent to the trigger block C. The outputs from the trigger blocks A and B are combined in the trigger block C to generate the negatively charged particle trigger by the IH and OH hit correlation. Finally, a combination of the triggers is used as the main trigger for the DAQ start. 4.3. Performance of negatively charge particle trigger by OH A set of data was taken with the trigger to select the negatively charged particles by the three adjacent hits of the OH modules described in Section 3.2, and compared with the minimum bias data. Fig. 11 shows distributions of the transverse momentum per unit charge for all particles obtained with the two different trigger conditions, the minimum biased trigger (histogram) and the negatively charged particle trigger (data points). The distributions are normalized with the integrated beam current, and simple geometrical acceptances are corrected for. The positive values of the horizontal axis correspond to the yield of the positively charged particles, and most of the particles are protons in this experiment. The negative values correspond to the negatively charged particles and these are mainly negative-pions. Negative Charge
104
Positive Charge
Relative Yield (a.u.)
103
102
10
1 -400
-300
-200
-100
0
100
200
300
400
Transverse Momentum per Charge (MeV/c/e)
Fig. 10. Logic diagram of the trigger circuit made of programmable logic modules. The OH signals are discriminated with a low and a high thresholds. The IH and OH discriminator signals are fed into the trigger blocks A, B and C, to generate the trigger signals. See text for more details.
Fig. 11. Distributions of the transverse momentum per unit charge for all particles obtained with the two different trigger conditions, the minimum biased trigger (histogram) and the negatively charged particle trigger (data points).
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As one can see, the positively charged particles are reduced by about 2 orders of magnitude by the negatively charged particle trigger while the number of the negatively charged particles is changed little in the pT region from 50 to 150 MeV/c. So, the negatively charged particle trigger is working well as designed. The rate of the negatively charged particle trigger is listed in Table 3 together with the rate of the minimum bias trigger. The trigger reduction 1 factor from the minimum biased trigger is 30k=2:2M 73 , and it is consistent with the proton reduction factor of the pT distribution in Fig. 11 and consistent with the possible inefficiency of the proton rejection due to the proton large angle scattering in the OH modules (a few % level of inefficiency). As discussed in Section 3.2, the trigger efficiency of the negative-pions is a function of pT . To see the pT dependence, the ratio of the pT spectra with and without the negatively charged particle trigger is shown in Fig. 12. The points correspond to the experimental data, and the solid curve is the result of the geometrical trigger efficiency calculation (essentially same as the curve in Fig. 7). The general tendency of the pT dependence of the trigger efficiency is well reproduced by the simple calculation, but about 20% of inefficiency is observed for the experimental data even at the low pT region, pT 70 MeV=c. An apparent inefficiency of the trigger partly comes from the contamination of the background protons into the negatively charged particle region in Fig. 11. If a highmomentum proton is scattered in CDC, the CDC hit pattern may become similar with that of a negative-pion. The signature of such miss calculations of pT and particle charge can be seen in the pT =chargeo 300 MeV=c=e region in Fig. 11 because a negatively charged particle with such high transverse momentum is quite scarce kinematically. This proton contamination effect is relatively large at the high pT region, pT 300 MeV=c. A real inefficiency of the trigger may come from the discriminator threshold of OH modules. Since the PMT signals from the OH modules by the pion hits are relatively low, 5–10% level of the pion hits may have signals below Table 3 Online trigger rate, fraction of negative-pions and fraction of multi-track events for different triggers, (A) negatively charged particle trigger by OH, (B) trigger by IH–OH hit correlation, (C) two-track trigger, (D) trigger by energy loss in OH and combinations of them Trigger types
Trigger/ nC
p /all (%)
Multi-track fraction (%)
Minimum bias trigger Neg. charge trigger (A) IH–OH correlation (B) and (A) Two-track trigger (C)
2.2M 30k 15k
p0:21 8.2 16
11.0 14.4 –
1.3M
–
16.5
(A) and (C) (A), (B) and (C) Energy loss in OH (D) and (A)
– 12k 9.0k
– 15 30
19.3 19.2 –
1.2
1
0.8 Trigger Efficiency
182
0.6
0.4
0.2
0 0
50
100 150 200 Transverse Momentum (MeV/c)
250
300
Fig. 12. The trigger efficiency is extracted by dividing the pT spectrum with the negatively charged particle trigger by that from the minimum biased data (data points). The curve is essentially same as the curve in Fig. 7 that shows the trigger efficiency by the simple geometrical calculation.
the discriminator threshold. The minimum bias trigger requires only one OH signal above the discriminator threshold, and the negatively charged particle trigger by OH requires at least three OH signals above the threshold. It means the negatively charged particle trigger may have additional 10–20% inefficiency due to the low signal pulse height in comparison with the yield of the minimum bias trigger. This inefficiency may explain the trigger efficiency lower than the simple geometrical estimation in the low pT region. The fractions of p tracks in the acquired data are also listed in Table 3. The sign of inequality for the minimum bias trigger is coming from the possible proton contamination into the negative-pion events. The improvement of the fraction of p tracks from the minimum bias trigger to the negatively charged particle trigger is 8:2%=0:21% 39, and this value is roughly a half of the factor of the trigger rate change, about 73, mentioned previously. This difference can be explained by the trigger efficiency of the negative-pions averaged with the transverse momentum distribution. The averaged efficiency is about 60%. 4.4. Performance of trigger by IH and OH hit correlation Fig. 13 shows the distributions of the transverse momentum per unit charge. The histogram is the distribution obtained with the negatively charged particle trigger by OH. By requiring the additional trigger condition of the IH–OH hit correlation, the distribution with the data
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1400 Negative Charge
Positive Charge
1200
Relative Yield (a.u.)
1000
800
600
400
200
0 -200
-100 0 100 200 300 Transverse Momentum per Charge (MeV/c/e)
400
Fig. 13. Distribution of the transverse momentum per unit charge. The histogram shows the distribution obtained with the negatively charged particle trigger by OH. By requiring the additional trigger condition of the IH–OH hit correlation, the distribution by the data points is obtained.
points is obtained. The clear reduction of the yield of the positively charged particles, mainly protons, indicates a successful reduction of the background protons by the IH–OH correlation trigger. The reduction factor of the background protons is about a half and it is consistent with the cut criteria. The yield of the negatively charged particles is changed a little, the efficiency of the trigger is about 93%. The trigger rate and fraction of p tracks for the IH–OH correlation trigger is also listed in Table 3. The trigger rate reduction factor is 15k=30k ¼ 12, the improvement of the fraction of p tracks is 16%=8:2% 2:0 and these values are also consistent with the cut criteria. 4.5. Other triggers and overall trigger performance The triggers discussed above are based on the difference of the signs of the particle charges. We also tested other types of triggers dedicated for either of the rare hadronic reactions, the ðp; pþ p Þ reaction or the nðp; LÞp reaction followed by the L ! pp decay. For the ðp; pþ p Þ reaction, we evaluated the performance of the two-track trigger. The result is listed in Table 3. The fraction of the multi-track events is 11.0% for the minimum bias trigger data. The two-track trigger improves the fraction to 16.5%. The fraction is improved already in the negatively charged particle trigger by OH to 14.4%, and the combination of the two types of trigger results in the multi-track event fraction of 19.3%. The fraction is improved by a factor 19:3%=11:0% 1:8. For the ðp; pþ p Þ reaction measurement, we can use the
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ðAÞ ðBÞ ðCÞ trigger condition in Table 3. The trigger condition reduces the trigger rate by the factor 1 and improves the fraction of p tracks 12k=2:2M 180 by a factor 15%=0:21% 71. The trigger rate 12k trigger/nC allows us to set the beam intensity to about 0.1 nA for the 50 mm Cu target if we assume the capability of the DAQ system is about 1k event/s. This beam intensity corresponds to the accessible cross-section of roughly 0.3 nb/nucleon in a day for the ðp; pþ p Þ reaction. In the estimation of the accessible crosssection, we used an effective nucleon number 5 and an overall efficiency of 0.03 due to the trigger efficiency and the detector acceptance for pþ p pairs. The accessible cross-section is large enough to measure the Cuðp; pþ p ÞX reaction within a few days of beamtime. For the nðp; LÞp reaction measurement, we evaluated the performance of the trigger by the energy loss in OH. The trigger rate of the energy loss trigger is listed in Table 3. The energy loss cut is made for all three adjacent OH modules by using the signals from the high threshold discriminators as vetos, and the cut rejects about a half of proton hits as shown in Fig. 9. If the distributions of the pulse heights of the three OH modules for the background protons mainly come from pulse height fluctuations, we expect a factor of 18 ¼ ð12 Þ3 background reduction by the threefold coincidence of the OH signals. On the other hand, if the pulse height of three OH modules have strong correlations, the reduction factor should be close to 12. The measured trigger rates are 9.0k and 30k and the fractions of p tracks are 30% and 8.2% with and without the trigger by the energy loss in OH, respectively. From the trigger rates and the fractions of p tracks, we can make a simple estimation of the background reduction factor r as follows: 30k ð1 0:082Þ r ¼ 9:0k ð1 0:30Þ
(1)
and we obtain the background reduction factor r 1=4:4. The trigger efficiency of the negative-pions can be estimated by the same manner as Eq. (1), and the efficiency is close to 100%. The obtained reduction factor r is considerably smaller than 12. The proton background reduction factor can be estimated more precisely from the ratio of the transverse momentum spectra with and without the OH energy loss trigger. Fig. 14 shows the ratio of the proton yields as a function of the transverse momentum. There is a strong pT dependence of the ratio. Since the averaged transverse momentum of protons is about 250 MeV/c, the typical ratio of the background proton distributions with and without the energy loss trigger is 0:14 17. The larger values of the ratio for the lower transverse momentum protons come from the contribution of the quasi-free Nðp; p pÞN reaction. Protons associated with the quasi-free negativepion production are also accumulated in the ratio calculation. It is an apparent inefficiency of the proton rejection and the actual proton rejection factor is believed to be close to 17. The large reduction factor means the
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performance of the trigger system was evaluated by using T p ¼ 430 MeV proton beams. An excellent reduction, more than 2 orders of magnitude reduction, of the background events was achieved, and the efficiency and the inefficiency of the trigger system were well understood by the simple considerations.
0.5 0.45
Reduction Factor of Proton Tracks
0.4 0.35
Acknowledgments
0.3
We acknowledge the outstanding work of the RCNP Accelerator group for delivering excellent quality of proton beams. We thank H. Toki, K. Hatanaka, K. Sato, S. Ninomiya, K. Nagayama and M. Uraki for the support of the construction of the beam line and the spectrometer system in the ring-cyclotron experimental facility of RCNP. We also thank M. Nomachi for the development of the programmable logic module (ULM) at Osaka University. This research was supported in part by the Ministry of Education, Science, Sports and Culture of Japan.
0.25 0.2 0.15 0.1 0.05 0 0
50
100
150
200
250
300
350
400
Transverse Momentum (MeV/c) Fig. 14. Ratio of the proton distributions with and without the OH energy loss trigger as a function of the transverse momentum.
distributions of the OH signal pulse heights from the background protons mainly come from event-by-event and module-by-module fluctuations. For the nðp; LÞp reaction measurement, we can use the trigger condition (A)(D) in Table 3. The trigger condition 1 reduces the trigger rate by a factor 9.0k/2.2M 240 , and the fraction of p tracks is improved by a factor 30%/0.21%140. 5. Summary We developed a trigger system for the measurement of the proton-induced rare hadronic reactions around the beam kinetic energy T p ¼ 400 MeV in which negativepions were created in the final states. The trigger system was based on the highly segmented trigger scintillation detectors and the programmable logic modules, and was designed to enhance events with the negative-pion production by the difference of the curvatures of the particle tracks in the magnetic field. Since the production crosssection of the negative-pion by the proton-induced reactions is expected to be smaller by about 3 orders of magnitude than the total cross-section at the beam energy, we expected large improvement by the trigger. The construction of the trigger system was not trivial due to a large detector acceptance that was inevitable to measure the rare reactions. The modules of the OH were arranged with a tilt angle so that the hit multiplicity of adjacent OH modules can identify the charge sign of the particles from the target. The hit position correlation between the IH and OH was also used to enhance the negatively charged particles. The
Appendix A. Efficiency calculation of trigger by outer hodoscope Most of the parameters of the negatively charged particle trigger by the outer hodoscope (OH) can be calculated analytically if only the geometrical cuts are considered. Low momentum charged particles cannot reach to OH if the radius of the curvature of the track projected on the transverse plane is smaller than Rmin =2. The condition introduces a threshold of the transverse momentum as follows: Rmin 2 where B is the strength of the solenoidal magnetic field. Since the condition just grants single hit of the OH module, the trigger condition of the three adjacent hits in OH introduces a slightly higher cut of the transverse momentum as follows: pT 40:3B
pT 40:3B
Rmin 2 cos yOH
where yOH is the angular separation between the adjacent OH modules defined previously in Fig. 6. The trigger efficiency reaches 100% as the increase of pT at pT ¼ 0:3B
Rmin . 2 cos 32 yOH
Since these pT values are very close to each other if yOH is small, the pT dependence of the trigger efficiency has a sharp rise at the low pT end. The trigger efficiency is kept 100% up to the transverse momentum: pT ¼ 0:3B
AB 2 sin b
(A.1)
where AB and b are the distance between points A and B and the angle ffAOB in Fig. A1, respectively. Above the
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OH modules
trigger efficiency is written as follows: 1¼
B
b ðdB dA Þ yOH
dB ¼ sin1 A
Rmax
β Rmin
185
Rmax ; 2r
dA ¼ sin1
Rmin 2r
where r is the radius of the curvature of the particle track projected on the transverse plane, r ¼ pT =0:3B. Rmax and Rmin are defined in Fig. 6. These calculations on the negatively charged particle trigger describe the pT dependence of the trigger efficiency shown in Fig. 7. References
O Fig. A1. Schematic beam view of the OH geometrical parameters for the efficiency calculation of the negatively charged particle trigger. The point O corresponds to the beam line. The points A and B are the inner and the outer edges of the OH modules, respectively.
transverse momentum given by Eq. (A.1), the trigger efficiency decreases gradually as the increase of pT , and the
[1] A. Sakaguchi, et al., RCNP proposal E251, 2004. [2] T. Kishimoto, et al., RCNP proposal E122, 1998. [3] T. Kishimoto, Nucl. Phys. A 629 (1998) 369; T. Kishimoto, et al., Nucl. Phys. A 663 (2000) 509; S. Ajimura, et al., Nucl. Phys. A 691 (2001) 312. [4] A. Parren˜o, A. Ramos, N.G. Kelkar, C. Bennhold, Phys. Rev. C 59 (1999) 2122. [5] T. Inoue, K. Sasaki, M. Oka, Nucl. Phys. A 684 (2001) 478. [6] B.J. VerWest, R.A. Arndt, Phys. Rev. C 25 (1979) 1979. [7] M. Daum, et al., Eur. Phys. J. C 23 (2002) 43.