Porous CsI multiwire dielectric detectors by Hayk Lorikyan

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Nuclear Instruments and Methods in Physics Research A 510 (2003) 150–157

Porous CsI multiwire dielectric detectors$ M.P. Lorikyan* Yerevan Physics Institute, 2 Brothers Alikhanian Str., 375036 Yerevan, Armenia

Abstract The multiwire porous dielectric detectors developed earlier worked stable in combination with a pulsed voltage supply, which limited their application. In this work a new type of multiwire porous dielectric detector has been developed and investigated. Such detectors operate at a constant voltage supply and thus in a permanent mode. These detectors are shown to exhibit a good time stability and a highly efﬁcient detection of both heavily ionizing particles and soft X-rays and have standard deviation of a spatial resolution of better than s ¼ 718 mm: r 2003 Elsevier B.V. All rights reserved. PACS: 29.40.Gx; 29.40.Wk; 78.55.Mb Keywords: Porous multiwire detectors

1. Introduction In 1979 a group of researchers of the Yerevan Physics Institute observed the phenomenon of drift and multiplication of electrons (EDM) in porous dielectrics (PD) under the influence of an external electric field [1–5], which allowed them to create a new class of radiation detectors—porous detectors (PD). In these detectors as a working material, porous dielectrics KCl, CsI and so on are used and they work on the basis of the abovementioned phenomenon. The EDM in PD in an external electric field has been demonstrated as controllable by the electric field and provides a very high yield s of secondary electrons per incident heavily or minimum ionizing particles. $ The work is supported by the International Science and Technology Center *Tel.: +374-2-26315; fax: +374-2-224633. E-mail address: lorikyan@star.yerphi.am (M.P. Lorikyan).

[1–9]. When the porous emitter is traversed by minimum-ionizing particles, the secondary emission gain factor s is 250 [6]. In the case of aparticles the gain factor is several thousands [7– 12]. Later the secondary electron emission in an external electric ﬁeld has also been investigated [13,14]. Afterwards, the same group of researchers developed and studied multiwire and microstrip porous detectors [12,15]. In earlier works, it was shown that the time and spatial resolution of such detectors are less than 60 ps and 250 mm: The rise time and width of pulse are p1 ns and E2 ns; respectively [15,16]. These detectors are operated in a vacuum of p10 2 Torr and can have a little quantity of material on the particles’ path. Qualitatively, the microscopic mechanism of the highly effective EDM in porous dielectric media under the presence of an external electric ﬁeld can be represented in the following manner. In the pore walls, primary particles stimulate electrons of

ARTICLE IN PRESS M.P. Lorikyan / Nuclear Instruments and Methods in Physics Research A 510 (2003) 150–157

different energy bands in the dielectric material to escape from their bands and enter into a free band. Thus, electrons and positive holes are formed. The electrons of energies higher than the electron affinity w of the material can escape from the pore walls and can be accelerated in the pores under the action of an external electric field. When the energy acquired by the accelerated electrons is enough they produce new secondary electrons and holes in the medium. If the coefficient of secondary-electron emission in each act of electron collisions with the pore walls is si > 1; an efficient EDM process occurs in a porous medium and this process produces an avalanche multiplication of electrons (holes). Under usual conditions, when the value of w in dielectrics is sufficiently high, electrons of very small energies cannot take part in the emission process and a high coefficient si is practically impossible. Probably, as in the case of p-type semiconductors, in pore walls under the action of an electric field the electron affinity noticeably decreases. The decrease in w also increases the thickness of a layer (escape depth) from which these electrons can reach the surface and escape into vacuum [17]. All these effects provide a high si and efficient EDM processes in a porous medium. The electrons and holes under the influence of an external electric field move in opposite directions and on the anode a negative pulse arises. The process of EDM is schematically shown in Fig. 1.

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However, these processes are suppressed by the presence of defects in the crystal structure of the porous medium, as far as electrons and holes can be trapped by defects, and space and surface charges arise, i.e., the medium becomes polarized. Of note is the fact that detectors with dielectric working media have a notable density of carriertrapping centers and thus will have a poor spatial and energy resolution, and eventually an unstable performance [18]. It is well known that the anomalous secondary electron emission (ASEE, Malter effect), where the processes of the drift and multiplication of electrons occur due to the presence of large space and surface charges in the porous medium, is unstable, inertial and noncontrollable [19–25]. In case of ASEE for minimum-ionizing particles, low coefficient of the secondary electron emission was observed ðsE1Þ [24,25]. That is why the ASEE has never been used for the radiation detection. In earlier studies [15,16,26–33], we observed a rapid decrease of particle detection efficiency of multiwire porous detectors (MWPDD) with time and the performance of MWPDD was stable only in the mode of pulsed voltage supply. In [34–36] the MWPDD was assembled under technological conditions that were as clean as possible, therefore, the polarization effects are minimized and the MWPDDs’ performance is stable in permanent mode operation with good stability. Moreover they feature high detection efficiency and good spatial resolution. In this work the MWPDD with the porous CsI with a density of E1:1% of the single-crystal bulk density is investigated. A MWPDD under clean technological conditions was assembled.

2. The multiwire porous detector (MWPDD) and conditions of measurements

Fig. 1. Schematic view of drift and multiplication of ionizing electrons in porous dielectrics in the presence of an external ﬁeld.

The schematic view of MWPDD is shown in Fig. 2. The anode wires ð+25 mmÞ denoted with (1) in this ﬁgure are made of gilded tungsten. They are stretched across the ﬁberglass laminate frame (2). The anode wires in the MWPDD are spaced with a pitch of b ¼ 0:25 mm: The sensitive area of MWPDD was 22:0 22:0 mm2 : A porous CsI

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Fig. 2. Schematic view of the MWPDD: (1) anode wires, (2) ﬁberglass laminate frame, (3) frame, which determines the detector’s gap, (4) cathode, (5) porous CsI layer.

layer (5), having a thickness of 0:8 mm and a density of 0.7% is deposited on the cathode (4) which is made of an aluminum foil of 65 mm thickness. The frame (3) determines the detector’s gap, which is 0:5 mm: So, after assembling the detectors, the CsI layer was compacted by a factor of 1.6. The porous CsI layer was prepared by thermal deposition in an Ar atmosphere [37] at a pressure of p ¼ 3 Torr: The measurements were taken under 7 10 3 Torr vacuum at room temperature ðE20 CÞ: The purity of CsI was 99.99%. Fast current amplifiers with a rise time of 2 ns; a conversion ratio of 30 mV=mA and an input impedance of E250 O were used to amplify the MWPDD pulses. The detection threshold after amplification was Vthr ¼ 35 mV: In the pauses between measurements of each point, the detectors were exposed to radiation and a voltage was applied. The energies of X-rays and a-particles were 5:9 keV and 5 MeV; respectively. The intensity of a-particles was 676 min 1 cm 2 and the intensity of X-rays was 576 min 1 cm 2 : The Xrays were incident to the cathode, while a-particles impinged parallel to the anode wires. In all cases, measurement errors are statistical and are not indicated in figures. The whole system’s intrinsic noise was E0:1–0:2 s 1 :

We did not carry out a strong control of treatment and other relevant parameters of the experiment. That is why we observed large differences in the counting characteristics obtained at different times after assembling an MWPDD. However, after exposing the MWPDD to vacuum, all the multiwire porous detectors became insensitive to such parameters and had the same counting characteristics that did not change furthermore. Apparently, these effects are associated to changes in the structure of CsI crystals in the porous layer. This problem needs more comprehensive studies, which are described in the following. In the first experiment the anode wires in the MWPDD are all connected to one input of the amplifier. Fig. 3 shows the dependence of the number of detected aparticles Na versus the voltage measured 5 h after the deposition of the porous CsI layer. One can see that this curve forms a plateau at the level of 100% of the registration efficiency. Once the 100% level was reached a stable registration efficiency Z of the MWPDD was obtained as shown in Fig. 4. In order to evaluate the spatial sensitivity of the detector and determine the upper limit of the spatial resolution, in the second experiment the anode wires of the MWPDD were divided into two groups (Fig. 5). The first group included the even-numbered and the second group included the odd-numbered

3. Results of measurements Our investigations have shown that the initial characteristics of porous detectors based on porous CsI are very sensitive to the treatment of the porous CsI layers and the operation of an MWPDD. Therefore sometimes its counting properties are not reproducible.

Fig. 3. The dependence of the total number of detected aparticles Na versus voltage ﬁve hours after the CsI layer was deposed.

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Fig. 4. Stability of the registration efﬁciency Z of the MWPDD observed immediately after the measurements shown in Fig. 3.

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NðIIÞ NðcÞ: The ratio of the number of coincidence pulses to the total number of detected particles Ss ¼ NðcÞ=N determines the spatial sensitivity of the detector. In the case of a very low spatial resolution NENðcÞ; as in the case of NbNðcÞ; the detector’s spatial resolution is better than the spacing b: We assume, that the detector’s spatial resolution is better than b in the case of Ss ¼ NðcÞ=Np0:37 and it is worse than b in the case of Ss > 0:37: Fig. 6 shows the dependence of the number of detected a-particles on voltage measured in 5 hours after porous CsI layer’s deposition. The crosses correspond to Na (I), points—to Na (II), triangles—to Na (c), and squares—to Na ¼ Na ðIÞ þ Na ðIIÞ Na ðcÞ: From these results it follows that all the curves form a plateau, and within the errors each group of wires counts an equal number of aparticles and in the region of plateau the number of coincidence pulses Na ðcÞ is very small ðE3%Þ compared with the total number of a-particles. Therefore, the spatial resolution of this MWPDD is better than the distance between the anode wires, i.e., 7125 mm: Fig. 7 shows the time-stability functions for Na (squares) and Na (c) (triangles) observed immedi-

Fig. 5. The scheme of the measurement.

anode wires. Anode wires in each group were connected with each other and the numbers of signals from the ﬁrst group NðIÞ and the second group NðIIÞ were separately ampliﬁed and counted. The number of coincidence pulses from both groups of wires NðcÞ was also counted It is evident that in a spatially sensitive detector each particle is detected by only one anode wire (in the presented experiment by only one group of wires), without producing coincidence pulses and when the detector has no spatial sensitivity, the same particle is detected by two adjacent anode wires and coincidence pulses can arise. The total number of detected particles is N ¼ NðIÞ þ

Fig. 6. The dependence of the number of detected a-particles by the ﬁrst group of anode wires Na ðIÞ (crosses), by the second group of anode wires Na ðIIÞ (points), the number of coincidence pulses Na ðcÞ from both groups of anode wires (triangles) and the total number of a-particles, Na ¼ Na ðIÞ þ Na ðIIÞ Na ðcÞ (squares) on voltage measured in 5 h after porous CsI layer’s deposition.

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Fig. 7. The time-stability functions for the total number of aparticles Na (squares) and for the number of coincidence pulses Na ðcÞ from both groups of anode wires (triangles) observed immediately after obtaining the results of Fig. 6.

Fig. 8. The results of the 15-h-long time-stability measurements carried out on the sixth day, squares—the total number of aparticles, Na ¼ Na ðIÞ þ Na ðIIÞ Na ðcÞ; triangles—the number of coincidence pulses Na ðcÞ from both groups of anode wires.

ately after obtaining the results of Fig. 6. Similar results were obtained throughout. The results of the 13-h-long time-stability measurements carried out on the sixth day are presented in Fig. 8. It follows from Figs. 7 and 8 that the MWPDD has a stable registration efﬁciency and stable spatial resolution. Similar measurements were conducted with Xrays. Here, all qualitative characteristics of the

Fig. 9. The dependence of the number of detected X-quanta by the ﬁrst group of anode wires Nx ðIÞ (crosses), by the second group of anode wires Nx ðIIÞ (points), the number of coincidence pulses Nx ðcÞ from both groups of anode wires (triangles) and the total number of X-quanta, Nx ¼ Nx ðIÞ þ Nx ðIIÞ Nx ðcÞ (squares) registered in 5 h after assembling the MWPDD as a function of the voltage.

MWPDD were the same as in the case of aparticles. Fig. 9 shows the number of X-quanta registered 5 h after assembling the MWPDD as a function of the voltage. The crosses correspond to Nx (I), points—to Nx (II), triangles—to Nx (c) and squares—to the total number of X-quanta Nx : In contrast to a-particles, in this case the plateau on these curves is not so indicative. However, like in the case of a-particles, each group of wires counts an equal number of X-quanta and the number of coincidence pulses Nx (c) is always very small compared to the total number of X-quanta. Thus, the spatial resolution of MWPDD for X-rays is better than 7125 mm: Results of the same kind of measurements conducted on the 14th day are shown in Fig. 10. The comparison of Fig. 9 with Fig. 10 shows that during 14 days no signiﬁcant change in the counting response of the MWPDD had taken place, and in both cases the registration efﬁciency of X-rays on the plateau of this curve was 0.7. The results of the time-stability measurements are presented in Fig. 11, where the average numbers of Nx ¼ Nx ðIÞ þ Nx ðIIÞ Nx ðcÞ (squares) and Nx (c) (triangles) per day are shown as a function of days. This MWPDD was investigated for 14 days, for 9 h a day, being turned off for the

ARTICLE IN PRESS M.P. Lorikyan / Nuclear Instruments and Methods in Physics Research A 510 (2003) 150–157

Fig. 10. The dependence of the number of detected X-quanta by the ﬁrst group of anode wires Nx ðIÞ (crosses), by the second group of anode wires Nx ðIIÞ (points), the number of coincidence pulses Nx ðcÞ from both groups of anode wires (triangles) and the total number of X-quanta, Nx ¼ Nx ðIÞ þ Nx ðIIÞ Nx ðcÞ (squares) conducted on the 14th day.

Fig. 11. The results of the time-stability measurements carried out for 14 days, squares—the total number of a-particles Na and triangles—the number of coincidence pulses Na ðcÞ from both groups of anode wires.

remaining 15 h of the day. On the second, fourth and ninth days the MWPDD was off. As seen in Fig. 11, Nx (the X-ray registration efﬁciency) had decreased by about 30% on the third day of the measurements, then within the errors remained constant. Also no signiﬁcant changes in the number of events with the same particle being registered by two adjacent anode wires were

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Fig. 12. The U-dependencies of the total number of a-particles Na —squares and the number of coincidence pulses from both groups of anode wires Na ðcÞ—triangles on the second day after assembling of the detector with b ¼ 0:125 mm:

observed, i.e. the spatial resolution of MWPDD had not changed with time. Similar investigations have also been carried out with an MWPDD having b ¼ 0:125 mm The sensitive area of this MWPDD was 10 20 mm2 : The intensity of incident a-particles was 1950 min 1 cm 2 : The U-dependencies of Na and Na ðcÞ on the second day after assembling of this detector are shown in Fig. 12. As it is seen, in this case Na ðcÞ=Na ¼ 0:03 at the detection efﬁciency of a-particle of 0.9. Thus the standard deviation of spatial resolution of the MWPDD is better than s ¼ 718 mm: It should be noted that upon ﬁnishing the series of these measurements, all detectors were disassembled, and revealed no changes in the porous CsI layer. Indeed, we did not register any deterioration of the MWPDD’s counting response during the testing period, which sometimes lasted more than hundred days.

4. Conclusions It is necessary to note that the available experimental data are insufﬁcient for a ﬁnal quantitative interpretation of the obtained effects. But it has been indicated that in certain time after

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assembling the MWPDDs, they spontaneously acquire high time-stability and spatial sensitivity. It is evident that using CsI of higher purity one will obtain better results. The experimental results form a basis for further studies of the properties of porous detectors under the influence of an electric field and ionizing radiation. In addition to their practical significance, these results will promote the development of the theory of electron drift and multiplication in porous dielectric media under the action of strong electric fields and ionizing radiation. These results broaden and deepen the knowledge in the field of porous dielectric physics and open up new perspectives for non-traditional use of dielectric media in engineering.

Acknowledgements The author is grateful to the founders of the International Science and Technology Center, to collaborators of ISTC proposal project Professors M.A. Piestrup and Ch.K.Gary for valuable collaboration, to ISTC ofﬁcers L. Horowicz and S. Temeev for their support. The author also expresses his gratitude to G. Ayvazyan, R. Aivazyan, H. Vardanyan, for their assistance in the process of this study and professor F. Sauli and A.K. Odian for rendering of large support, G. Grigoryan for help in data processing and G. Tadevosyan for help in the preparation of this paper.

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