Journal of the Korean Physical Society, Vol. 62, No. 5, March 2013, pp. 839∼844
A Pulse Shape Discrimination Method with CsI Using the Ratio of Areas for Identifying Neutrons and Gamma Rays Jong-Kwan Woo,∗ Je Wou Ko, Silin Na and Yong Joo Kim Department of Physics, Jeju National University, Jeju 690-756, Korea
HyoSang Lee Department of Physics, Pusan National University, Pusan 609-735, Korea (Received 11 October 2012, in final form 17 January 2013) Identification of gamma rays and neutrons is a fundamental technique used in elementary particle physics experiments, such as the search for dark matter, double beta decay, etc. The pulse shape discrimination (PSD) methods with liquid and crystal scintillators, a well-known technology, have been under development for several decades. The PSD methods with crystal scintillators are known to be less stable and less effective than PSD methods with liquid crystals. Here, an improved PSD technique with a CsI crystal scintillator based on the well-known PSD methods with liquid scintillators is introduced. In this study, we found the possibility that the proposed PSD with a CsI crystal scintillator provides a performance to discriminate gamma rays and neutrons equivalent to that of a PSD with a liquid scintillator. PACS numbers: 95.35.+d, 20.40.Mc, 87.66.-a Keywords: Pulse shape discrimination, PSD, Crystal scintillator, CsI, Gamma ray, Neutron DOI: 10.3938/jkps.62.839
I. INTRODUCTION
KL → π0 ν ν¯. Including the two examples mentioned above, many experiments need fine technology to identify MIPs and HIPs. We will introduce a PSD method for the gamma ray, an MIP, and a neutron, an HIP, using a CsI crystal scintillator. A PSD method with a liquid scintillator, a well-known technology, and its applications have been under development and study for several decades. Even though PSD with a liquid scintillator gives a more reliable and stable output than that with a crystal scintillator, more careful consideration is warranted. PSD with a crystal scintillator can compensate for the disadvantages of PSD with a liquid scintillator giving a better solution in some areas. The disadvantages of PSD with a crystal scintillator are the lower particle discrimination efficiency and the very weak signal from electrons in the target material. We will introduce an improved PSD method with a CsI crystal scintillator in this study.
Identification of gamma rays and neutrons is a fundamental technique in elementary particle physics experiments, such as the search for dark matter, double beta decay, etc. As the first example, in the scattering between a target nucleus and a weakly interaction massive particle (WIMP), a strong candidate for non-baryonic supersymmetric dark matter [1], the WIMP behaves as a heavy ionization particle (HIP). In the WIMP search, a major background comes from gamma rays that behave as minimum ionization particles (MIPs). The WIMP and the neutron, categorized as HIP particles, produce similar signals. Particle identification, especially that of a neutron, an HIP, and a gamma ray, an MIP, plays a key role in the WIMP search experiment. As the second example, we can use a pulse shape discrimination (PSD) method in elementary particle physics experiments based on an accelerator, for example, an experiment measuring the branching ratio of KL → π0 ν ν¯ decay. A major mechanism of detection for KL → π0 ν ν¯ decay is to measure the two gamma rays from the π0 → γγ decay following KL → π0 ν ν¯ decay [2]. Thus, separation of the gamma ray from the neutron background induced in the beam line plays a key role in measuring the branching ratio of ∗ E-mail:
II. BACKGROUNDS OF PSDS WITH LIQUID AND CRYSTAL SCINTILLATORS 1. PSD with a Liquid Scintillator
A liquid scintillator, especially a liquid xenon scintillator, provides many advantages, for example, the discrimination of HIPs (neutrons, alphas, Muons, WIMPs,
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etc.) from MIPs (gamma ray, bosons, etc.) [3–5]. Many studies of PSD with crystal and liquid scintillators have been reported over several decades [6–9]. We can categorize the types of changes in the target nucleus when the incident particle hits a target nucleus in the liquid state as excitation and ionization. Those two types of processes, excitation and ionization, occur simultaneously. The target nucleus emits a photon through the excitation process, the so-called scintillation process. This has been studied in detail for crystal and liquid scintillators [8–11]. The excitation process in a liquid scintillator follows the sub-processes described below: P + X + X → (excitation) → P + X ∗ + X → P + X2∗ → P + X + X + scintillation,
mechanism, we can identify the incident particles. Already, PSD methods with liquid scintillators have been proven [8–11] and have shown clear differences between the pulse shapes from gamma rays and from neutrons. The primary signal comes from the excited target nucleus and from recombination of the ionized target and ionized electrons while the secondary signal comes only from drift electrons. We can measure the very weak secondary signal with a crystal scintillator because ionized electrons hardly drift in a crystal scintillator. HIPs and MIPs provide different ratios of the amplitudes of the primary signal to the secondary signal, which gives a clue to identifing the two types of particles [11,12]. We can magnify the primary and the secondary signals by using various techniques [13–15].
(1)
where X and P stand for a target nucleus and an incident particle while X ∗ and X2∗ stand for an excited target nucleus and an excimer target molecule, respectively. The excimer target nucleus, X2∗ , finally emits a photon in a process called scintillation. A scintillator or scintillation detector is a material emitting a scintillation photon through elastic scattering with an incident particle. In the ionization process, a target atom is ionized through elastic scattering with an incident particle. The ionization process in a liquid scintillation process has been studied in detail. The ionization process in a liquid scintillator follows the sub-processes [8–11] described below: P + X + X → (ionization) → P + X + X + + e → P + X2+ + e → (recombination) → P + X2∗ → P + X + X + scintillation, (2) where X + , X2+ , and e are an ionized target nucleus, an ionized target molecule, and an electron, respectively. The ionized nucleus recombines immediately with a normal target atom in a process we call a recombination process. One third of the ionized electrons are recombined with ionized target nuclei while 2/3 of the ionized electrons drift [8–11]. We can raise the recombination ratio a little by applying an electric field, increasing the pressure, etc. An ionized molecule X2+ combines with an electron and becomes an excimer molecule X2∗ . An excimer molecule X2∗ emits a scintillation photon when it decays into two normal nuclei [8–11]. Also, we call this photon in the ionization process as a scintillation photon. The excimer state target molecule X2∗ consists of two normal target atoms, 2X, that are combined in two types of states, singlet and triplet states. Because the triplet state is much more unstable than the singlet state, in other words, the triplet state decays much faster, 27 ns, than the triplet state, 2.2 s, this causes a difference in the ratio of the number of photons from the singlet to that from the triplet state for the incident particles. Using this
2. PSD with a Crystal Scintillator
In a crystal scintillator, compared with a liquid scintillator, we cannot easily use the ionized electrons because the ionized electrons do not move fast enough to make a good signal. In a classical way, we use only the scintillation photons from the primary signal in excitation and ionization, excluding the signal from the drift electrons in the ionization process. The same as the excitation process in a liquid scintillator, that in crystal scintillator occurs through the sub-processes described below: P + X + X → (excitation) → P + X ∗ + X → P + X2∗ → P + X + X + scintillation, (3) where X and P stand for a crystal target nucleus and an incident particle while X ∗ and X2∗ stand for an excited target nucleus and an excimer target molecule, respectively. The excimer target nucleus X2∗ emits a scintillation photon. We estimate that the coupling energy of the excimer target nucleus X2∗ in a crystal is lower than that in a liquid scintillator by several eV [10]. Even though the excimer target molecule Xn∗ can consist of several nucleus, we restrict our PSD with a crystal scintillator to only the excimer target nucleus X2∗ . In the ionization process, a crystal target atom is ionized through elastic scattering with an incident particle. The ionization process in a crystal scintillator occurs through the sub-processes described below: P + X + X → (ionization) → P + X + X + + e → P + X2+ + e → (recombination) → P + X2∗ → P + X + X + scintillation. (4) An ionized nucleus shifts to a normal target nucleus and recombines immediately with a normal target atom, the so-called shift-recombination process. We can assume that the recombination ratio of electrons in the crystal is bigger than that in a liquid scintillator, 1/3. The ratio
A Pulse Shape Discrimination Method with CsI Using the Ratio · · · – Jong-Kwan Woo et al.
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Table 1. Specifications of the neutron beam provided by the MC-50 cyclotron at KIRAMS. Specification Unit Beam current 40 µA Average input proton energy 35 MeV Target Beryllium Neutron production rate 2.75*1011 /hr/cm2 Energy of neutron < 5 MeV for 99% of neutrons Fig. 1. (Color online) Excitation and ionization in the crystal scintillator through elastic collisions with incident particles. X ∗ − X and X + − X stand for the excitation state and the ionization state, respectively.
of recombined nuclei will be studied in the near future. An ionized molecule X2+ combines with an electron and becomes an excimer molecule X2∗ . The excimer molecule X2∗ emits a scintillation photon when it decays into two normal nuclei. Both the processes of excitation and ionization produce an excimer state target molecule X2∗ . The excimer state target molecule X2∗ consists of two target particles 2X. These two target atoms are combined in two types of states, the singlet and the triplet states. The triplet state decays much faster than the singlet state because the triplet state is much more unstable than the singlet state, which causes a difference in the ratio of photons from the singlet and to the form of the triplet states for incident particles colliding with target crystal nuclei. Using this mechanism, we can identify the incident particles by using PSD with a crystal scintillator. From previous studies on liquid scintillators, the ratio of the amplitude of the signal from the singlet state, NS , to that of the signal from the triplet state, NT , differs with the type of incident particle [8,12]. In other words,
NS NT
M IP
>
NS NT
Fig. 2. (Color online) (left) Expected patterns of the signal induced by the MIP and the HIP, and (right) detected typical pulse shape induced the MIP (gamma ray) and the HIP (neutron). A red signal (upper side one in the right figure) stands for the signal from the MIP while a blue signal (lower side) stands for the signal from the HIP.
HIP
,
(5)
where NT and NS stand for the amplitude of the fast signal from the triplet state and the amplitude of the slow signal from the singlet state, respectively. We assume that the crystal scintillator provides the same pattern as a liquid one. Through elastic scattering, an HIP produces the fast signal more than an MIP does, so an HIP provides a broader signal than an MIP. Please refer to the left one in Fig. 2. The fast signal appears at around 27 ns for a liquid scintillator, which indicates the HIP emits more scintillation around 27 ns [8–11]. As a result, the signal pattern from an HIP drops slower than that from an MIP in several 10 ns. In contrast, the singlet state contributes less than the triplet state because the average decay time of the singlet state is on the level of a few seconds. We expect the signal patterns from a crystal scintillator to be the same as those from a liquid scintillator.
Fig. 3. (Color online) Experimental setup using the neutron beam provided by MC-50 cyclotron at KIRAMS.
III. EXPERIMENTAL SETUP Neutrons and gamma rays were provided through elastic scattering between a Be target and 50-MeV proton beam provided by the MC-50 cyclotron at the Korea Institute of Radiological and Medical Sciences (KIRAMS). After passing a collimator placed 150 cm away from the Be target, the neutron beam spread to a 54 × 54 cm2 region located 120 cm away from the collimator. We attached a photomultiplier tube (PMT) at the end of the CsI crystal scintillator (7.5 × 7.5 × 20 cm3 ) that was
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Fig. 4. (Color online) Integrated pulse and the fitted function. x = time in ns, and y = amplitude of the pulse.
Fig. 5. (Color online) Plot of the data taken at a distance of 10 cm away (D10) from the center of the neutron beam. The x-axis stands for the area of the tail while the y-axis stands for the area of the head. Both of the x and the y axes are in arbitrary units.
placed at a square region on the bottom. The distances between the CsI crystal and the center of the neutron beam were 10 cm (named D10) and 27 cm (named D27). We placed the CsI crystal 50 cm away from the beam center when we measured the cosmic background. Please refer to Fig. 3. Table 1 shows the specifications of the neutron beam provided by the MC-50 cyclotron at KIRAMS. Figure 2 shows the two typical pulses induced by gamma rays and neutrons. IV. ANALYSIS Using a CsI crystal scintillator, we measured 269 signals from neutrons, gamma rays, and cosmic backgrounds with high accuracy. We reproduced the shape of the signal and found a breakpoint to separate the signal for discriminating between the slow signal from the singlet state and the fast signal from the triplet state. Using a least-squares fit, we fitted the data points from 0 ns to 30 ns to a line y = ax + b while we fitted the data points from 60 to 100 ns to a line y = cx + d. Then, we found the cross point of the two lines, xc = −(b−d)/(a−c). We took this cross point, xc , as the breakpoint for separating the head from the tail of the pulse shape. Figure 4 shows a real pulse and the function fitted with the least-squares method. The fluctuating blue line (bold) stands for the real pulse while the red straight lines stand for the fitted functions. y = 2 ∗ 106 x − 0.1065 with χ2 = 0.7476 for the head while y = 175867x−0.0206 with χ2 = 0.3605 for the tail. The breakpoint of the line in Fig. 4 is about 40.7 ns, which is almost 1.5 times the average lifetime, 27 ns, of the triplet state of the excimer state, X2∗ , in a liquid scintillator. The areas of the head and the tail come mostly from the fast signal and the head slow signal, respectively. The ratio of areas, Area Areatail = 5.501 for the signal in Fig. 4, which is a typical type of HIP (neutron or Muon) signal. Now, let us define a rahead tio, RHT ≡ Area Areatail . For another example of an MIP
Fig. 6. (Color online) Plot of the data taken at a distance of 27 cm away (D27) from the center of the neutron beam. The x-axis stands for the area of the tail while the y-axis stands for the area of the head. Both the x and the y axes are in arbitrary units.
(gamma) signal, the right upper graph in Fig. 2, the breakpoint is about 20.4 ns, which is 0.75 times the average lifetime, 27 ns, of the triplet state of the excimer state, X2∗ , in a liquid scintillator. The ratio of areas is RHT = 27.0. With a brief comparison, we can find a clear difference in the two ratios, RHT s, for signals induced by HIPs and MIPs, the ratio for MIPs being much larger than that for HIPs. This means that the fast signal from the triplet state is included in the head and the tail simultaneously. Thus, the area of the tail in an HIP signal is much larger than the area of the tail in an MIP signal. Figures 5 – 7 show the experimental results. The xaxis stands for the area of the tail of the pulse, which comes mostly from the fast signal from the triplet state, while the y-axis stands for the area of the head. Figures 5 and 6 describe the ratios of the areas of the tail to the
A Pulse Shape Discrimination Method with CsI Using the Ratio · · · – Jong-Kwan Woo et al.
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Fig. 7. (Color online) Plot of the data taken under cosmic rays without a neutron beam. The x-axis and the y-axis stand for the area of the tail and the area of the head, respectively. Both the x and the y axes are in arbitrary units.
head for pulses taken from CsI located 10 cm (named D10) and 27 cm (named D27) away from the center of the neutron beam. The graphs contain the signals from neutrons and gamma rays because the MC-50 cyclotron at KIRAMS provides a neutron beam and gamma rays simultaneously. The data in Fig. 5 (D10) was taken at a position closer to the center of the neutron beam than the data of Fig. 6 (D27); Compared to D27, D10 contains data from neutrons that are more monotonic. We clearly see that data from a neutron-rich (D10) experiment is closer to the y-axis than data from a gamma ray-rich one (D27). The fitted lines are y = 3.59x + 0.66 with χ2 = 0.72 for D10 and y = 2.63x + 0.55 with χ2 = 0.50 for D27. We find that the incident particles of the D10 data are provided by a neutron beam that is more monotonic than that of D27; χ2 for D10 is bigger than χ2 for D27. The data in Fig. 7 were taken from the cosmic background at the same place after taking the data for D10 and D27. We can assume that the data in Fig. 7 contain the signal from cosmic muons and from gamma rays from the laboratory. Figure 8 shows histograms of experimental data, with the x axis standing for the ratio RHT , which is the same as the slopes in Figs. 5 – 7. The average of RHT is 3.17 with a standard deviation of σ = 1.13 for D10 data while the average of RHT is 3.81 with a standard deviation of σ = 3.17 for D27 data. These results also agree with the fact that the D10 data are provided by a neutron beam more monotonic than that used to provided the D27 data. Also, the average of RHT is 5.94 with a standard deviation of σ = 5.65 for the data taken from cosmic background. We can see deep points at bins from 7 to 8, the exact bin value being between 7.26 and 8.28, in the ratio RHT on the x-axis for the cosmic-ray data in
Fig. 8. (Color online) Histograms for the experimental data. The x-axis stands for the ratio RHT = area of head/area of tail while the y-axis stands for the frequency.
Fig. 8. We can take the deep point at 7.8 as a trial one. We can define two clusters, one is below the deep point 7.8 while the other is above the deep point. The cluster to the left of the deep point in the cosmic ray graph, bottom in Fig. 8, stands for the data induced by HIPs while the cluster to the right of the deep point stands for the data induced by MIPs. We fit the two signals in Figs. 2 and 4, and the ratios, area of the head to area of the tail, were 5.5 for neutrons and 27 for gamma rays. These ratios (RHT s) agree with the slopes in Figs. 5 – 7. In other words, when the ratio RHT is bigger than 7.8, we can deduce that the signal is induced by an MIP, and when the ratio is smaller than or equal, it is induced by an HIP. We could not categorize one event for the cosmic data out of the 58 events in Fig. 8 because we took the bin size to be 1.0 for the cosmic data in the Fig. 8. We could not categorize one event out of the 105 events for D27 in Fig. 8 while we could categorize all events for the D10
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data out of the 106 events in Fig. 8. The overall efficiency for discriminating an MIP cluster from an HIP cluster is 267/269 = 99.3% for a bin size of 1.0.
of all those concerned with the publication of this paper are duly acknowledged.
V. CONCLUSION
REFERENCES
We separate the signal into head and tail parts by using a least-squares method for identifying whether the signals are induced by HIPs or MIPs. By comparing the ratios RHT = area of head/area of tail, we were able to discriminate between the types of incident particles. We could separate the signals into two clusters by using a deep point, RHT = 7.8. Finally, we achieved a discrimination efficiency of 99.3% for separating a cluster induced by HIPs from a cluster induced by MIPs by taking a bin sze of section 1.0 for 269 events taken from the MC-50 cyclotron at KIRAMS. A PSD method with a CsI crystal provided a very high accuracy in discriminating particle types, but a long time is required for data acquisition. Our PSD method is appropriate for rare event experiments with low background, not for experiments producing many events. Conclusively, we found that a CsI crystal scintillator with our PSD method may be able to discriminate whether the signal came from an HIP or an MIP. We need more study to enhance the accuracy of PSD with a CsI crystal scintillator. Also, we plan to perform an experiment with the charged particles to improve PSD with a CsI crystal scintillator.
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ACKNOWLEDGMENTS This work was supported by the research grant of the Jeju National University in 2009. Also, the contributions
