γ{\hbox-}Flash suppression using a gated photomultiplier assembled with an LaBr3(Ce) detector to measure fast neutron capture reactions

Nuclear Instruments and Methods in Physics Research A 723 (2013) 121–127

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

γ-Flash suppression using a gated photomultiplier assembled with an LaBr3(Ce) detector to measure fast neutron capture reactions K.Y. Hara a,n, H. Harada a, Y. Toh a, J. Hori b a b

Japan Atomic Energy Agency, 2-4 Shirakata Shirane, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan

art ic l e i nf o

a b s t r a c t

Article history: Received 26 October 2012 Received in revised form 16 April 2013 Accepted 3 May 2013 Available online 10 May 2013

A gated photomultiplier tube (PMT) assembled with an LaBr3(Ce) detector was applied toward the prompt γ-ray measurement of fast neutron capture reactions. Time-of-flight measurements of the neutron capture reactions of Cl and Al were performed using the 46-MeV electron linear accelerator at the Kyoto University Research Reactor Institute (KURRI) as a pulsed neutron source. The photomultiplier gating technique effectively suppressed the saturation of the PMT output and extended the energy region of the TOF measurement. & 2013 Elsevier B.V. All rights reserved.

Keywords: Fast neutron Time-of-flight method LaBr3(Ce) scintillator Photomultiplier gating technique

1. Introduction Neutron capture cross-sections in the keV energy region are important in nuclear engineering and nuclear astrophysics. In nuclear engineering, the neutron capture cross-sections of minor actinides and structural materials are required in the energy region from 0.025 eV to several hundred keV [1,2] to study nuclear transmutation based on accelerator-driven systems or fast reactors. In astrophysics, the neutron capture cross-sections of the nuclides beyond iron are required for investigating the abundance patterns and the stellar environments (i.e., the temperature and the neutron density) in sprocess nucleosynthesis [3,4]. The interesting energy region for the neutron capture cross-sections is 0.1–200 keV to obtain the Maxwellian-averaged cross-section at thermal energies of kT¼ 8– 30 keV that is used in the nucleosynthesis network calculations. When a time-of-flight (TOF) method is used for the prompt γ-ray measurements, countermeasures against the so-called γ-flash are required to derive the energy dependent capture cross-sections for fast neutrons [5]. The γ-flash consists of intense γ-ray pulses involved in the neutron beam production. In many cases, the neutrons are produced by photoneutron reactions with bremsstrahlung of electron beams or by spallation reactions with proton beams. While the former is associated with the bremsstrahlung generated by incident electrons, the latter is associated with the γ-rays from target nuclei and spallation fragments excited

n

Corresponding author. Tel.: +81 29 282 5796. E-mail address: [email protected] (K.Y. Hara).

0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.05.011

by incident protons. Besides, neutron capture γ-rays generated in the neutron production target and the moderator also contribute to the γ-flash. The scattering γ-flash on a sample causes overloading of the detector and the assembled circuits. At the initial TOF, therefore, the neutron TOF measurement is usually disturbed by the γ-flash [6]. Such as a length of the flight-path, a beam collimation system, and a shadow bar, some efforts are needed to reduce the intensity of the γ-flash, although these methods decrease the intensity of neutron beam. Fast time-response properties of a γ-ray detector have an advantage to quickly recover from the effect of the γ-flash. In this work, we have applied an LaBr3(Ce) detector to the TOF measurements for fast neutron capture reactions. The LaBr3(Ce) detector has become an attractive device for γ-spectroscopy measurements [7] because it has a fast decay time (∼20 ns) and a very high light output (∼60;000 ph=MeV) [8]. Currently, detectors such as BaF2 [9,10], C6D6 [11,12], NaI(Tl) [13,14], BGO [15,16] and Ge detector types [17,18] have been used to measure the neutron capture reactions in TOF facilities. The time response of the LaBr3(Ce) detector is comparable to that of the BaF2 and C6D6 detectors and is better than that of the other detectors mentioned. Moreover, the LaBr3(Ce) detector offers the best energy resolution among these detectors, except for the Ge detector. As well as the time-response properties of the γ-ray detector, the γ-ray energy resolution is important. When a sample contains a large amount of chemical or isotopic impurities, the γ-ray energies are used to distinguish the γ-rays emitted from individual nuclides. In this work, we have applied for the first time the gating technique [19] to a combination of a photomultiplier tube (PMT)

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and an LaBr3(Ce) scintillator. The PMT gating technique is to turn on/off the PMT output by controlling the reverse bias between the cathode and the dynodes with a gate pulse. To avoid overloading of the PMT and the assembled circuits, the PMT is turned off during the γ-flash by the PMT gating operation. Previously, the PMT gating technique has been applied to TOF experiments such as neutron inelastic scattering measurements with an NaI(Tl) detector at RPI [20] and neutron transmission measurements with a plastic scintillation detector at HELIOS [21]. On the other hand, the PMT gating technique has not been used for measurements of the neutron capture reaction. We demonstrate how effectively the procedure circumvents the γ-flash problem.

2. Experiment 2.1. Experimental setup at KURRI-Linac The TOF measurements of fast neutron capture reactions were performed with the 46-MeV electron linear accelerator (linac) at the Kyoto University Research Reactor Institute (KURRI) [22]. The experimental setup is shown in Fig. 1. The linac was operated at an electron energy of ∼30 MeV, a repetition rate of 200 Hz, a pulse width of 100 ns, and a peak current of ∼4 A. The electron beam hit a watercooled tantalum target and induced bremsstrahlung photons. The neutrons were produced in the target by a photonuclear reaction and were moderated in the water tank coupled with the target. The neutron beam was passed along a 12-m flight path from the moderator to a capture sample. Air was evacuated from the flight tube. The neutron collimation system, which composed of borated paraffin, Li2CO3, Pb, B4C, and H3BO3, was positioned at 1351 with respect to the electron beam axis. It tapered from approximately 12 cm in diameter at the entrance of the flight tube to approximately 2 cm in diameter at the capture sample. To suppress the frame-overlap component due to the low-energy neutrons from the previous high-frequency pulse, a cadmium sheet of 0.5-mm thickness was inserted in the flight path. A Pb shadow bar (a block of diameter 5 cm and length 10 cm) is usually placed between the tantalum target and the entrance of the flight tube to reduce the intensity of the γ-flash, but it leads to a decrease in the intensity of the neutron beam. In this measurement, the Pb shadow bar was taken off the beam axis to keep the intensity of the neutron beam as high as possible. The PMT saturation due to the γ-flash is suppressed by applying the PMT gating technique. Sodium chloride powder (NaCl, 3.6 g) inside an aluminum case of 2.5-cm inner diameter and an aluminum disc of 3.5-cm diameter (Al, 25 g) were used as the capture samples. The NaCl sample was used to calibrate the TOF spectrum (the neutron energy) using the neutron resonances of 35Cl and the pulse height spectrum (the γ-ray energy) using the γ-rays of the 35Cl(n, γ) reaction. The Al sample was used to measure the neutron resonances in a few hundred keV energy region. In addition,

KURRI Linac (pulsed electron beam) eTa-Target

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a carbon block was used to evaluate the background caused by the scattering neutrons on a sample. The isotopic compositions of these samples were natural. An LaBr3(Ce) detector was located 5 cm from the sample. 2.2. Detector setup and photomultiplier operation The detector consisted of an LaBr3(Ce) crystal (Saint-Gobain Crystals, BrilLanCe380) [23], a PMT (Hamamatsu Photonics, R32902) [24], and a gated voltage divider (Hamamatsu Photonics, C1392-11MOD) [19,24]. A magnetic shield case (Hamamatsu Photonics, E989-62) and PMT housing (Hamamatsu Photonics, E1341-01) were attached to the PMT. Because LaBr3 is hygroscopic, the cylindrical crystal was encased in an aluminum housing and hermetically sealed with a glass window. The LaBr3(Ce) scintillator (1.5  1.5 in.) was mounted on the PMT windows (diameter 2 in.) and fastened onto the PMT housing with an aluminum adapter. Silicone grease (Saint-Gobain Crystals, BC-630) was used for the optical coupling between the scintillator and the PMT window. The LaBr3(Ce) scintillator offers superior energy resolution among scintillation detectors because it can produce a high light output within a very short decay time, but the properties result in instantaneous high currents in the photocathode and the dynode chain of the PMT [25]. Therefore, with standard PMTs, these anode outputs of the LaBr3(Ce) detectors are saturated even below the nominal voltage at the high-energy γ-ray (∼10 MeV). Lowering the bias voltage or reducing the number of dynodes in the PMT has been employed for the LaBr3(Ce) scintillator in some studies [8,26– 28]. To measure the γ-ray up to 10 MeV without the saturation of the PMT output, the bias voltage (HV1) for our detector was chosen as −750 V, which was lower than the nominal voltage, −1500 V. The energy resolution (FWHM) was approximately 3% for 1.3-MeV γ-rays from a 60Co source at −750 V. It was approximately 4% in the measurement of the neutron capture reaction with the PMT gating operation. The gated voltage divider operates the PMT with a stable gain unless a gate signal is externally input to the divider. By switching the dynode voltage with the gate pulse, the photoelectron multiplication is suspended by the reverse bias between the cathode and the dynodes. Therefore, when the PMT gating operation is repeated in the same period as the pulsed neutron beam, an excessive PMT output during the γ-flash will be avoided. The supplied voltage for the reverse bias (HV2) had a nominal value of +200 V. A transistor–transistor logic (TTL) pulse derived from the accelerator trigger was used as the external gating signal to turn off the PMT output, where the width of the TTL pulse was a nominal value of 2 μs and its frequency was 200 Hz as with the pulsed beam. 2.3. Timing adjustment of the PMT gate pulse The neutron capture γ-rays emitted from the NaCl and Al samples were measured with the TOF method. A block diagram of the electronics is shown in Fig. 2. The pulse from the LaBr3(Ce) detector was processed using a timing filter amplifier (TFA, ORTEC474), where the TFA was set at 500 ns differential time constant and the integral time constant was set at Out position (equivalent to 4 ns). The output from the TFA was input to a multichannel analyzer module (MCA, Yokogawa WE7562) to record the pulse height and the peak timing of each pulse. A start signal for triggering the MCA was generated by a linac electron pulse loaded on the tantalum target. The neutron TOF was derived from the time difference between the trigger (start signal) and the output from the TFA (stop signal). The dead time of the MCA depends on the pulse width.

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For the PMT gating operation, the timing of the gate pulse (T A ) was adjusted with respect to the arrival time of the γ-flash (T 0 ), where a time interval, t s ¼ T 0 −T A , is defined as the setting parameter. For example, a timing chart for the gate pulse and the PMT output is shown in the inset of Fig. 2. The PMT output is turned off at T A by inputting the gate pulse, and it is then turned on after a short time, i.e., at T B . The PMT output is shut off during the time interval, t g ¼ T B −T A , that depends on the gate pulse width and the time constant of the capacitance-coupled circuit. Because t g is fixed, the time interval between T 0 and T B increases with decreasing t s . The TOF data become available after T C because of a finite restoration time of the PMT gain, t r ¼ T C −T B . To measure the fast neutron capture reactions, the requirement for the gating system is that the PMT gain quickly and reliably recovers after passing through the γ-flash. At least, one requirement is T A o T 0 o T B . Under our detector setup (with a gate pulse width of 2 μs), the time interval t g is approximately 11 μs. Therefore, the gate pulse should precede the γ-flash by 0–11 μs, i.e., 0 o t s o 11 μs. First, in order not to detect the γ-flash, t s was set at 9:7 μs for the NaCl sample. As shown in Section 3.2, the setting corresponds to that of the time interval between T 0 and T B is 1:6 μs. This time interval is long enough to avoid the influence of the γ-flash. Then the effectiveness of the PMT gating technique is presented by comparing the measurement without the PMT gating operation. Second, in order to know to what extent T B is able to close to T 0 , i.e., in order to obtain the upper limit of the neutron energy in this TOF measurement, t s was set at 10:9 μs for the Al sample. The setting corresponds to that the time interval between T 0 and T B is 0:4 μs. The interval is a very short time because the duration time of the γ-flash is approximately 0:2 μs. In this case, the time interval between T 0 and T C will be approximately 1 μs because the typical t r is a few hundred nanoseconds in the specification sheet. Therefore, the measured energy region is expected to be of the order of several hundred keV.

3. Results and discussion 3.1. γ-Flash suppression with the PMT gating operation The TOF spectra are shown in Fig. 3, where the solid and dashed lines show the spectra that were measured with and without the PMT gating operation, respectively. In the measurements without the

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Fig. 3. TOF spectra for the NaCl (a) and Al samples (b). The horizontal axis is the neutron TOF, and the vertical axis is the number of counts per beam pulse per TOF channel. The solid and dashed lines show the measurements with and without the PMT gating operation, respectively. The arrows indicate the reference timing T 0 for the TOF measurements.

PMT gating operation, large peaks due to the γ-flash are observed in the TOF range of 0:7–1:7 μs in Fig. 3(a) and (b). From the calibration using the narrow neutron resonances of 35Cl, the reference timing for the TOF measurements (T 0 ) is given as 0:7 μs. Here, the corrected flight time t m is defined as TOF–T0. In addition to overloading of the detector and the assembled circuits due to the γ-flash, the data acquisition is inhibited for 2–3 μs corresponding to the width of the saturated pulse due to the γ-flash. As mentioned in Section 2.3, the dead time depends on the pulse width. By applying the PMT gating technique, the peak due to the γ-flash disappears, and then the measured TOF spectrum extends toward faster neutron region. Similarly, the pulse height spectra in Fig. 4 are significantly different between the measurements with and without the PMT gating operation. In the measurements without the PMT gating operation, the pulse height above 5800 ch corresponds to the peak due to the γ-flash in the t m of 0–1 μs (the TOF range of 0:7–1:7 μs in Fig. 3). In fact, the pulse height spectra during the initial 4 μs are shown in Figs. 5 and 6 by a TOF time width of 1 μs. For the NaCl and Al samples, the saturated spectra due to the γ-flash are presented by the dashed lines in Figs. 5(a) and 6(a), respectively. The difference in the pulse-height distribution of the γ-flash for the two samples arises from the γ-ray scattering which depends on the atomic number and the mass of the sample. After the elapse of the dead time, the pulse height spectra are observed in Figs. 5 (d) and 6(d), but the energy resolution is not fully recovered. In the measurements with the PMT gating operation, on the other hand, the saturation due to the γ-flash is excluded. The time variation of pulse height after the PMT gating operation is described in more detail in Sections 3.2 and 3.3. 3.2. Restoration time of the PMT gain after the PMT gating operation For the measurements with the PMT gating operation, the pulse height spectra for the NaCl sample are shown at five representative points in Fig. 7, where the t m times are 1.6–1.7 (a), 1.8–1.9 (b), 2.0–2.1 (c), 2.2–2.3 (d), and 2:4–2:5 μs (e). Because the start timing of the PMT-gain recovery after the PMT gating operation is sufficiently delayed from the γ-flash, the influence of the γ-flash on the PMT-gain recovery is completely eliminated. Therefore, the restoration time required for the PMT gain to

K.Y. Hara et al. / Nuclear Instruments and Methods in Physics Research A 723 (2013) 121–127

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Fig. 5. Time variation of the pulse height spectra for the NaCl sample at a t m of 0–1 μs (a), 1–2 μs (b), 2–3 μs (c), and 3–4 μs (d). The solid and dashed lines show the measurements with and without the PMT gating operation, respectively. In the measurement without the PMT gating operation, the data acquisition is inhibited during 2–3 μs (b)(c).

recover a normal gain can be obtained from the spectra. As shown in Fig. 7(a), the PMT gain begins to recover at t m ¼ 1:6 μs, but no peak is observed in the spectrum. It means that the restoration of the PMT gain is not enough yet. In the successive spectra, the peaks of the 511-keV γ-rays are observed, where the solid arrows indicate the peak positions of the 511-keV γ-rays in Fig. 7(b)–(e). The peak position of the 511-keV γ-rays is returned to over 95% of the stable position within 0:5 μs from the recovery start time. The

restoration time t r ¼ 0:5 μs is consistent with a typical value given in the specification sheet of the gated voltage divider. The pulse height spectra for the Al sample are shown at five representative points in Fig. 8, where the t m times are 0.4–0.5 (a), 0.9–1.0 (b), 1.4–1.5 (c), 1.6–1.7 (d), and 1:8–1:9 μs (e). Since the faster gate timing was set for demonstrating the measurement close to the γ-flash, the PMT gain begins to recover at t m ¼ 0:4 μs. According to the restoration time of the PMT gain (the t r of 0:5 μs), the PMT gain has already recovered at t m ¼ 0:9 μs, but the peak of the 511-keV γ-rays is not clearly observed in Fig. 8(b). In t m o 1:4 μs, the detector system seems to be still suffering from the residual effect of the γ-flash or the effect of the neutron burst. The capture γ-rays, which are induced by the sample and high energy neutrons (neutron burst), concentrate just after the γ-flash arrival at the sample. Therefore, it will be reasonable that only the data after 1:4 μs are used, where the time corresponds to 400 keV in neutron energy. 3.3. Time dependence of the baseline shift To present the baseline shift, the time dependences of the pulse height are compared between the measurements with and without the PMT gating operation. The variation of the pulse height is caused by two effects: (1) the saturation of the PMT gain due to the γ-flash and (2) the restoration of the PMT gain after the PMT gating operation. The peak positions of the 511-keV γ-rays are shown as a function of time in Fig. 9. Fig. 9(a) and (b) correspond to the NaCl and Al samples, respectively. The circles and the diamonds indicate the measurements with and without the PMT gating operation, respectively. Here, the pulse height measured without

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the PMT gating operation is shifted by −20 channels, which is the difference in the zero offset between the measurements with and without the PMT gating operation. In the measurements without the PMT gating operation, the time dependence of the pulse height decreases until 20 μs and recovers to a stable position after 80 μs. The baseline shift is significantly impacted by the saturation of the PMT gain due to the γ-flash. In the measurement with the PMT gating operation, in contrast, the fluctuations of the pulse height in Fig. 9(a) and (b) are within 4% for t m ≥2 μs, where there is a slight difference in t m o2 μs because the PMT gate timing t s is off for 1 μs. By applying the PMT gating operation, the pulse height is more stable than that without the PMT gating operation. The baseline shift complicates the data analysis because the effective discrimination level for the pulse height is changed as the time variation. In Fig. 3, therefore, the counting rate seems to be changing in synchronization with the time dependence of the baseline shift. Moreover, the baseline shift distorts the pulse height spectrum. For example, the γ-ray energy spectra for the 0.4-keV neutron resonance of 35Cl are shown in Fig. 10, where the resonance corresponds to the t m of 44–45 μs in Fig. 3. The peaks of the γ-rays emitted in the 35Cl(n, γ) reaction are indicated by the arrows, where the values in the figures are the γ-ray energies in keV [29]. In the measurement without the PMT gating operation (Fig. 10(a)), the γ-ray peaks have asymmetric shapes that were caused by the baseline shift. Consequently, the widths of the γ-ray

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peaks in Fig. 10(a) are larger than those in Fig. 10(b). In the measurement with the PMT gating operation (Fig. 10(b)), the γ-ray peaks have symmetric shapes.

K.Y. Hara et al. / Nuclear Instruments and Methods in Physics Research A 723 (2013) 121–127

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Fig. 10. Pulse height spectra for the 0.4-keV neutron resonance of 35Cl. The neutron resonance corresponds to the t m of 44–45 μs. The spectra were measured without the PMT gating operation (a) and with the PMT gating operation (b). These data were obtained for nearly the same measurement times. The arrows indicate the peaks of the γrays emitted in the 35Cl(n, γ) reaction. The values in the figures stand for the γray energies in keV [29]. t m [μs] 10

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101 E n [keV]

102

103

104

Fig. 11. TOF spectra for the NaCl sample gated in the pulse height range 800– 5800 ch. The solid and dashed lines show the measurements with and without the PMT gating operation, respectively. The neutron resonances of 23Na and 35,37Cl are indicated by the solid arrows at 0.4, 2.8, 4.3, and 8.3 keV. The neutron resonances of 27 Al are also indicated by the dashed arrows at 5.9, 34.8, 86.2, 143, and 203 keV. The neutron resonances of Br due to background neutrons are represented by the circles.

3.4. Measurable energy region in the TOF spectrum The TOF spectra for the NaCl and Al samples are shown in Figs. 11 and 12, respectively, where the horizontal axis is the neutron energy and the vertical axis is the counts per beam pulse per TOF channel; the upper horizontal axis is the time, t m . To discriminate the γ-flash and the background events, these spectra are gated by the pulse height between 800 and 5800 ch in Fig. 4 (γ-ray energy of 1–10 MeV). The solid and dashed lines are the spectra measured with and without the PMT gating operation, respectively. The resonance peaks are observed at the energies of the known neutron resonances [30]. As shown in Fig. 11, the neutron resonances of 23Na and 35,37Cl are observed at 0.4, 2.8, 4.3, and 8.3 keV. In addition, the neutron resonances of 27Al, which were caused by the aluminum case of the NaCl sample, are also observed in the same figure at 5.9, 34.8, 86.2, 143, and 203 keV. The resonances of 23Na and 35,37Cl are indicated by the solid arrows, and those of 27Al are indicated by the dashed arrows. Similar to Fig. 11, the neutron resonances of 27Al are also observed

at 5.9, 34.8, 86.2, 143, and 203 keV in Fig. 12. In Figs. 11 and 12, other structures are seen below 1 keV. As shown in Fig. 13, these structures are also seen in the TOF spectrum for the carbon sample (dot-dashed line) but not seen in the spectrum for the blank (dotted line). The structures are derived from nuclear reactions caused by the background neutrons in detector materials such as Br and La. For example, the neutron resonances of 79Br at 0.035 and 0.054 keV and those of 81Br at 0.10 and 0.14 keV are represented by the circles in Figs. 11–13. The background would be reduced by setting an appropriate shield for the detector from the scattering neutrons on the sample. As can be seen from Figs. 11 and 12, the TOF spectra are confined to below 60 keV in the measurements without the PMT gating operation, whereas the TOF spectra are extended to much higher energy region by the PMT gating operation. Since the pulse height data become available after t m ¼ 1:4 μs as described in Section 3.2, the measurable energy range is extended up to 400 keV.

4. Conclusion We applied a gated PMT assembled with an LaBr3(Ce) detector for the measurements of fast neutron capture γ-rays under an intense γ-flash. The TOF measurements on NaCl and Al samples were performed from 10 eV to a few MeV with the pulsed neutron beam at the KURRI-Linac. The PMT gating technique effectively suppressed the saturation of the PMT output due to the γ-flash. In the measurement for the NaCl sample, the influence of the γ-flash

K.Y. Hara et al. / Nuclear Instruments and Methods in Physics Research A 723 (2013) 121–127

on the PMT-gain recovery was completely eliminated. The restoration time of the PMT gain after the PMT gating operation was 0:5 μs. The time dependence of the baseline shift presented the reliable stability. Besides, in the measurement for the Al sample, the upper limit of the neutron energy for this TOF measurement was determined. As a result, compared with the measurement without the PMT gating operation, the measurable neutron energy region was extended from 60 keV to 400 keV. Therefore, the PMT gating technique is an effective procedure for the prompt γ-ray measurement of fast neutron capture reactions with the LaBr3(Ce) detector. The procedure will be used for the measurement of the neutron capture cross-section in the keV energy region, which is interesting in nuclear engineering and nuclear astrophysics. Acknowledgments

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