Investigation of the dielectric particle detector

NUCLEAR

INSTRUMENTS

AND

METHODS

167 (1979)

427-430;

(~) N O R T H - H O L L A N D

PUBLISHING

CO.

INVESTIGATION OF T H E DIELECTRIC PARTICLE D E T E C T O R

S. M. GUKASIAN, R. L. KAVALOV, M. P. LORIKIAN and Yu. L. M A R K A R I A N Yerevan Physics Institute, Yerevan 375036, Markarian St. 2, Armenia, U.S.S.R. Received 28 March 1979 and in revised form 17 July 1979 A dielectric detector is described, in which low density MgO served as a working medium. Some characteristics of the detector are given and the dependence of detector efficiency on the applied voltage for different densities of MgO is discussed.

1. Introduction In our previous report l) a novel detector of charged particles was described, in which the use of the drift and multiplication of &electrons in porous low-density dielectric materials in strong electric fields was made 2< ~). It was shown that when minim u m ionization particles pass through the porous dielectric layer a controllable secondary electron emission results if a uniform electric field is applied to the dielectric. In the dielectric detector (DD) a strong electric field is established in the vicinity of anode wires immersed in the porous dielectric. The secondary electrons are collected on these wires. c~-particles with energy of about 5 MeV were observed to produce on the anode wires pulses with amplitudes up to 10 mV and full width < 10 ns. In the present work we give the results of the more detailed investigation of a DD with MgO as a working medium. 2. The detector and the experimental arrangement The detector in question does not differ in its de,sign from that previously reported. It is represented diagrammatically in fig. 1. Its external electrodes are fine metal meshes with 2 0 0 × 2 0 0 / ~ m 2 cells and 40/~m wires. The Space be,tween these electrodes is filled with the porous dielectric. In the mid-plane of the dielectric 250/~m distant from each mesh, the 25/~m gold plated tungsten anode wires are stretched to a 500/~m separation. The detector is 2.5 cm in diameter. All the signal wires were electrically connected to provide a c o m m o n 50 ohm output. The output pulse is fed to an amplifier having a gain of 150. The lower threshold of recorded pulses was 0.3 inV. The detector was installed in a chamber evacuated with a titanium pump to 10 7 Torr. A radioactive ~-source was also installed in the

vacuum chamber and during the test it was placed in the working position above the detector by means of a manipulator. The particles were collimated to transfer only the ones directed normal to the detector surface. The scintillation counter (SC) below the detector recorded the observable particle and produced the master pulse. To investigate the detector operation in the case of minimum ionization particles, a fl-source outside the vacuum chamber was used. The electrons passed through a vacuum-tight Mylar window. For the selection of m i n i m u m ionization events the scintillation counter was calibrated so as to detect only the electrons with energy >~ 1 MeV. The electric field in the detector (without regard for the dielectric filling) was calculated in the case of continuous cathodes by the formula •

Vo

V(x, y ) = 2

2 In

- In (4 sin 2 (Trx/S) + 4sh 2 (=y/S)]], ¢

3 z¢

T

S~

,5 Sc.

j

~

:

6"

Fig. 1. Detector and the experimental setup: 1. Vacuum chamber; 2. Mylar window; 3. Radioactive source; 4. Collimator; 5. Dielectric detector (DD); 5 (a) Cathode electrodes of D D ; 5 (b) Anode wires of DD ; 5 (c) Low-density dielectric ; 6. Scintillation counter (SC).

428

s.M. GUKASIAN et al.

TABLE 1

Air pressure (atm)

Density of MgO (g/cm 3)

Density of MgO in % of bulk density

0.033 0.026 0.013

0.058 0.029 0.018

1.6% 0.8 % 0.5%

obtained from the expression for the complex potential given in Ref. 12. In this formula Vo and / are respectively the voltage and the distance between the external cathode electrode and the anode wire plane, S is the spacing between the wires and d is the wire diameter. According to calculations, the electric field has a cylindrical configuration at 50 ~ m distance from the wires and is of the order of 105 V / c m in this region for 1000 V potential difference between the wires and the electrodes. The field between the wires sharply decreases and is less than 2×104 V / c m in the (_+S/4) interval. The real field pattern in the layer is apparently much different from the calculated one owing to the comparability of the anodeto-cathode distance ( ~ 250/zm) with the dimensions of the mesh cathode cells ( ~ 2 0 0 / z m ) . This distorts the field in the layer and impairs the conditions for the electron drift. Besides, the field configuration is also affected by the presence of the porous dielectric. It is worthwhile to note that the calculation of electric fields in the case of mesh electrodes is not an easy task, and to solve it one should avail oneself of the volume modelling technique. Consideration of the porous dielectric under the conditions of strong ionization and polarization makes the problem still more complicated. MgO of different densities was obtained by burning metallic Mg in the air at different pressures. In table 1 we give the values of mean densities and the corresponding air pressures. The quantity of burnt Mg does not influence the density of formed MfO, but affects only the layer thickness. The density p was determined within an accuracy of ~20%.

a# ,

o7-

,

[

;,

9

~

o.~-

/"

I

/. ~.O.ce% 1

~. ?.a,z p' ....... ~.e-a~z l ' .e- o.az j, ,,.. . . . . .

~ /

MgO

:~}

l-....,~,"

/

C~3o,8 o./ ,~o

~__L-=-

f O0 0

. ~ ' ~

,

/200

/400

,

/6o0

f ~O0

,

,,

2~o

Fig. 2. The curves of detector efficiency versus the supply voltage.

have also tested the time stability of the detector operation. The detection efficiency r/ was determined as the ratio of the number of twofold coincidences between the DD and CC pulses to that of the CC master pulses. In fig. 2 we give the data on the measurement of the V-dependence of r/ for different layer densities for both electrons and ~z-particles. For all the MgO densities, r/ is seen to rise with V and then to smoothly reach a plateau. Beyond the plateau some increase in r/ is observed for ~z-particles with the further increase of the field in the layer. It follows from the figure that the increase with V is stronger for lesser MgO densities, as well as that the maximum value of r/ for p = 0.5% and 0.8% is much l

0,8 0,7 o

0.6 ~

i

V=llO0~

V= 1200~ o

G3"

V = 1530~

3. Results

The dependence of the efficiency of particle detection r/ for 0.5%, 0.8%, 1.6% densities of MgO on the voltage V applied between the external electrodes and the anode wires was investigated. We

0,4

0,9

/2

:6

Fig. 3. The curves of detector efficiency versus the density of MgO layer.

DIELECTRIC

PARTICLE

429

tor with MgO did not differ from those with KCI. The output pulses in both cases are of triangular shape with full width ~<6 ns and pulse height of about 10 inV.

QgQ8-

0,705;

t t t t ,,,

,,,o

Q5

2. j~ ~ G&7. •~. f

a.- seu~e

= 1,17.

c~k 0.3

ae

DETECTOR

b

io

~o

do

do

1bo

t~c,

t~,

Fig. 4. The operation time dependence of the detector efficiency.

greater than that for p = 1.6%. For p = 0.5% and 0.8% the detection efficiency is practically independent of the density in the plateau region, both in the case of o:-particle and electron irradiation, though before the plateau an increase in r/ is observed with decreasing p for V fixed (see fig. 3). It is interesting to note that the efficiency curves for B-rays reach the plateau at higher values of V, and the rise of 17 beyond the plateau is not observed up to 2 kV. In fig. 4 the results of r/ measurement in the plateau region are given for long-term operation of the detector. Along the abscissa the operation time in minutes is plotted. During these measurements the detector was continuously irradiated with a~particles with an intensity of 5× 102 ~z-particles per second. The points in the figure correspond to 1.6 % density, the crosses and circles to 0.8% and 0.5% densities respectively. The detector with MgO of 1.6% density operated at 1700V, and those with p = 0.8% and 0.5%, at 1350 V. The values of V were chosen so that the detector operated in each case within the plateau region. The detector is seen from fig. 4 to be less efficient for p = 1.6% than for p = 0.8% and 0.5%, but to operate in a more stable manner in time. The variations in efficiency with time for p = 0.8% and 0.5% are apparently connected with polarization effects in the MgO layer, These effects are appreciable at higher multiplications and, possibly, at lower densities of MgO, owing to the reduction in electric conductivity. The oscillographic observations have shown that the time and amplitude characteristics of the detec-

4. Discussion of results In the dielectric detector the ionization, the drift and multiplication of electrons take place in the porous dielectric and the information is read from the anode wires immersed in the dielectric material. The theory of these phenomena in dielectrics in the presence of strong electric fields is not yet well developed and their quantitative interpretation encounters some difficulties. The pulse formation in the detector may be depicted as follows: the 6electrons are accelerated under the action of an electric field and acquire in the dielectric pores energy sufficient for the ionization and the production of secondary electrons in pore walls. If the multiplication coefficient per pore is > 1, and the number of pores in the path is sufficient for the development of an avalanche, then appreciable current pulses arise on the anode wires. On the other hand, strong ionization of the material takes place as the avalanches develop, and the ionization density follows the field growth pattern. As drift of positive ions in the material is difficult and most probably even impossible, then depending On the primary particle-current density, a screening field of positive ions is formed in time which prevent the further drift and multiplication of secondary electrons. Also, the recombination of electrons strongly increases with the increase of a positive space charge. These phenomena result eventually in the attenuation of the anode signal. One can account for the behaviour of the Vdependence of r/ observed in fig. 2 in the following manner: the increase of curves at the origin is connected with the improvement of conditions of 6-electrons collection as well as the increase of the coefficient of internal multiplication in the layer. The plateau presumably corresponds to the fulfillment in the layer of the conditions for 6-electrons collection optimal for the given detector structure. The further increase of curves beyond the plateau region with the increase in voltage is difficult to explain. It may be due to the decrease of the 6electron absorption factor with the increase in the field. Such being the case, the absence of a rise beyond the plateau can hardly be understood.

430

s M. GUKASIAN et al.

The field topography in our case is not favourable for the efficient drift and multiplication of secondary electrons. Besides, there are regions with extreme distortions and weakening of the field on account of the mesh electrodes and the non-optimal choice of the ratio of the interelectrode distance to the wire spacing, I/s. Hence, the physical conditions are different throughout the layer and this also makes the interpretation of results difficult. One can considerably improve the detector characteristics and increase its service life by providing a better field configuration, the appropriate choice of low-density dielectric material and the elimination of space charge in the layer. It should be noted that the detector operation is strongly influenced by the purity and structure of the dielectric as well as the technology of its formation. Hence, much experimental and theoretical work is required for the elucidation of the role played by these factors. These investigations are naturally of both practical and theoretical interest for the understanding of phenomena in low-density dielectrics in strong electric fields (105-106 V/cm) during irradiation with charged particles. Though the processes taking place in the dielectric detector and its performance are not well known, nevertheless the results of the study of its operation allow us to assert that the proposed prin-

ciple of particle detection may be successfully utilized in physics experiments. In conclusion the authors wish to thank Prof. A. Ts.Amatuni for his interest and R.J. Khachatrian and N.N. Trofimchuk for assistance. References 1) S.N. Gukasian, R.L. Kavalov, M.P. Lorikian and N.N. Trolimchuk, Scientilic Report EFI-280t5)-I978. 2) M.P. Lorikian, R. L. Kavalov, N. N. Trolimchuk and E.E. Davidian, lzv. Akad. Nauk Arm. SSR, Fiz. 6 (197l) 298. 3) M. P. Lorikian, R. L. Kavalov, N. N. Trofimchuk and V. L. Serov, lzv. Akad. Nauk Arm. SSR, Fiz. 7 (1972) 118. 4) M. P. Lorikian, R. L. Kavalov and N. N. Trolimchuk, Pis'ma v Zh. Eksp. i Teor. Fiz. 16 (1972) 320. -~) M. P. Lorikian, R. L. Kavalov and N. N. Trotimchuk, Nucl. Instr. and Meth. 122 (1974) 377. 0) M. P. Lorikian, R.L. Kavalov and N.N. Trolimchuk, Izv. Akad. Nauk Arm. SSR, Fiz. 8 (1973) 33. 7) M. P. Lorikian, R. L. Kavalov, N. N. Trofimchuk and A. N. Arvanov, Scientific Report EFI-84 (1974). s) N.N. Trolimchuk, M.P. Lorikian, R.L. Kavalov, A.N. Arvanov and V.G. Gavalian, Zh. Eksp. i Teor. Fiz. 69 (1975~ 640. ~) N. N. Trofimchuk, M. P. Lorikian and R. L. Kavalov, Pribori i Tekhn. Eksp. 5 (1974) 149. 10) M.P. Lorikian, R.L. Kavalov, N.N. Trofimchuk, A.N. Arvanov and V.G. Gavalian, Scientilic Report EFI-131 (1975). ll) M. P. Lorikian and NN. Trofimchuk, Nud. Instr. and Meth. 140 (1977) 505. 12) G.A. Erskine, Nud. lnstr, and Meth. 105 (1972) 565.