Nuclear Instruments and Methods in Physics Research A 454 (2000) 257}259
The porous multiwire detector夽 Martiros Lorikyan* Physics Institute 2, Alikhanian Brothers Str., Yerevan 375036, Armenia
Abstract A porous multiwire detector is described and preliminary results of investigations are given. 2000 Published by Elsevier Science B.V. All rights reserved.
1. Introduction In 1969, we have found that under the action of an external electric "eld an e!ective drift and multiplication of electrons arise in layers of some porous dielectrics and highly e!ective controllable secondary electron emission takes place [1}5]. In contrast to the Malter e!ect this e!ect is controllable and instantaneous (see review [15]).
2. Basic principle The process of electron drift and multiplication in a porous dielectric arises as follows: The ionizing radiation produces electrons in the walls of porous material. These electrons in the pores are accelerated by the applied electric "eld and produce secondary electrons on the walls of pores. This takes place with all generations of secondary electrons and when the mean gain of secondary electron emission becomes larger than 1, an avalanche-like multiplication of electrons is developed [4]. 夽 Work supported by the International Science and Technology Center. * Tel.: 3742-26-31-5; fax: 3742-22-46-33. E-mail address: lorikyan@moon.yerphi.am (M. Lorikyan).
We have used the e!ect electron drift and multiplication in a porous dielectric under the action of external electric "eld for the development of emission and multiwire porous detectors.
3. Emission detector The emitter (Fig. 1) consists of a metallic substrate (1), porous KCl layer (2) and a metallic grid (3) to which the positive voltage is applied [6]. The electrons come out through the openings in the grid. The secondary emission gain p is determined as the ratio of the number of secondary electrons to the number of b-particles passing through the detector. In Fig. 2, the dependence of the secondary electron emission gain p on the applied voltage under the action of 2 MeV b-particles is shown: The porous KCl emitter was 50 lm and 400 lm thick, respectively, with a density of 3% of the single crystal density.
4. Multiwire porous detector The multiwire porous detector is like a Charpak multiwire chamber, but here instead of gas the
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Fig. 1. Schematic view of the emitter. Metallic substrate: 1; porous KCl: 2; metallic grid: 3.
Fig. 4. The time diagram of applied voltages.
Fig. 2. Secondary emission gain p as function of the applied voltage for porous KCl of 50 lm and 400 lm thickness. Fig. 5. Dependence of the number of detected 5 MeV a-particles on the operating voltage.
Fig. 3. Schematic view of the multiwire porous detector. Anode wires: 1; porous KCl: 2; and cathode: 3.
porous dielectric is used. The gap and distance between the anode wires are also very small [7}15]. In Fig. 3, the schematic view of the multiwire porous detector used is shown. It has anode wires (1), porous KCl (2) and cathode (3). The gap of the detector is 400 lm, the distance between the anode wires is 500 lm and the diameter of these wires is 25 lm. The detector had porous KCl of the density of 1% of the density of single crystals. These detectors operate in vacuum better than 10\ Torr and have 100% detection e$ciency for strong and minimum ionizing particles and soft X-rays, the space and time resolutions are less than $100 lm and $60 ps, respectively.
The operation of these detectors is stable when they are supplied by voltage pulses of operation and depolarization following each other. In Fig. 4, the time diagram of the voltages applied is given. The operation and depolarization voltages have 3 and 17 ms duration, respectively. The amplitude of the depolarizing pulse was 700 V. The signals were ampli"ed by a fast ampli"er having a gain of +200 and 2.5 ns rise time. In Fig. 5, the dependence of the number of detected 5 MeV a-particles N on the operating voltage is given. The reaching of a plateau means that the detection e$ciency becomes 100%.
5. Neutron detection The porous detectors permit the detection of the thermal neutrons with 100% e$ciency, as all the thermal neutrons absorbed in the porous material
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Acknowledgements The author would like to thank R. Aivasyan, G. Aivasyan and O. Vinnitskij for their help.
References
Fig. 6. Dependency of secondary emission gain p of a porous LiF emitter on the electric "eld.
are detected and it is possible to choose the thickness and density of the porous material such that the thermal neutrons and reaction products are fully absorbed. For example, for full absorption of thermal neutrons in porous LiF of 4% relative density, a 400 lm thick layer is needed. From this point of view very interesting are the results of investigations of the dependence of secondary emission gain on the voltage applied to the LiF emitter, exposed to 5.5 MeV a-particles (Fig. 6). 6. Outlook I should like to note that we have not used high-purity dielectrics, and in case of high-purity dielectrics we expect that the polarization e!ect requiring pulsed high voltage will be strongly reduced.
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