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A neutron detector on the basis of a boron-containing plastic scintillator
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Nuclear Instruments and Methods in Physics Research A 550 (2005) 343–358 www.elsevier.com/locate/nima
A neutron detector on the basis of a boron-containing plastic scintillator G.I. Britvicha , V.G. Vasil’chenkoa,, Yu.V. Gilitskya , A.P. Chubenkoa,b, A.E. Kushnirenkoa , E.A. Mamidzhanyana,b, V.P. Pavluchenkoa,b, V.A. Pikalova , V.A. Romakhina,b, A.P. Soldatova , O.V. Sumaneeva , S.K. Chernichenkoa , I.V. Sheina , A.L. Shepetova,b a
Institute for High Energy Physics, 142281 Protvino, Moscow Region, Russia b P.N. Lebedev Physical Institute, Moscow, Russia
Received 10 December 2004; received in revised form 21 March 2005; accepted 1 April 2005 Available online 5 July 2005
Abstract Characteristics of construction elements and investigation results of a prototype of a fast neutron detector are described. The detector prototype is based on a boron-containing molded plastic scintillator SC-331 manufactured in IHEP. The use of this scintillator for neutron detection ensures a capacity to work under high loading rates with a relatively low level of registered background events. The newly created detector may be used for neutron flux measurements in extensive air showers. r 2005 Elsevier B.V. All rights reserved. Keywords: Neutrons; Boron-containing plastic scintillator; Detector; Extensive air showers
1. Introduction At the present time, neutron measurements are widely used in cosmic ray experiments. There exists a world-wide net of 120 neutron stations [1] used for studying cosmic ray intensity variations Corresponding author. Fax: +7 095 2302337.
E-mail addresses: [email protected] (V.G. Vasil’chenko), [email protected] (A.P. Chubenko).
which define our knowledge in a wide range of cosmic ray astrophysics: the solar–terrestrial relationship, the influence of galactic radiation and solar flares on the Earth’s magnetosphere, the Solar system as a whole (cosmic weather), and so on [2,3]. These stations, situated at various latitudes around the globe and at various heights above the sea level, are carrying out continuous measurements with the use of neutron monitors, the most widely used is the NM64-type neutron supermonitor [4].
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.04.083
ARTICLE IN PRESS G.I. Britvich et al. / Nuclear Instruments and Methods in Physics Research A 550 (2005) 343–358
In the past, detectors similar to neutron monitors have been used to determine the number of hadrons in extensive air showers (EASs) created by primary cosmic particles with energies above 100 TeV [5]. These standard neutron monitors have also been joined with shower installations and, together with cosmic ray intensity monitoring, have been used as an EAS hadron component detector [6,7]. Investigations being carried out at the NM64type neutron supermonitor based on the shower installation of the Tien–Shan mountain station of the Lebedev Physical Institute, have led to the discovery of an anomalous delay of the neutron signals in EAS cores [8,9]. The essence of this phenomenon consists of the following. The NM64 monitor (just as all other types of neutron monitors) registers the evaporation neutrons from the nuclear fission produced by high-energy hadrons (with energies above 100 MeV) within the lead of the shower installation. Before detection, these evaporation neutrons, created with energies of a few MeV, are slowly thermalized (to energies of about 102 eV), diffusing inside the monitor. The diffusion process results in an exponential time distribution of the neutron signals with a characteristic relaxation time of about 360–400 ms. Such a picture is reproduced with great accuracy when an EAS with an energy below 3 PeV passes through the monitor. However, at energies above 3 PeV, a small part of the shower (about 2–3% of the whole) has a prolonged shape for the time distribution of the intensity of neutron signals. The fraction of such anomalous showers rapidly increases with increasing energy, and reaches about 20–30% at an EAS energy of about 10 PeV. Fig. 1 shows the experimental time distribution of the intensity of neutron signals DM exp =Dt from the Tien–Shan neutron supermonitor registered after the passage of a 20 PeV EAS core by a group of six neutron counters. Here DM exp are the number of counts of neutron signals in the time intervals Dt. The expected diffusion curve of the intensity of neutron signals M corr F ðtÞ for the 20 PeV EAS is also shown on the plot, where M corr is the total number of neutron signals under the diffusion curve and F ðtÞ is the probability
µ
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µ
Fig. 1. The experimental time distribution of the intensity of neutron signals DM exp =Dt from the Tien–Shan neutron supermonitor registered after the passage of a 20 PeV EAS core ðÞ. Solid line M corr F ðtÞ is the expected diffusion curve of the intensity of neutron signals.
distribution of neutron signals for the diffusion curve [8,9]. It is seen that the experimental time distribution of neutron signals shown in Fig. 1 has an anomalously prolonged shape and its deviation from the expected diffusion curve reaches from three to two orders of magnitude in the time interval from 10 to 500 ms. An attempt to explain this effect as a result of neutron signal losses in registration channels [10], leads to a contradiction with the existing models of EAS development in the atmosphere, because the hadron multiplicities, necessary for an explanation of the experimental time distribution of the intensity of neutron signals, prove to be three to two orders of magnitude higher than the values predicted by the models [11]. It should be noted that the observed effect has a threshold character: it starts to become apparent in the energy region above 3 PeV, where a characteristic knee in the primary cosmic ray spectrum, of still unknown nature, is present. In the same energy region, a wide range of unexplained phenomena are also observed in mountain and stratospheric cosmic ray experiments (for example, the halo events [12], Centauros [13] and Anticentauros [14], and the coplanar events in X-ray chambers [15]). Thus, the effect of prolongation of neutron intensity time distributions is representative of the whole series of
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anomalous processes, which points to the necessity of a thorough investigation in the energy range above 10 PeV. Historically, neutron detection in monitors has employed expensive ionization counters. Experience shows that these counters are relatively slow—with output pulses of duration of about 1 ms. This results in a limitation of the maximum trustworthy counting rate for these detectors due to pile up events, which cannot exceed a value of about 5 105 s1 . Moreover, after the passage of an EAS core through the ionization counter, the dead time could considerably increase, which might lead to an additional distortion of the registered neutron counting rate. This circumstance is of vital importance in the exploration of the cores of EASs with primary energies above 10–100 PeV. In fact, our estimates show that for such a purpose, there are detectors capable of operating with neutron fluxes of about 2–3 106 s1 . However, the cases when high-energy EAS cores are passing through the experimental installation are relatively scarce. The overwhelming majority of EASs develop some distance away from the monitor, so the detectors register only their peripherals. Therefore, it is necessary to have detectors with maximum effective areas, capable of registering the peripheral low-intensity neutron fluxes of about 1 cm2 s1 . For the study of cosmic ray intensity variations, effective registration of neutron fluxes with intensities 103 cm2 s1 or even lower is necessary. Thus, the problem of cosmic ray neutron registration comes to two different tasks, one of which demands the creation of a fast neutron detector having a relatively small effective area, capable of operating in the high loading rates in EAS cores, and the second one, the development of a relatively cheap detector with the maximum effective area for registration of low-intensity neutron fluxes.
2. Elements of the neutron detector 2.1. Selection of the active medium Depending on the energy of the investigated neutrons, various methods are used for their
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detection. Detection of neutrons with energies E410 MeV is accomplished via interactions with C nuclei in carbon-rich substances. Detection of neutrons having energies Eo0:1 MeV is accomplished through the scattering of neutrons in a hydrogen-containing medium with the subsequent registration of knocked out protons. In the energy range below 0.1 MeV, neutrons may be slowed down to energies E ’ 0:5 keV with subsequent registration of nðp; dÞg reaction products [16,17]. Further neutron deceleration down to thermal energies ðE ’ 102 eVÞ could even be accomplished as necessary. Plastic scintillators enriched by 6Li or 10B [18,19] may be used as an active medium for the detection of decelerated neutrons. Inside a boroncontaining plastic scintillator, the registration of charged products of the reaction 10Bðn; aÞ7 Li takes place. In 6% of the events this reaction produces a ground state 7Li nucleus along with a 1.78 MeV a-particle. In 94% of the events the lithium nucleus is created in an exited state 7 Li , which is accompanied by emission of a 1.47 MeV a-particle and a 0.478 MeV g-quantum. In contrast to lithium, the use of more inexpensive boron in plastic scintillators is preferable, because the percentage of the necessary 10B isotope in natural boron is high enough (19.9%) [20], permitting one to avoid the expensive procedure of its enrichment. Cross-sections s of 1 eV neutrons on 6Li and 10B are 149 barn and 609 barn, respectivelyp[21]; ffiffiffiffiffiffi with increasing energy they change as s1= E n . It is seen that the use of 10B for the neutron registration is four times more effective than that of 6Li (but for the case of lithium, the energy release of the 6Li ðn; aÞt reaction is 4.8 MeV and no g-quanta are emitted.) In this work we used a boron-containing molded polystyrene (PS) scintillator SC-331 which is manufactured in IHEP. The light output of the scintillator is about 56–60% of that for anthracene, the luminescence maximum wavelength is about lmax ¼ 420 nm, and it contains K ¼ 2–3% by weight of natural boron [19]. It is known that the light output of a-particle-induced scintillations is considerably lower than that from b-particles of corresponding energy. Our measurements have
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shown that the SC-331 scintillator light output for the reaction 10Bðn; aÞ7 Li is equivalent to that from a b-particle with an energy of about 110–130 keV. At the same time, the SC-331 scintillator light output for an minimum ionizing particle (MIP) is equivalent to that from a b-particle depositing an energy of about 2000 keV per 1 cm of the thickness of the boron-containing scintillator. As opposed to boron ionization counters, the boron-containing plastic scintillators do not have the defects mentioned above. Indeed, the radiation decay times for such scintillators are only about a few nanoseconds, and there is no problem with the light output linearity in the full range of EAS energy deposition. However, the use of these detectors would be defensible only if the subsequent photoreceivers and electronics, in their turn, could ensure proper operation rates under high loading conditions. 2.2. Possible layouts of fast neutron detectors Scintillation neutron detectors of various types may be used to solve the astrophysical problems formulated above. These detectors could be divided into two main types: detectors with an external moderator and a thin scintillator and those with a thick scintillator which simultaneously plays the role of the neutron moderator. It is obvious that the characteristics of these detectors will be considerably different. Before dealing with a further development of the detector prototype construction, one has to decide which of the two principal configurations would be optimal for the task in question. Preliminary measurements of some characteristics of the model detectors belonging to both construction types are necessary for this purpose. 2.2.1. Model detector constructions with thin scintillators Fig. 2a shows a simple construction of the fast neutron detector with a thin scintillator (3) having a thickness of d ¼ 5 mm and lateral diameter equal to that of the diameter of the sensitive area S of the photocathode of a photomultiplier (PMT) (5). By means of an optical glue, the boron-containing scintillator is fastened to the surface of the
polymethylmetacrylate (PMMA) light mixer (4), which also acts as a neutron reflector. The other side of the light mixer is attached to the PMT’s entrance window with a silicon glue. The polyethylene (PE) housing (1), together with a 5-cm thick PE moderator (2) for the incoming neutrons, covers the entire set-up from the outside. To increase the efficiency of light collection on the glued photocathode surface, the scintillator (3) and light mixer (4) are wrapped in a reflective Tyvek paper [22]. Results of the amplitude spectrum measurements of moderated neutrons (to the thermal energy region) are shown in Fig. 3a. To suppress the background g-quanta during the measurements, the detector was shielded by a lead sheet of 5-cm thickness. The spectrum has a mean amplitude value at channel A ¼ 221 and a relation of the r.m.s. peak width DA to A of d ¼ DA=A ¼ 0:09. In such a case, the mean number of photoelectrons (ph.e.) Z for the neutron peak may be estimated as [23,24] pffiffiffiffi d ¼ 1:14= Z , (1) where 1.14 is a parameter necessary to take into account the influence of the coefficient of secondary photoemission of the PMT used (FEU-184Ti) on the width of the amplitude spectrum. Hence, for the selected PMT FEU-184Ti (with a diameter of its sensitive area of about 63 mm and with a quantum efficiency Y ¼ 26% at the wavelength of 410 nm), the mean number of photoelectrons in the neutron peak is about Z ’ 160 ph:e: The accuracy of these measurements is about 10%. This scintillation counter is a detector with a relatively low effective area of neutron detection, defined as a product SF Z, where F is the 10B ðn; aÞ7 Li reaction probability per one incident fast neutron for the given detector construction, S is the sensitive area of the photoreceiver and Z is the registration efficiency of this reaction by the photoreceiver. Thanks to the high Z value of the construction, it is relatively simple to obtain the efficiency of Z ¼ 1. This model of a neutron detector permits us to separate neutron signals from MIP and g-quanta ones. The considered detector could be used both for pulse counting and for amplitude analysis.
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Fig. 2. Layouts of fast neutron detectors using thin scintillators in combination with polyethylene moderators. (a) Detector with a small effective area (cross-section) for the high loading rate operation conditions; (b) Detector with an enhanced effective area. A is a top view (the polyethylene moderator is not shown) and B is a cross-section; (c) Detector with the maximum effective area using wavelength shifting fibers or a wavelength shifting light conductor. A is a top view (the polyethylene moderator is not shown) and B is a cross-section. 1, polyethylene housing; 2, polyethylene moderator; 3, thin scintillator; 4, light conductor/light mixer which serves simultaneously as a neutron reflector; 5, PMT; 6, voltage multiplier and other electronics; 7, wavelength shifting fibers or a wavelength shifting light conductor.
Fig. 2b demonstrates the construction elements of a neutron detector with a large boron-containing scintillator (designations are the same as in Fig. 2a). Thanks to an extended scintillator (3), the effective neutron registration area in such a detector is considerably higher. The maximum lateral cross-section of the scintillator in this configuration also cannot exceed the sensitive area S of the photocathode of a PMT (5). By means of
optical glue, the scintillator’s butt-end is fastened to an adiabatic light conductor (4), which in turn, is connected to the entrance window of the PMT. Both the glued scintillator (3) and light conductor (4) are wrapped with the reflective Tyvek paper. The wrapped scintillator is coupled with a PE neutron moderator (2). A measurement of the light attenuation—see Fig. 3b—shows that the last condition is fulfilled
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Fig. 3. Some characteristics of the neutron detector models. (a) Amplitude spectrum from the interactions of moderated neutrons with the scintillator for the Fig. 2a construction (the FEU-184Ti type PMT is used as a photoreceiver). (b) Light attenuation curve in a strip scintillator ð5 72 mm2 Þ of the Fig. 2b construction with a FEU-110 type PMT as a photoreceiver. (c) Amplitude spectrum from the neutron interactions in the near scintillator range (the Fig. 2c construction with a XP-2232B type PMT). 1, the total spectrum; 2, spectrum after the PMT noise subtraction.
for a 5-mm thick SC-331 scintillator having a length up to 30 cm. The mean path for the light attenuation length in such a scintillator is about l att ’ 50 cm. For an effective registration of the 10Bðn; aÞ7 Li reaction, the mean number of photoelectrons in the neutron peak must be above 5 ph.e. But under
the real conditions of neutron measurements, the intensive background of g-quanta and other particles is always present. In such a case, the requirement for the minimum photoelectron number should be enhanced and must be formulated especially for a given task. Hence, in our case, to effectively separate the neutron peak from the
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g-background of the Pu–Be source and the g-quanta from the 10Bðn; aÞ7 Li reaction, while ensuring reliable operation of the detector during a continuous time, the mean photoelectron number must exceed 20 ph.e. This threshold level also determines the minimum number of photoelectrons required from the far end of an extended scintillator z ¼ 20 ph:e: Because this detector configuration permits the values Z ’ z, the registration efficiency Zo1. As a result, the maximum difference between pulse amplitudes obtained from the near and far end of the scintillator may reach Z=z ¼ 160 20 ¼ 8. The maximum scintillator length in such a case could be up to L ¼ l att lnðZ=zÞ ¼ 105 cm. The considered detector may be used both in pulse counting mode and, partially, for amplitude measurements (if pulses from the near and far parts of the scintillator do not differ by more than a factor of two from each other). Fig. 2c shows the construction elements of a neutron detector with the maximum effective area. By the means of wavelength shifting fibers (WSFs) (7) glued in grooves of the boron-containing scintillator (3), the scintillation light is re-emitted to the long wavelength region and then transported to a photoreceiver. The WSFs are connected to the entrance window of a PMT (5). Estimation of the maximum lateral size of the scintillator’s sensitive area may be based on an empirical fact, in order to achieve the highest degree of light collection, the pitch between the WSFs cannot exceed 6–10 mm. To enhance the total light output, the boron-containing scintillator, together with the WSFs, should be wrapped by the reflecting Tyvek paper. The wrapped scintillator is coupled with a PE neutron moderator (2). The trapping efficiency of WSFs is defined according to the formula [25,26]: ¼ 12 ½1 1=n2 ,
(2)
where n ¼ nc =nr ; nr is the reflection index of the fiber’s cladding, and nc is that of fiber core. Thus, for the double Y-11 type cladding WSF (with a diameter of about 1 mm and lmax ¼ 476 nmÞ; ¼ 0:054 [27].
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Fig. 3c demonstrates an amplitude spectrum for this model of a neutron detector (1) as well as the spectrum after subtracting the PMT’s internal noise (2). Measurements have shown that the neutron peak from the Pu–Be neutron source, which should be observed around the value Z ’ 3:9 ph:e:, cannot be seen over an intensive g-background. The mean value of the spectrum (2) is situated around channel 73, corresponding to Z ¼ 8:4 ph:e: (The quantum efficiency of the XP2232B type used PMT [28] is Y ¼ 11% and the light attenuation length in the WSFs is above l att ¼ 350 cm [27].) In this measurement the far ends of WSFs had reflecting coatings and the source of moderated neutrons was situated close to their near ends, i.e. 50 cm away from the PMT. The Z measurement accuracy in this experiment is about 30%. The value of Z obtained in the last configuration is far enough from the necessary value 20 ph.e., that the characteristics of the detector based on the construction of such a type are far away from those required for our purpose. However, the replacement of the PMT in the construction of Fig. 2c by an avalanche photodiode (APD) [29] with a quantum efficiency in the green spectrum range up to Y ¼ 90%, makes it possible to increase the number of registered photons up to 8.2 times, thus achieving values Z ’ 32 of electron-hole pairs (ehp). The minimum number of ehp required from the farthest end of a scintillator in the pulse counting mode could also be set to z ¼ 20 ehp. The resulting difference between the amplitudes from the near and far regions is about Z=z ’ 1:6. This circumstance permits us to achieve the maximum length for the neutron-sensitive detector range up to L ’ 170 cm with the use of a single APD. Because this detector construction allows for low values for Z; Zo1. The considered detector can only operate in pulse counting mode. Instead of WSFs, the light from a boroncontaining scintillator may also be re-emitted by a wavelength shifting light conductor (WSLC) based on PMMA and then transported to a photoreceiver. The maximum lateral size of the WSLC cannot exceed the sensitive area of photoreceiver. The light attenuation length in WSLC
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may reach l att ¼ 150 cm. For an effective neutron detector, it is necessary to use photoreceivers having the highest quantum sensitivity in the long wavelength (green) range of the WSLC emission spectrum. It is obvious that the maximum area of a boron-containing scintillator cannot exceed the operation surface of the WSLC. So, for WSLC of a rectangular shape, the trapping efficiency can be defined according to formula [30]: h pffiffiffiffiffiffiffiffiffiffiffiffiffi i ¼ 12 1 n2 1=n . (3) In the calculation of n according to Eq. (3), the nr index must be set to the reflection index of the air, i.e. nr ¼ 1. For the adiabatic PMMA WSLC may achieve the maximum trapping efficiency ¼ 0:12–0.25 (depending on the contact type of the WSLC with the photoreceiver used) [30]. Measurements have shown that in the last case (not shown in the figures), the neutron peak from the Pu–Be source is also absent against the g-quanta background. According to our estimates, the average number of ph.e. when the neutron source was situated at the near end of the WSLC (50 cm away from the PMT), was about Z ’ 3:5 ph:e: for the mean quantum sensitivity of the PMT Y ¼ 10%; for the WSLC luminescence spectrum lmax ¼ 490 nm. The end of the WSLC furthest from the PMT had no reflecting coating, and because that, the true value of , according to our estimates, was about ¼ 0:06. The value of Z in this case is considerably lower than the minimum photoelectron number required z ¼ 20 ph:e: The accuracy of this determination is about 30%. Hence, the characteristics of the neutron detector based on the construction of such a type are far away from those required for our purpose. It should be noted that the boron-containing scintillators could be easily designed on the basis of PS with an emitted luminescence maximum lmax p400 nm. Our experimental samples of such scintillators with lmax ¼ 380 nm have shown a light output around 50–52% of that for anthracene, with luminescence decay times of about 2 ns. Such scintillators may be used with WSLC emitting light in the blue spectrum region with lmax ¼ 4302440 nm (and ¼ 0:1220:25), where
the quantum sensitivity of the usual PMT achieves its highest values up to Y ¼ 26%, and they permit us to achieve Z ¼ 20–40 ph.e. Because this detector construction allows for low values for Z, Zo1. The considered detector can only operate in pulse counting mode. 2.2.2. Model detector construction with thick scintillators A simple construction of the fast neutron detector with a thick scintillator is shown in Fig. 4. The size of the scintillator is equal to the size of the sensitive area of the FEU-110 PMT having its diameter of about 63 mm. By means of a silicate optical glue, the scintillator (2) is fastened directly to the entrance window of the PMT (3). In contrast to the analogous construction for the fast neutron detector Fig. 2a, the scintillator thickness in the current design is 75 mm and an external moderator is absent. Moderation of fast neutrons takes place immediately inside the scintillator volume (2) by the creation of knocked out proton cascades of various energies. The PMT registers scintillations caused by knocked out protons. To increase the light collection efficiency, the scintillator is wrapped by the reflective Tyvek paper. The detector ensures a uniform distribution of output signal over the PMT photocathode. An amplitude spectrum of the knocked out protons (1) observed in the SC-331 scintillator when it was exposed to fast neutrons from the Pu–Be source is shown in Fig. 5. Against the dominant background from knocked out protons a weak peak is observable from the interactions of
Fig. 4. Layout of a neutron detector (cross-section) with a thick scintillator. 1, the housing; 2, thick scintillator; 3, PMT; 4, voltage multiplier and other electronics.
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Fig. 5. Amplitude spectrum from neutron interactions inside a 7.5-cm thick scintillator. The FEU-110 type PMT is used as a photoreceiver. 1, for the fast neutrons; 2, for the moderated neutrons.
the neutrons, moderated inside the scintillator. In order to determine Z for this weak peak, the spectrum (2) from preliminary moderated neutrons was also measured and it is shown in Fig. 5. The mean values of these neutron peaks are situated in about channel 124 and d ’ 0:2. This procedure has permitted us to determine Z ’ 32 ph:e: for the weak peak. This is an universal detector, capable of registering both fast neutrons and other particles, including g-quanta. Effective measurement of the relative numbers of fast neutrons in EAS by means of counting of knocked out protons is possible without any difficulties. This means that, in principle, the Z ¼ 1 level may be achieved with the considered detector, but determination of the absolute number of fast neutrons, which have been interacted in the scintillator and have produced cascades of scintillation flashes, demands a precise procedure of detector calibration. A separate problem is to define the connection between the number of registered flashes (mostly from knocked out protons) and the number of fast neutrons which have passed through the scintillator. A
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calibrated source of fast neutrons is necessary for this purpose, which should have a precisely known intensity and an energy spectrum similar to that of the investigated neutron fluxes. For many tasks such a calibration source is hard to find. Because of that the true value of Z may be p1 in such cases. It should be noted that the construction elements in Fig. 4 and the operation principles of this detector are analogous to the detector shown in Fig. 2a. It is obvious that for the creation of a fast neutron detector with a thick scintillators having a large effective area, one can use the model detector construction of the types of Figs. 2b and c. Our estimates show that the scintillator thickness in such constructions must be about 10 cm. Note that thick scintillator considerably increases the registration efficiency of g-quanta commonly attending the neutron radiation. For separation of these g-quanta, one can use the delayed coincidences of the signals from knocked out protons and the signals from the interactions of moderated neutrons within the scintillator’s boron [17]. Measurement results of the characteristics of various neutron detector models presented above show that the simple construction of Fig. 2a with a thin scintillator is the most adequate for the creation of a prototype of fast neutron detectors aimed for the solution of astrophysical tasks in the high energy EAS cores. Relatively high values of the number of photoelectrons registered by the PMT in this model detector construction permit us to select the neutron signals (from MIPs and g-quanta) by means of fast and simple threshold electronics, with results that could be easily interpreted. A detailed design of the neutron detector prototype is presented below. 2.3. Choice of a photoreceiver for the neutron detector prototype Generally speaking, there exist a set of PMTs with appropriate spectral sensitivities of their photocathodes which could be used in the neutron detector prototype: FEU-84, FEU-110, FEU184Ti, FEU-167, FEU-200, and others. However, the low light output from the 10Bðn; aÞ7 Li reaction in PS scintillators requires the use of photoreceivers with quantum efficiency Y 410%. It should
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be mentioned that APDs [28] having Y ¼ 80290% are more appropriate for this purpose than PMTs. It is also important to note that the non-uniformity of quantum sensitivity over the surface of PMT photocathode does not exceed 20–30%. Otherwise, light conductors and mixers must be used to increase the size of the light spots on the PMT photocathodes. Because all aspects of the PMTs used in the neutron detector prototype are quite analogous for various PMT types, all further considerations will be carried out on the example of a FEU-110 type PMT.
2.4. PMT power supply scheme for the neutron detector prototype A variety of methods for the dynode system potential stabilization are widely used for the creation of a stable PMT power supply scheme, capable of operating under high loading rates. In laboratory conditions, passive high voltage dividers with additional high-precision power supply sources are used for the stabilization of the potentials in the last dynode stages. Also possible is the use of active dividers based on Zener diodes [31] or high-b transistors [32] for the feeding of the last dynode stages. Today many photoreceivers employ a power supply scheme based on the Cockroft–Walton voltage multiplier [33]. This is a consequence both of the intensive development of its necessary elemental base and of the intrinsic advantages of the scheme: low consumption of external power coupled with high dynode current, low-voltage external feeding ð 12 VÞ, and the possibility of using individual voltages for tuning and other purposes. The FEU-110 PMT power supply scheme of our detector is shown in Fig. 6. The scheme consists of a capacity-diode voltage multiplier and a control device which permits us to tune the output voltage in the limits 1–2 kV. The high voltage stability is about 103 . Both the voltage multiplier and the registration electronics are situated inside the lightproof detector housing. On the bottom lid of the housing, two low-voltage connectors ð 12 VÞ are placed which are also used for signal output.
Fig. 6. PMT feeding scheme for the neutron detector prototype. 1, PMT; 2, voltage multiplier; 3, transformer; 4, switches; 5, generator; 6, control circuit; 7, digital-to-analogue converter (DAC); 8, amplifier; 9, comparator; 10, negative voltage stabilizer; 11, positive voltage stabilizer; data, digital code of the output high voltage; clock, DAC clock; V out , amplitude of the comparator’s output pulse; 12 V , the external feeding voltage.
It is commonly accepted that a PMT operates in the linear regime if the average anode current i does not exceed 10% of the current flowing through its divider. Normalized amplitudes of the output signals A=A0 from an a-source with activity 8 103 s1 placed on the scintillator are shown in Fig. 7 as a function of the anode current i. A0 is the amplitude at i ¼ 180 nA; the current was varied through an additional illumination of the PMT photocathode. Hence, the applied asource creates an anode current of about i ¼ 180 nA by the use of a Cockroft–Walton voltage multiplier. The Cockroft–Walton scheme ensures a sufficiently high stability level and may be used until the anode current values reach up to i ¼ 1 mA. Therefore, this scheme would be operating reliably until the equivalent loading rates reach up to 5 108 a-particles/s. For comparison, analogous characteristics for a passive voltage divider with a total resistance of 2 MO is also shown in Fig. 7. Photoreceivers used in the neutron scintillation detector must have sufficiently low levels for their intrinsic noise. Low-noise requirements are of special importance if the low-intensity neutrons of natural background are to be used for the
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Fig. 7. Dependencies of the normalized amplitude of FEU-110 type PMT output signal A=A0 on the value of the anode current i. , for the Cockroft–Walton feeding scheme; m, for the passive 2 MO voltage divider. The curves are drawn to guide the eye.
detector calibration. To decrease the level of PMT noise, another feeding scheme is possible when the positive high voltage is connected to the PMT anode (and the free end of the voltage divider is grounded).
3. Results 3.1. Neutron detector model testing with a Pu– Be neutron source Testing of a neutron detector model was performed on a construction similar to that shown in Fig. 2a without the PMMA light mixer/neutron reflector (4). An ordinary FEU-110 PMT with quantum sensitivity Y ¼ 12% was used as a photoreceiver. The 5-mm thick SC-331 scintillator was glued directly to the PMT photocathode. Characteristics of this neutron detector model were determined by the registration of previously moderated neutrons from a Pu–Be source with a 105 Bq activity. The source was placed near a 5-cm thick PE moderator.
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Fig. 8. Amplitude spectra from the interactions of the moderated neutrons with various elements of the detector’s layout. 1, spectrum of a boron-containing SC-331 scintillator situated on a FEU-110 type PMT; 2, spectrum of a SC-301 scintillator (without boron); 3, spectrum from the input window and the PMT intrinsic noise.
Fig. 8 demonstrates the 10Bðn; aÞ7 Li reaction amplitude spectra from the detector with boroncontaining scintillator SC-331 (1), from the detector an ordinary SC-301 scintillator having analogous sizes (2), and the background spectrum from g-quanta and the intrinsic PMT noise (3). The sampling time for all spectra was equal to 5 min. The intensity of g-quanta during the measurements was quite high; for this reason the detector was shielded from the Pu–Be source by a 5-cm thick piece of lead. The measurements have shown that the mean value of the neutron peak A corresponds to channel 164 with d ¼ 0:16 and Z ¼ 50 ph:e: The value of Z is defined according to formula (1). The amplitude dispersion of output pulses over the photocathode surface results in an increase of d, i.e. in some Z decrease compared to the value which should be expected for Y ¼ 12%. Comparison of the curves (1) and (2) in the region over channel 400 shows that the fraction of 478 keV g-quanta from the 10Bðn; aÞ7 Li reaction to the background pulse intensity is about 23%.
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counts. For the values d ¼ 5 mm and the boron concentration K ¼ 3% in our scintillator, N=N max ¼ 0:53. The measurements show that the minimum number of background events N b (i.e. the minimum of N b =N) is achieved for small scintillator thickness, dp3 mm. But it is also desirable to keep the relation N=N max as high as possible. Thus, the r value (and consequently K) must proportionally increase with decreased d. Natural limits for K growth are the capacity of the PS scintillator matrix to contain the boron admixture and the dropping of the scintillator light output with increasing K. At the present time, stable boroncontaining scintillators with Kp6% have been created. Fig. 9. Dependencies of the number of registered neutrons N ðÞ and the number of background events N b ðmÞ on the SC331 scintillator thickness d. Dashed line—the maximum value N max , obtained in fitting of the experimental data by the function (4). The curves are drawn to guide the eye.
Fig. 9 shows the dependencies of the numbers of registered moderated neutrons N and the background events N b (defined in the 3s interval around the neutron peak mean) on the scintillator thickness d. The accuracy of these measurements is about 3%. The use of a boron-containing PS scintillator creates a certain level of the background events. The background intensity is defined mostly by the interactions of both of the Pu–Be source g-quanta and the 478 keV g-quanta from the 10Bðn; aÞ7 Li reactions inside the scintillator and the PMT entrance window. The intrinsic PMT noise is significant only in the low-amplitude region. As d increases the background grows roughly linearly. The dependence of the number of registered neutrons on the scintillator thickness may be expressed as [16,17] N ¼ N max ½1 expðrdÞ ,
(4)
where N max is the maximum neutron number which could be registered from the given neutron source and r is an absorption coefficient. Fitting the experimental results by Eq. (4) shows that in this case r ¼ 0:151 mm1 and N max ¼ 2:06 105
3.2. Neutron detector prototype testing with a calibrated Pu– Be neutron source To estimate the detector prototype registration probability, the number of the 10Bðn; aÞ7 Li reactions inside the scintillator per one incident fast neutron needs to be determined. This number is defined by the following functional F of the neutron field [16,17]: Z E max Z F¼ FðE; rÞsðEÞ d3 r dE. (5) 0
V
Here E is the neutron energy; FðE; rÞ is the energy and spatial density of the neutron flux; and sðEÞ is the cross-section of the 10Bðn; aÞ7 Li reaction for neutrons with the energy E [21]. The integration are performed over the entire energy range of neutrons (from 0 to E max ) and the entire detector volume V. It is clear that the detector registration probability F Z is the energy dependent characteristic. We measured this characteristic for our detector prototype as the registration probability of a primary fast neutron from a calibrated Pu–Be source with a 5:35 105 Bq activity. The fast neutrons were normally incident to the external surface of the PE moderator of the neutron detector prototype (of the Fig. 2a type design). These measurements were performed in an open geometry at a height of about 6 m above the concrete floor. The detector registration
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probability was defined as a relation of the integral number of pulses under the neutron peak in the amplitude spectrum to the total number of the fast Pu–Be-neutrons that passed through the neutron detector prototype. Taking into account that in our case Z ¼ 1, the detector registration probability of fast neutrons from Pu–Be source F Z ’ 0:95% was obtained. In the flux measurements of the fast neutrons another characteristic is usually used; the detector sensitivity, which takes into account the moderator size ðS ¼ 95 cm2 Þ. In our case, the detector sensitivity for fast neutrons from Pu–Be source is equal to 0:9 pulse=cm2 =neutron with a statistical error in the measurement of about 10%.
3.3. Neutron detector prototype testing under conditions of Tien– Shan mountain cosmic ray station The neutron detector prototype (of the Fig. 2a type design) was tested in the real operation conditions at the Tien–Shan mountain cosmic ray station (3340 m above sea level). In autumn 2003 the neutron detector prototype was installed inside the NM64-type neutron supermonitor [9], and amplitude spectra of its pulses were obtained in two operation modes: (1) the simple cosmic ray intensity monitoring (with registration of all the pulses coming from the prototype without any selection) and (2) with triggering by shower master signals which were generated when EASs were passing through the monitor. Fig. 10a and b show a part of the amplitude spectrum for output pulses in the range around the neutron peak when the prototype was operating in the simple monitoring mode. The neutron peak, having the maximum in channel 231 and root mean square s1 ¼ 36 channels, is distinctly seen against a wide monotonic background (Fig. 10a). Fig. 10b shows the same spectrum with the
Fig. 10. Amplitude spectrum of our neutron detector prototype in the simple monitoring mode. (a) Total spectrum. (b) Spectrum after the background subtraction. Solid lines correspond to the Gaussian fit of our experimental data points.
(a)
(b)
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background subtracted. The integral pulse intensity below the neutron peak is 0:34 s1 with a measurement error of about 10%. Fig. 11a and b show an analogous spectrum obtained when an amplitude discriminator scheme was triggered by the EAS installation’s shower master signals. The pulses from the prototype were analyzed only during time gates 10–1500 ms after each shower trigger. (This time interval overlaps the period when thermalized neutrons from cosmic ray hadrons induced nuclear fission are diffusing inside the monitor.) In the amplitude spectrum a neutron peak is present, its average being at channel 226 with root mean square s1 ¼ 38 channels. As should be expected, the measured integral intensity in this case (the area below the neutron peak) is 22 times higher as in the previous one, and reaches 7:5 s1 . These measurements have demonstrated the adequacy of the characteristics of the neutron detector prototype in real operation conditions, and have demonstrated the possibility of its use in shower experiments at the Tien–Shan station.
(a)
4. Conclusion This work has confirmed the possibility of creation of fast neutron detectors based on boron-containing molded polystyrene scintillators SC-331 and set-up constructions like those shown in Figs. 2a–c and 4. An analysis of the basic characteristics of the model detectors has shown: (1) The creation of fast neutron detectors with large effective operation areas is possible on the basis of Fig. 2c construction, with the use of wavelength shifting fibers and photoreceivers with high quantum sensitivity (about 80–90%) in the long wavelength spectrum range (for example, those of the avalanche photodiode type). Fig. 11. Amplitude spectrum of our neutron detector prototype under triggering by the EAS master signals. (a) Total spectrum. (b) Spectrum after the background subtraction. Solid lines correspond to the Gaussian fit of our experimental data points.
(b)
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(2) For fast neutron detector with moderate effective operation areas, a construction of Fig. 2b may be used. (3) For fast neutron detectors with relatively small effective operation areas capable of operating reliably under extremely high loading rate conditions, the construction of Fig. 2a is the most appropriate. (4) Use of 6Li instead of 10B in plastic scintillators for the neutron detection would increase the energy output up to three times. This circumstance would give a principle possibility for a considerable enlargement of the effective operation areas in the neutron detector constructions of Fig. 2b and c. (5) Measurements with the neutron detector prototype have shown that the registration probability of a fast neutron from a Pu–Be source is about 0.95%. The detector ensures a 20% stability of output pulses until the anode current reaches values of about 1 mA. (6) Our investigations at the Tien–Shan mountain station have shown the adequacy of neutron detector prototype characteristics for the detection of cosmic ray neutrons. The boron-containing molded plastic scintillator SC-331 may also be used for the creation of hadronic calorimeters in high energy physics. Detectors based on Fig. 2a and b constructions, may be used both in neutron radiation dosimetry and in various geological, geophysical, and seismological works. The neutron detector prototype is capable of ensuring operation of the control systems in the case of a significant short-time increase of the neutron intensity, therefore the scintillation neutron detectors may be used for monitoring of nuclear fuel waste storage.
Acknowledgements This work is supported by the RFFI Grant 0502-16655. The authors are thankful to the Department of the Atomic Science and Technique of the Russian Atomic Ministry for the financial support and to our IHEP colleague—A.S. Solov’ev for his help in the work.
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References [1] C.J. Hatton, Prog. Elem. Phys. Part. Cosmic Ray Phys. 10 (1971) 3. [2] H. Mavomichalaki, V. Yanke, L. Dorman, et al., Neutron monitor network and space weather, in: I.A. Daglis (Ed.), Proceedings of the Effects of Space Weather on Technology Infrastructure, NATO Science Series II, vol. 176, Kluwer Academic Publishers, Dordrecht, 2004, p. 301. [3] J. Feynman, S.B. Gabrial, J. Geophys. Res. 104 (5) (2000) 10543. [4] C.J. Hatton, H. Carmichel, Can. J. Phys. 42 (1964) 2443. [5] T.V. Danilova, E.V. Denisov, S.I. Nicolscky, Sov. Phys. JETP 46 (5) (1964) 1561. [6] E. Boehm, Proceedings of the 11th ICRC, vol. 8, Budapest, 1969, p. 487. [7] V.M. Migunov, G.V. Skripin, Extensive air showers, Proceedings of IKFI, So Ran Publ., Yakutsk, 1987, p. 99 (in Russian). [8] V.P. Antonova, G.B. Zhdanov, N.M. Nesterova, et al., Bull. Lebedev Phys. Inst. 12 (1997) 1. [9] V.P. Antonova, A.P. Chubenko, S.V. Kryukov, et al., J. Phys. G: Nucl. Part. Phys. 28 (2002) 251. [10] Yu.V. Stenkin, V.N. Bakatanov, D.V. Smirnov, et al., Study of neutron bursts with baksan array, Proceedings of 27th ICRC, vol. 3, Hamburg, 2001, p. 1449. [11] A.P. Chubenko, A.L. Shepetov, V.P. Antonova, et al., Proceedings of 28th ICRC, vol. 1, Tsukuba, 2003, p. 69. [12] A.S. Borisov, Z.M. Guseva, V.G. Denisova, et al., Nucl. Phys. B (Proc. Suppl.) 52B (1997) 185. [13] H. Wilczynski, for JACEE Collaboration, Nucl. Phys. B (Proc. Suppl.) 52B (1997) 81. [14] L.W. Jones, Nucl. Phys. B (Proc. Suppl.) 52B (1997) 103. [15] A.S. Borisov, Z.M. Guseva, V.G. Denisova, et al., Bull. Russ. Acad. Sci. Ser. Phys. 49 (1985) 1285. [16] M.F. L’Annunziata (Ed.), Handbook of Radioactivity Analysis, second ed., Academic Press, Amsterdam, Boston, Heidelberg, 2003. [17] K. Berkuz, K. Virtz, Neutron Physics, Springer, Berlin, 1964. [18] S. Normond, B. Mouanda, S. Haan, M. Louvel, IEEE Trans. Nucl. Sci. NS-49 (2) (2002) 577. [19] G.I. Britvich, V.G. Vasil’chenko, V.N. Kirichenko, et al., Insrum. Exp. Tech. 45 (5) (2002) 644. [20] C.M.Lederer, S.V. Shirley (Eds.), Table of Isotopes, seventh ed., A Wiley-Interscience, New York, USA, 1978. [21] V. McLane, C.L. Dunford, Ph.F. Rose, Neutron Cross Sections, vol. 2, Academic Press, London, 1988. [22] Product Handbook for DuPont-Tyvek, DuPont, P.O. Box 80728, Wilmington, DE 19880-0728, USA, 2003. [23] L.S. Peak, P. Soler, Nucl. Instr. and Meth. A 301 (1991) 341. [24] H. Gemmeke, A. Grimm, J. Kleinfeller, et al., IEEE Trans. Nucl. Sci. NS-35 (1) (1988) 377. [25] H. Leutz, Nucl. Instr. and Meth. A 364 (1995) 422. [26] H.-G. Unger, Planar Optic Waveguides and Fibers, Clarendon Press, Oxford, 1977.
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[27] Catalogue of KURARAY Corporation Ltd, Chuo-ku, Tokio, Japan 1997. [28] Philips Data Handbook. Electron Tubes. Book T9, Eindhoven, The Netherlands, 1987. [29] T. Okusawa, Y. Sasayama, M. Yamasaki, T. Yoshida, Nucl. Instr. and Meth. A 440 (2000) 348. [30] G. Keil, Nucl. Instr. and Meth. A 89 (1970) 111.
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Nuclear Instruments and Methods in Physics Research A 550 (2005) 343–358 www.elsevier.com/locate/nima
A neutron detector on the basis of a boron-containing plastic scintillator G.I. Britvicha , V.G. Vasil’chenkoa,, Yu.V. Gilitskya , A.P. Chubenkoa,b, A.E. Kushnirenkoa , E.A. Mamidzhanyana,b, V.P. Pavluchenkoa,b, V.A. Pikalova , V.A. Romakhina,b, A.P. Soldatova , O.V. Sumaneeva , S.K. Chernichenkoa , I.V. Sheina , A.L. Shepetova,b a
Institute for High Energy Physics, 142281 Protvino, Moscow Region, Russia b P.N. Lebedev Physical Institute, Moscow, Russia
Received 10 December 2004; received in revised form 21 March 2005; accepted 1 April 2005 Available online 5 July 2005
Abstract Characteristics of construction elements and investigation results of a prototype of a fast neutron detector are described. The detector prototype is based on a boron-containing molded plastic scintillator SC-331 manufactured in IHEP. The use of this scintillator for neutron detection ensures a capacity to work under high loading rates with a relatively low level of registered background events. The newly created detector may be used for neutron flux measurements in extensive air showers. r 2005 Elsevier B.V. All rights reserved. Keywords: Neutrons; Boron-containing plastic scintillator; Detector; Extensive air showers
1. Introduction At the present time, neutron measurements are widely used in cosmic ray experiments. There exists a world-wide net of 120 neutron stations [1] used for studying cosmic ray intensity variations Corresponding author. Fax: +7 095 2302337.
E-mail addresses: [email protected] (V.G. Vasil’chenko), [email protected] (A.P. Chubenko).
which define our knowledge in a wide range of cosmic ray astrophysics: the solar–terrestrial relationship, the influence of galactic radiation and solar flares on the Earth’s magnetosphere, the Solar system as a whole (cosmic weather), and so on [2,3]. These stations, situated at various latitudes around the globe and at various heights above the sea level, are carrying out continuous measurements with the use of neutron monitors, the most widely used is the NM64-type neutron supermonitor [4].
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.04.083
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In the past, detectors similar to neutron monitors have been used to determine the number of hadrons in extensive air showers (EASs) created by primary cosmic particles with energies above 100 TeV [5]. These standard neutron monitors have also been joined with shower installations and, together with cosmic ray intensity monitoring, have been used as an EAS hadron component detector [6,7]. Investigations being carried out at the NM64type neutron supermonitor based on the shower installation of the Tien–Shan mountain station of the Lebedev Physical Institute, have led to the discovery of an anomalous delay of the neutron signals in EAS cores [8,9]. The essence of this phenomenon consists of the following. The NM64 monitor (just as all other types of neutron monitors) registers the evaporation neutrons from the nuclear fission produced by high-energy hadrons (with energies above 100 MeV) within the lead of the shower installation. Before detection, these evaporation neutrons, created with energies of a few MeV, are slowly thermalized (to energies of about 102 eV), diffusing inside the monitor. The diffusion process results in an exponential time distribution of the neutron signals with a characteristic relaxation time of about 360–400 ms. Such a picture is reproduced with great accuracy when an EAS with an energy below 3 PeV passes through the monitor. However, at energies above 3 PeV, a small part of the shower (about 2–3% of the whole) has a prolonged shape for the time distribution of the intensity of neutron signals. The fraction of such anomalous showers rapidly increases with increasing energy, and reaches about 20–30% at an EAS energy of about 10 PeV. Fig. 1 shows the experimental time distribution of the intensity of neutron signals DM exp =Dt from the Tien–Shan neutron supermonitor registered after the passage of a 20 PeV EAS core by a group of six neutron counters. Here DM exp are the number of counts of neutron signals in the time intervals Dt. The expected diffusion curve of the intensity of neutron signals M corr F ðtÞ for the 20 PeV EAS is also shown on the plot, where M corr is the total number of neutron signals under the diffusion curve and F ðtÞ is the probability
µ
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µ
Fig. 1. The experimental time distribution of the intensity of neutron signals DM exp =Dt from the Tien–Shan neutron supermonitor registered after the passage of a 20 PeV EAS core ðÞ. Solid line M corr F ðtÞ is the expected diffusion curve of the intensity of neutron signals.
distribution of neutron signals for the diffusion curve [8,9]. It is seen that the experimental time distribution of neutron signals shown in Fig. 1 has an anomalously prolonged shape and its deviation from the expected diffusion curve reaches from three to two orders of magnitude in the time interval from 10 to 500 ms. An attempt to explain this effect as a result of neutron signal losses in registration channels [10], leads to a contradiction with the existing models of EAS development in the atmosphere, because the hadron multiplicities, necessary for an explanation of the experimental time distribution of the intensity of neutron signals, prove to be three to two orders of magnitude higher than the values predicted by the models [11]. It should be noted that the observed effect has a threshold character: it starts to become apparent in the energy region above 3 PeV, where a characteristic knee in the primary cosmic ray spectrum, of still unknown nature, is present. In the same energy region, a wide range of unexplained phenomena are also observed in mountain and stratospheric cosmic ray experiments (for example, the halo events [12], Centauros [13] and Anticentauros [14], and the coplanar events in X-ray chambers [15]). Thus, the effect of prolongation of neutron intensity time distributions is representative of the whole series of
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anomalous processes, which points to the necessity of a thorough investigation in the energy range above 10 PeV. Historically, neutron detection in monitors has employed expensive ionization counters. Experience shows that these counters are relatively slow—with output pulses of duration of about 1 ms. This results in a limitation of the maximum trustworthy counting rate for these detectors due to pile up events, which cannot exceed a value of about 5 105 s1 . Moreover, after the passage of an EAS core through the ionization counter, the dead time could considerably increase, which might lead to an additional distortion of the registered neutron counting rate. This circumstance is of vital importance in the exploration of the cores of EASs with primary energies above 10–100 PeV. In fact, our estimates show that for such a purpose, there are detectors capable of operating with neutron fluxes of about 2–3 106 s1 . However, the cases when high-energy EAS cores are passing through the experimental installation are relatively scarce. The overwhelming majority of EASs develop some distance away from the monitor, so the detectors register only their peripherals. Therefore, it is necessary to have detectors with maximum effective areas, capable of registering the peripheral low-intensity neutron fluxes of about 1 cm2 s1 . For the study of cosmic ray intensity variations, effective registration of neutron fluxes with intensities 103 cm2 s1 or even lower is necessary. Thus, the problem of cosmic ray neutron registration comes to two different tasks, one of which demands the creation of a fast neutron detector having a relatively small effective area, capable of operating in the high loading rates in EAS cores, and the second one, the development of a relatively cheap detector with the maximum effective area for registration of low-intensity neutron fluxes.
2. Elements of the neutron detector 2.1. Selection of the active medium Depending on the energy of the investigated neutrons, various methods are used for their
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detection. Detection of neutrons with energies E410 MeV is accomplished via interactions with C nuclei in carbon-rich substances. Detection of neutrons having energies Eo0:1 MeV is accomplished through the scattering of neutrons in a hydrogen-containing medium with the subsequent registration of knocked out protons. In the energy range below 0.1 MeV, neutrons may be slowed down to energies E ’ 0:5 keV with subsequent registration of nðp; dÞg reaction products [16,17]. Further neutron deceleration down to thermal energies ðE ’ 102 eVÞ could even be accomplished as necessary. Plastic scintillators enriched by 6Li or 10B [18,19] may be used as an active medium for the detection of decelerated neutrons. Inside a boroncontaining plastic scintillator, the registration of charged products of the reaction 10Bðn; aÞ7 Li takes place. In 6% of the events this reaction produces a ground state 7Li nucleus along with a 1.78 MeV a-particle. In 94% of the events the lithium nucleus is created in an exited state 7 Li , which is accompanied by emission of a 1.47 MeV a-particle and a 0.478 MeV g-quantum. In contrast to lithium, the use of more inexpensive boron in plastic scintillators is preferable, because the percentage of the necessary 10B isotope in natural boron is high enough (19.9%) [20], permitting one to avoid the expensive procedure of its enrichment. Cross-sections s of 1 eV neutrons on 6Li and 10B are 149 barn and 609 barn, respectivelyp[21]; ffiffiffiffiffiffi with increasing energy they change as s1= E n . It is seen that the use of 10B for the neutron registration is four times more effective than that of 6Li (but for the case of lithium, the energy release of the 6Li ðn; aÞt reaction is 4.8 MeV and no g-quanta are emitted.) In this work we used a boron-containing molded polystyrene (PS) scintillator SC-331 which is manufactured in IHEP. The light output of the scintillator is about 56–60% of that for anthracene, the luminescence maximum wavelength is about lmax ¼ 420 nm, and it contains K ¼ 2–3% by weight of natural boron [19]. It is known that the light output of a-particle-induced scintillations is considerably lower than that from b-particles of corresponding energy. Our measurements have
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shown that the SC-331 scintillator light output for the reaction 10Bðn; aÞ7 Li is equivalent to that from a b-particle with an energy of about 110–130 keV. At the same time, the SC-331 scintillator light output for an minimum ionizing particle (MIP) is equivalent to that from a b-particle depositing an energy of about 2000 keV per 1 cm of the thickness of the boron-containing scintillator. As opposed to boron ionization counters, the boron-containing plastic scintillators do not have the defects mentioned above. Indeed, the radiation decay times for such scintillators are only about a few nanoseconds, and there is no problem with the light output linearity in the full range of EAS energy deposition. However, the use of these detectors would be defensible only if the subsequent photoreceivers and electronics, in their turn, could ensure proper operation rates under high loading conditions. 2.2. Possible layouts of fast neutron detectors Scintillation neutron detectors of various types may be used to solve the astrophysical problems formulated above. These detectors could be divided into two main types: detectors with an external moderator and a thin scintillator and those with a thick scintillator which simultaneously plays the role of the neutron moderator. It is obvious that the characteristics of these detectors will be considerably different. Before dealing with a further development of the detector prototype construction, one has to decide which of the two principal configurations would be optimal for the task in question. Preliminary measurements of some characteristics of the model detectors belonging to both construction types are necessary for this purpose. 2.2.1. Model detector constructions with thin scintillators Fig. 2a shows a simple construction of the fast neutron detector with a thin scintillator (3) having a thickness of d ¼ 5 mm and lateral diameter equal to that of the diameter of the sensitive area S of the photocathode of a photomultiplier (PMT) (5). By means of an optical glue, the boron-containing scintillator is fastened to the surface of the
polymethylmetacrylate (PMMA) light mixer (4), which also acts as a neutron reflector. The other side of the light mixer is attached to the PMT’s entrance window with a silicon glue. The polyethylene (PE) housing (1), together with a 5-cm thick PE moderator (2) for the incoming neutrons, covers the entire set-up from the outside. To increase the efficiency of light collection on the glued photocathode surface, the scintillator (3) and light mixer (4) are wrapped in a reflective Tyvek paper [22]. Results of the amplitude spectrum measurements of moderated neutrons (to the thermal energy region) are shown in Fig. 3a. To suppress the background g-quanta during the measurements, the detector was shielded by a lead sheet of 5-cm thickness. The spectrum has a mean amplitude value at channel A ¼ 221 and a relation of the r.m.s. peak width DA to A of d ¼ DA=A ¼ 0:09. In such a case, the mean number of photoelectrons (ph.e.) Z for the neutron peak may be estimated as [23,24] pffiffiffiffi d ¼ 1:14= Z , (1) where 1.14 is a parameter necessary to take into account the influence of the coefficient of secondary photoemission of the PMT used (FEU-184Ti) on the width of the amplitude spectrum. Hence, for the selected PMT FEU-184Ti (with a diameter of its sensitive area of about 63 mm and with a quantum efficiency Y ¼ 26% at the wavelength of 410 nm), the mean number of photoelectrons in the neutron peak is about Z ’ 160 ph:e: The accuracy of these measurements is about 10%. This scintillation counter is a detector with a relatively low effective area of neutron detection, defined as a product SF Z, where F is the 10B ðn; aÞ7 Li reaction probability per one incident fast neutron for the given detector construction, S is the sensitive area of the photoreceiver and Z is the registration efficiency of this reaction by the photoreceiver. Thanks to the high Z value of the construction, it is relatively simple to obtain the efficiency of Z ¼ 1. This model of a neutron detector permits us to separate neutron signals from MIP and g-quanta ones. The considered detector could be used both for pulse counting and for amplitude analysis.
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1 6
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Fig. 2. Layouts of fast neutron detectors using thin scintillators in combination with polyethylene moderators. (a) Detector with a small effective area (cross-section) for the high loading rate operation conditions; (b) Detector with an enhanced effective area. A is a top view (the polyethylene moderator is not shown) and B is a cross-section; (c) Detector with the maximum effective area using wavelength shifting fibers or a wavelength shifting light conductor. A is a top view (the polyethylene moderator is not shown) and B is a cross-section. 1, polyethylene housing; 2, polyethylene moderator; 3, thin scintillator; 4, light conductor/light mixer which serves simultaneously as a neutron reflector; 5, PMT; 6, voltage multiplier and other electronics; 7, wavelength shifting fibers or a wavelength shifting light conductor.
Fig. 2b demonstrates the construction elements of a neutron detector with a large boron-containing scintillator (designations are the same as in Fig. 2a). Thanks to an extended scintillator (3), the effective neutron registration area in such a detector is considerably higher. The maximum lateral cross-section of the scintillator in this configuration also cannot exceed the sensitive area S of the photocathode of a PMT (5). By means of
optical glue, the scintillator’s butt-end is fastened to an adiabatic light conductor (4), which in turn, is connected to the entrance window of the PMT. Both the glued scintillator (3) and light conductor (4) are wrapped with the reflective Tyvek paper. The wrapped scintillator is coupled with a PE neutron moderator (2). A measurement of the light attenuation—see Fig. 3b—shows that the last condition is fulfilled
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Fig. 3. Some characteristics of the neutron detector models. (a) Amplitude spectrum from the interactions of moderated neutrons with the scintillator for the Fig. 2a construction (the FEU-184Ti type PMT is used as a photoreceiver). (b) Light attenuation curve in a strip scintillator ð5 72 mm2 Þ of the Fig. 2b construction with a FEU-110 type PMT as a photoreceiver. (c) Amplitude spectrum from the neutron interactions in the near scintillator range (the Fig. 2c construction with a XP-2232B type PMT). 1, the total spectrum; 2, spectrum after the PMT noise subtraction.
for a 5-mm thick SC-331 scintillator having a length up to 30 cm. The mean path for the light attenuation length in such a scintillator is about l att ’ 50 cm. For an effective registration of the 10Bðn; aÞ7 Li reaction, the mean number of photoelectrons in the neutron peak must be above 5 ph.e. But under
the real conditions of neutron measurements, the intensive background of g-quanta and other particles is always present. In such a case, the requirement for the minimum photoelectron number should be enhanced and must be formulated especially for a given task. Hence, in our case, to effectively separate the neutron peak from the
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g-background of the Pu–Be source and the g-quanta from the 10Bðn; aÞ7 Li reaction, while ensuring reliable operation of the detector during a continuous time, the mean photoelectron number must exceed 20 ph.e. This threshold level also determines the minimum number of photoelectrons required from the far end of an extended scintillator z ¼ 20 ph:e: Because this detector configuration permits the values Z ’ z, the registration efficiency Zo1. As a result, the maximum difference between pulse amplitudes obtained from the near and far end of the scintillator may reach Z=z ¼ 160 20 ¼ 8. The maximum scintillator length in such a case could be up to L ¼ l att lnðZ=zÞ ¼ 105 cm. The considered detector may be used both in pulse counting mode and, partially, for amplitude measurements (if pulses from the near and far parts of the scintillator do not differ by more than a factor of two from each other). Fig. 2c shows the construction elements of a neutron detector with the maximum effective area. By the means of wavelength shifting fibers (WSFs) (7) glued in grooves of the boron-containing scintillator (3), the scintillation light is re-emitted to the long wavelength region and then transported to a photoreceiver. The WSFs are connected to the entrance window of a PMT (5). Estimation of the maximum lateral size of the scintillator’s sensitive area may be based on an empirical fact, in order to achieve the highest degree of light collection, the pitch between the WSFs cannot exceed 6–10 mm. To enhance the total light output, the boron-containing scintillator, together with the WSFs, should be wrapped by the reflecting Tyvek paper. The wrapped scintillator is coupled with a PE neutron moderator (2). The trapping efficiency of WSFs is defined according to the formula [25,26]: ¼ 12 ½1 1=n2 ,
(2)
where n ¼ nc =nr ; nr is the reflection index of the fiber’s cladding, and nc is that of fiber core. Thus, for the double Y-11 type cladding WSF (with a diameter of about 1 mm and lmax ¼ 476 nmÞ; ¼ 0:054 [27].
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Fig. 3c demonstrates an amplitude spectrum for this model of a neutron detector (1) as well as the spectrum after subtracting the PMT’s internal noise (2). Measurements have shown that the neutron peak from the Pu–Be neutron source, which should be observed around the value Z ’ 3:9 ph:e:, cannot be seen over an intensive g-background. The mean value of the spectrum (2) is situated around channel 73, corresponding to Z ¼ 8:4 ph:e: (The quantum efficiency of the XP2232B type used PMT [28] is Y ¼ 11% and the light attenuation length in the WSFs is above l att ¼ 350 cm [27].) In this measurement the far ends of WSFs had reflecting coatings and the source of moderated neutrons was situated close to their near ends, i.e. 50 cm away from the PMT. The Z measurement accuracy in this experiment is about 30%. The value of Z obtained in the last configuration is far enough from the necessary value 20 ph.e., that the characteristics of the detector based on the construction of such a type are far away from those required for our purpose. However, the replacement of the PMT in the construction of Fig. 2c by an avalanche photodiode (APD) [29] with a quantum efficiency in the green spectrum range up to Y ¼ 90%, makes it possible to increase the number of registered photons up to 8.2 times, thus achieving values Z ’ 32 of electron-hole pairs (ehp). The minimum number of ehp required from the farthest end of a scintillator in the pulse counting mode could also be set to z ¼ 20 ehp. The resulting difference between the amplitudes from the near and far regions is about Z=z ’ 1:6. This circumstance permits us to achieve the maximum length for the neutron-sensitive detector range up to L ’ 170 cm with the use of a single APD. Because this detector construction allows for low values for Z; Zo1. The considered detector can only operate in pulse counting mode. Instead of WSFs, the light from a boroncontaining scintillator may also be re-emitted by a wavelength shifting light conductor (WSLC) based on PMMA and then transported to a photoreceiver. The maximum lateral size of the WSLC cannot exceed the sensitive area of photoreceiver. The light attenuation length in WSLC
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may reach l att ¼ 150 cm. For an effective neutron detector, it is necessary to use photoreceivers having the highest quantum sensitivity in the long wavelength (green) range of the WSLC emission spectrum. It is obvious that the maximum area of a boron-containing scintillator cannot exceed the operation surface of the WSLC. So, for WSLC of a rectangular shape, the trapping efficiency can be defined according to formula [30]: h pffiffiffiffiffiffiffiffiffiffiffiffiffi i ¼ 12 1 n2 1=n . (3) In the calculation of n according to Eq. (3), the nr index must be set to the reflection index of the air, i.e. nr ¼ 1. For the adiabatic PMMA WSLC may achieve the maximum trapping efficiency ¼ 0:12–0.25 (depending on the contact type of the WSLC with the photoreceiver used) [30]. Measurements have shown that in the last case (not shown in the figures), the neutron peak from the Pu–Be source is also absent against the g-quanta background. According to our estimates, the average number of ph.e. when the neutron source was situated at the near end of the WSLC (50 cm away from the PMT), was about Z ’ 3:5 ph:e: for the mean quantum sensitivity of the PMT Y ¼ 10%; for the WSLC luminescence spectrum lmax ¼ 490 nm. The end of the WSLC furthest from the PMT had no reflecting coating, and because that, the true value of , according to our estimates, was about ¼ 0:06. The value of Z in this case is considerably lower than the minimum photoelectron number required z ¼ 20 ph:e: The accuracy of this determination is about 30%. Hence, the characteristics of the neutron detector based on the construction of such a type are far away from those required for our purpose. It should be noted that the boron-containing scintillators could be easily designed on the basis of PS with an emitted luminescence maximum lmax p400 nm. Our experimental samples of such scintillators with lmax ¼ 380 nm have shown a light output around 50–52% of that for anthracene, with luminescence decay times of about 2 ns. Such scintillators may be used with WSLC emitting light in the blue spectrum region with lmax ¼ 4302440 nm (and ¼ 0:1220:25), where
the quantum sensitivity of the usual PMT achieves its highest values up to Y ¼ 26%, and they permit us to achieve Z ¼ 20–40 ph.e. Because this detector construction allows for low values for Z, Zo1. The considered detector can only operate in pulse counting mode. 2.2.2. Model detector construction with thick scintillators A simple construction of the fast neutron detector with a thick scintillator is shown in Fig. 4. The size of the scintillator is equal to the size of the sensitive area of the FEU-110 PMT having its diameter of about 63 mm. By means of a silicate optical glue, the scintillator (2) is fastened directly to the entrance window of the PMT (3). In contrast to the analogous construction for the fast neutron detector Fig. 2a, the scintillator thickness in the current design is 75 mm and an external moderator is absent. Moderation of fast neutrons takes place immediately inside the scintillator volume (2) by the creation of knocked out proton cascades of various energies. The PMT registers scintillations caused by knocked out protons. To increase the light collection efficiency, the scintillator is wrapped by the reflective Tyvek paper. The detector ensures a uniform distribution of output signal over the PMT photocathode. An amplitude spectrum of the knocked out protons (1) observed in the SC-331 scintillator when it was exposed to fast neutrons from the Pu–Be source is shown in Fig. 5. Against the dominant background from knocked out protons a weak peak is observable from the interactions of
Fig. 4. Layout of a neutron detector (cross-section) with a thick scintillator. 1, the housing; 2, thick scintillator; 3, PMT; 4, voltage multiplier and other electronics.
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Fig. 5. Amplitude spectrum from neutron interactions inside a 7.5-cm thick scintillator. The FEU-110 type PMT is used as a photoreceiver. 1, for the fast neutrons; 2, for the moderated neutrons.
the neutrons, moderated inside the scintillator. In order to determine Z for this weak peak, the spectrum (2) from preliminary moderated neutrons was also measured and it is shown in Fig. 5. The mean values of these neutron peaks are situated in about channel 124 and d ’ 0:2. This procedure has permitted us to determine Z ’ 32 ph:e: for the weak peak. This is an universal detector, capable of registering both fast neutrons and other particles, including g-quanta. Effective measurement of the relative numbers of fast neutrons in EAS by means of counting of knocked out protons is possible without any difficulties. This means that, in principle, the Z ¼ 1 level may be achieved with the considered detector, but determination of the absolute number of fast neutrons, which have been interacted in the scintillator and have produced cascades of scintillation flashes, demands a precise procedure of detector calibration. A separate problem is to define the connection between the number of registered flashes (mostly from knocked out protons) and the number of fast neutrons which have passed through the scintillator. A
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calibrated source of fast neutrons is necessary for this purpose, which should have a precisely known intensity and an energy spectrum similar to that of the investigated neutron fluxes. For many tasks such a calibration source is hard to find. Because of that the true value of Z may be p1 in such cases. It should be noted that the construction elements in Fig. 4 and the operation principles of this detector are analogous to the detector shown in Fig. 2a. It is obvious that for the creation of a fast neutron detector with a thick scintillators having a large effective area, one can use the model detector construction of the types of Figs. 2b and c. Our estimates show that the scintillator thickness in such constructions must be about 10 cm. Note that thick scintillator considerably increases the registration efficiency of g-quanta commonly attending the neutron radiation. For separation of these g-quanta, one can use the delayed coincidences of the signals from knocked out protons and the signals from the interactions of moderated neutrons within the scintillator’s boron [17]. Measurement results of the characteristics of various neutron detector models presented above show that the simple construction of Fig. 2a with a thin scintillator is the most adequate for the creation of a prototype of fast neutron detectors aimed for the solution of astrophysical tasks in the high energy EAS cores. Relatively high values of the number of photoelectrons registered by the PMT in this model detector construction permit us to select the neutron signals (from MIPs and g-quanta) by means of fast and simple threshold electronics, with results that could be easily interpreted. A detailed design of the neutron detector prototype is presented below. 2.3. Choice of a photoreceiver for the neutron detector prototype Generally speaking, there exist a set of PMTs with appropriate spectral sensitivities of their photocathodes which could be used in the neutron detector prototype: FEU-84, FEU-110, FEU184Ti, FEU-167, FEU-200, and others. However, the low light output from the 10Bðn; aÞ7 Li reaction in PS scintillators requires the use of photoreceivers with quantum efficiency Y 410%. It should
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be mentioned that APDs [28] having Y ¼ 80290% are more appropriate for this purpose than PMTs. It is also important to note that the non-uniformity of quantum sensitivity over the surface of PMT photocathode does not exceed 20–30%. Otherwise, light conductors and mixers must be used to increase the size of the light spots on the PMT photocathodes. Because all aspects of the PMTs used in the neutron detector prototype are quite analogous for various PMT types, all further considerations will be carried out on the example of a FEU-110 type PMT.
2.4. PMT power supply scheme for the neutron detector prototype A variety of methods for the dynode system potential stabilization are widely used for the creation of a stable PMT power supply scheme, capable of operating under high loading rates. In laboratory conditions, passive high voltage dividers with additional high-precision power supply sources are used for the stabilization of the potentials in the last dynode stages. Also possible is the use of active dividers based on Zener diodes [31] or high-b transistors [32] for the feeding of the last dynode stages. Today many photoreceivers employ a power supply scheme based on the Cockroft–Walton voltage multiplier [33]. This is a consequence both of the intensive development of its necessary elemental base and of the intrinsic advantages of the scheme: low consumption of external power coupled with high dynode current, low-voltage external feeding ð 12 VÞ, and the possibility of using individual voltages for tuning and other purposes. The FEU-110 PMT power supply scheme of our detector is shown in Fig. 6. The scheme consists of a capacity-diode voltage multiplier and a control device which permits us to tune the output voltage in the limits 1–2 kV. The high voltage stability is about 103 . Both the voltage multiplier and the registration electronics are situated inside the lightproof detector housing. On the bottom lid of the housing, two low-voltage connectors ð 12 VÞ are placed which are also used for signal output.
Fig. 6. PMT feeding scheme for the neutron detector prototype. 1, PMT; 2, voltage multiplier; 3, transformer; 4, switches; 5, generator; 6, control circuit; 7, digital-to-analogue converter (DAC); 8, amplifier; 9, comparator; 10, negative voltage stabilizer; 11, positive voltage stabilizer; data, digital code of the output high voltage; clock, DAC clock; V out , amplitude of the comparator’s output pulse; 12 V , the external feeding voltage.
It is commonly accepted that a PMT operates in the linear regime if the average anode current i does not exceed 10% of the current flowing through its divider. Normalized amplitudes of the output signals A=A0 from an a-source with activity 8 103 s1 placed on the scintillator are shown in Fig. 7 as a function of the anode current i. A0 is the amplitude at i ¼ 180 nA; the current was varied through an additional illumination of the PMT photocathode. Hence, the applied asource creates an anode current of about i ¼ 180 nA by the use of a Cockroft–Walton voltage multiplier. The Cockroft–Walton scheme ensures a sufficiently high stability level and may be used until the anode current values reach up to i ¼ 1 mA. Therefore, this scheme would be operating reliably until the equivalent loading rates reach up to 5 108 a-particles/s. For comparison, analogous characteristics for a passive voltage divider with a total resistance of 2 MO is also shown in Fig. 7. Photoreceivers used in the neutron scintillation detector must have sufficiently low levels for their intrinsic noise. Low-noise requirements are of special importance if the low-intensity neutrons of natural background are to be used for the
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Fig. 7. Dependencies of the normalized amplitude of FEU-110 type PMT output signal A=A0 on the value of the anode current i. , for the Cockroft–Walton feeding scheme; m, for the passive 2 MO voltage divider. The curves are drawn to guide the eye.
detector calibration. To decrease the level of PMT noise, another feeding scheme is possible when the positive high voltage is connected to the PMT anode (and the free end of the voltage divider is grounded).
3. Results 3.1. Neutron detector model testing with a Pu– Be neutron source Testing of a neutron detector model was performed on a construction similar to that shown in Fig. 2a without the PMMA light mixer/neutron reflector (4). An ordinary FEU-110 PMT with quantum sensitivity Y ¼ 12% was used as a photoreceiver. The 5-mm thick SC-331 scintillator was glued directly to the PMT photocathode. Characteristics of this neutron detector model were determined by the registration of previously moderated neutrons from a Pu–Be source with a 105 Bq activity. The source was placed near a 5-cm thick PE moderator.
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Fig. 8. Amplitude spectra from the interactions of the moderated neutrons with various elements of the detector’s layout. 1, spectrum of a boron-containing SC-331 scintillator situated on a FEU-110 type PMT; 2, spectrum of a SC-301 scintillator (without boron); 3, spectrum from the input window and the PMT intrinsic noise.
Fig. 8 demonstrates the 10Bðn; aÞ7 Li reaction amplitude spectra from the detector with boroncontaining scintillator SC-331 (1), from the detector an ordinary SC-301 scintillator having analogous sizes (2), and the background spectrum from g-quanta and the intrinsic PMT noise (3). The sampling time for all spectra was equal to 5 min. The intensity of g-quanta during the measurements was quite high; for this reason the detector was shielded from the Pu–Be source by a 5-cm thick piece of lead. The measurements have shown that the mean value of the neutron peak A corresponds to channel 164 with d ¼ 0:16 and Z ¼ 50 ph:e: The value of Z is defined according to formula (1). The amplitude dispersion of output pulses over the photocathode surface results in an increase of d, i.e. in some Z decrease compared to the value which should be expected for Y ¼ 12%. Comparison of the curves (1) and (2) in the region over channel 400 shows that the fraction of 478 keV g-quanta from the 10Bðn; aÞ7 Li reaction to the background pulse intensity is about 23%.
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counts. For the values d ¼ 5 mm and the boron concentration K ¼ 3% in our scintillator, N=N max ¼ 0:53. The measurements show that the minimum number of background events N b (i.e. the minimum of N b =N) is achieved for small scintillator thickness, dp3 mm. But it is also desirable to keep the relation N=N max as high as possible. Thus, the r value (and consequently K) must proportionally increase with decreased d. Natural limits for K growth are the capacity of the PS scintillator matrix to contain the boron admixture and the dropping of the scintillator light output with increasing K. At the present time, stable boroncontaining scintillators with Kp6% have been created. Fig. 9. Dependencies of the number of registered neutrons N ðÞ and the number of background events N b ðmÞ on the SC331 scintillator thickness d. Dashed line—the maximum value N max , obtained in fitting of the experimental data by the function (4). The curves are drawn to guide the eye.
Fig. 9 shows the dependencies of the numbers of registered moderated neutrons N and the background events N b (defined in the 3s interval around the neutron peak mean) on the scintillator thickness d. The accuracy of these measurements is about 3%. The use of a boron-containing PS scintillator creates a certain level of the background events. The background intensity is defined mostly by the interactions of both of the Pu–Be source g-quanta and the 478 keV g-quanta from the 10Bðn; aÞ7 Li reactions inside the scintillator and the PMT entrance window. The intrinsic PMT noise is significant only in the low-amplitude region. As d increases the background grows roughly linearly. The dependence of the number of registered neutrons on the scintillator thickness may be expressed as [16,17] N ¼ N max ½1 expðrdÞ ,
(4)
where N max is the maximum neutron number which could be registered from the given neutron source and r is an absorption coefficient. Fitting the experimental results by Eq. (4) shows that in this case r ¼ 0:151 mm1 and N max ¼ 2:06 105
3.2. Neutron detector prototype testing with a calibrated Pu– Be neutron source To estimate the detector prototype registration probability, the number of the 10Bðn; aÞ7 Li reactions inside the scintillator per one incident fast neutron needs to be determined. This number is defined by the following functional F of the neutron field [16,17]: Z E max Z F¼ FðE; rÞsðEÞ d3 r dE. (5) 0
V
Here E is the neutron energy; FðE; rÞ is the energy and spatial density of the neutron flux; and sðEÞ is the cross-section of the 10Bðn; aÞ7 Li reaction for neutrons with the energy E [21]. The integration are performed over the entire energy range of neutrons (from 0 to E max ) and the entire detector volume V. It is clear that the detector registration probability F Z is the energy dependent characteristic. We measured this characteristic for our detector prototype as the registration probability of a primary fast neutron from a calibrated Pu–Be source with a 5:35 105 Bq activity. The fast neutrons were normally incident to the external surface of the PE moderator of the neutron detector prototype (of the Fig. 2a type design). These measurements were performed in an open geometry at a height of about 6 m above the concrete floor. The detector registration
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probability was defined as a relation of the integral number of pulses under the neutron peak in the amplitude spectrum to the total number of the fast Pu–Be-neutrons that passed through the neutron detector prototype. Taking into account that in our case Z ¼ 1, the detector registration probability of fast neutrons from Pu–Be source F Z ’ 0:95% was obtained. In the flux measurements of the fast neutrons another characteristic is usually used; the detector sensitivity, which takes into account the moderator size ðS ¼ 95 cm2 Þ. In our case, the detector sensitivity for fast neutrons from Pu–Be source is equal to 0:9 pulse=cm2 =neutron with a statistical error in the measurement of about 10%.
3.3. Neutron detector prototype testing under conditions of Tien– Shan mountain cosmic ray station The neutron detector prototype (of the Fig. 2a type design) was tested in the real operation conditions at the Tien–Shan mountain cosmic ray station (3340 m above sea level). In autumn 2003 the neutron detector prototype was installed inside the NM64-type neutron supermonitor [9], and amplitude spectra of its pulses were obtained in two operation modes: (1) the simple cosmic ray intensity monitoring (with registration of all the pulses coming from the prototype without any selection) and (2) with triggering by shower master signals which were generated when EASs were passing through the monitor. Fig. 10a and b show a part of the amplitude spectrum for output pulses in the range around the neutron peak when the prototype was operating in the simple monitoring mode. The neutron peak, having the maximum in channel 231 and root mean square s1 ¼ 36 channels, is distinctly seen against a wide monotonic background (Fig. 10a). Fig. 10b shows the same spectrum with the
Fig. 10. Amplitude spectrum of our neutron detector prototype in the simple monitoring mode. (a) Total spectrum. (b) Spectrum after the background subtraction. Solid lines correspond to the Gaussian fit of our experimental data points.
(a)
(b)
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background subtracted. The integral pulse intensity below the neutron peak is 0:34 s1 with a measurement error of about 10%. Fig. 11a and b show an analogous spectrum obtained when an amplitude discriminator scheme was triggered by the EAS installation’s shower master signals. The pulses from the prototype were analyzed only during time gates 10–1500 ms after each shower trigger. (This time interval overlaps the period when thermalized neutrons from cosmic ray hadrons induced nuclear fission are diffusing inside the monitor.) In the amplitude spectrum a neutron peak is present, its average being at channel 226 with root mean square s1 ¼ 38 channels. As should be expected, the measured integral intensity in this case (the area below the neutron peak) is 22 times higher as in the previous one, and reaches 7:5 s1 . These measurements have demonstrated the adequacy of the characteristics of the neutron detector prototype in real operation conditions, and have demonstrated the possibility of its use in shower experiments at the Tien–Shan station.
(a)
4. Conclusion This work has confirmed the possibility of creation of fast neutron detectors based on boron-containing molded polystyrene scintillators SC-331 and set-up constructions like those shown in Figs. 2a–c and 4. An analysis of the basic characteristics of the model detectors has shown: (1) The creation of fast neutron detectors with large effective operation areas is possible on the basis of Fig. 2c construction, with the use of wavelength shifting fibers and photoreceivers with high quantum sensitivity (about 80–90%) in the long wavelength spectrum range (for example, those of the avalanche photodiode type). Fig. 11. Amplitude spectrum of our neutron detector prototype under triggering by the EAS master signals. (a) Total spectrum. (b) Spectrum after the background subtraction. Solid lines correspond to the Gaussian fit of our experimental data points.
(b)
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(2) For fast neutron detector with moderate effective operation areas, a construction of Fig. 2b may be used. (3) For fast neutron detectors with relatively small effective operation areas capable of operating reliably under extremely high loading rate conditions, the construction of Fig. 2a is the most appropriate. (4) Use of 6Li instead of 10B in plastic scintillators for the neutron detection would increase the energy output up to three times. This circumstance would give a principle possibility for a considerable enlargement of the effective operation areas in the neutron detector constructions of Fig. 2b and c. (5) Measurements with the neutron detector prototype have shown that the registration probability of a fast neutron from a Pu–Be source is about 0.95%. The detector ensures a 20% stability of output pulses until the anode current reaches values of about 1 mA. (6) Our investigations at the Tien–Shan mountain station have shown the adequacy of neutron detector prototype characteristics for the detection of cosmic ray neutrons. The boron-containing molded plastic scintillator SC-331 may also be used for the creation of hadronic calorimeters in high energy physics. Detectors based on Fig. 2a and b constructions, may be used both in neutron radiation dosimetry and in various geological, geophysical, and seismological works. The neutron detector prototype is capable of ensuring operation of the control systems in the case of a significant short-time increase of the neutron intensity, therefore the scintillation neutron detectors may be used for monitoring of nuclear fuel waste storage.
Acknowledgements This work is supported by the RFFI Grant 0502-16655. The authors are thankful to the Department of the Atomic Science and Technique of the Russian Atomic Ministry for the financial support and to our IHEP colleague—A.S. Solov’ev for his help in the work.
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