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The measurement of low alpha-activities using the spark counter
NUCLEAR INSTRUMENTS AND METHODS 56 (I967) 254-260; © NORTH-HOLLAND PUBLISHING CO.
THE M E A S U R E M E N T OF L O W A L P H A - A C T I V I T I E S USING T H E SPARK C O U N T E R ~. ~AR6 and V. SRKALOVA Department of Nuclear Physics, Comenius University Brat&lava, Czechoslovakia
Received 24 June 1967 The possibility of using a wire-plane spark counter with air filling at atmospheric pressure for the precise measurement of low alpha activities by presence of other types of radiation has been investigated. For this purpose some important properties of such a counter were examined. A moderate dependence of the efficiencyof the counter on the concentration of the water vapour in air has been found. Also, that the maximal efficiencyis reached about a relative humidity of 50-70%. The width of the sensitive
region of the counter does not depend very much on the anode wire diameter. The change of wire diameter from 0.03 mm to 2 mm changes the width of the sensitive region twice. A strong angular dependence for all the configurations of electrodes has been found. Spark counters can be used for the precise determination of low alpha activities for samples which differ from the alpha standard only by their activity.
1. Introduction The measurement of low alpha-activities requires, like other low-activity measurements, complicated counting apparatus and heavy shielding arrangement. There are two reasons for these additional complications: 1. Every detector of alpha particles is sensitive also to other types of particles causing the ionization; 2. The background of the counter, caused by cosmic radiation and by the natural activity of surrounding materials puts a lower limit on the activities that can be measured by the counter. Although it is possible to lower radically the background it is impossible to get off it completely. Spark counters with inhomogeneous field and with air-filling at atmospheric pressure are sensitive only to particles with large ionization power, like fragments of decayed nuclei and alpha particles. Their sensitivity to other types of radiation can be often neglected. This property of spark counters of the mentioned type enables us to use them for precise measurements of extremely low alpha activities also in the case when other types of radiation with lower ionization power is present. Using a spark counter it is necessary to apply neither a specialized electronic system as anticoincidence system, nor a shielding arrangement for suppressing the background. In many cases, especially by precise construction of the counter, the background tends to zero. In addition to the requirement of low background, the counter proposed for precise measurements of low activities must fulfil also some other conditions. First of all it has to be sufficiently stable. The stability of the counting characteristic of the spark counter in contradistinction to other types of counters, is a rather critical property because of the destructive effects of sparks on the surface of electrodes. In the present paper
the experimental investigation of some properties of the wire-plane spark counter is carried out, especially from the point of view of possibility to use a spark counter for precise measurements of low alpha activities.
2. Gas filling Connor 1) and Payne 2) have shown that the most advantageous filling of spark counters for the detection of alpha particles is air at atmospheric pressure. If the pressure is decreased under the value of atmospheric pressure the properties of the counter get worse and if the pressure is increased above this value, the counter becomes to be more sensitive also to particles with lower ionization power. In addition to the pressure, the efficiency of the counter depends also on the composition of the gas filling. The water vapour concentration has the largest effect on the efficiency of the air-filling spark counter3). In the present work the dependence of the efficiency of the wire-plane spark counter on the concentration of water-vapour in the filling gas has been measured. The diameter of the molybden anode wire was 0.2 mm, the distance between the anode wire and the cathode plane was 1.2, 1.4 and 1.5 mm successively. Plexiglass was used as the insulating material between the anode wire and cathode plane. The counter was placed in a hermetic climatisation box of 80 x 80 x 80 [cm], where the variations of temperature were less than 0.1%, and a constant composition of the gas-filling was held, too. The concentration of watervapour was changed by addition of water-vapour into the climatisation box through the valve. It has been found that the efficiency depends moderately on the concentration of water-vapour in the sensitive volume of the counter. It has to be noted that this dependence is lower than that, found in 3). As can be seen from fig. 1 the efficiency of the counter
254
THE MEASUREMENT OF LOW A L P H A - A C T I V I T I E S
cpm
1800 1,4ram
7600 1400
1200 1,5ram "1000 800 600 400
/
""""-~
"
~
1, 2 ,~m
200 0
RELAtiVE
.u
lolrY,
Fig. 1. Variation of the efficiency vs relative humidity (wire-plane counter, wire dia. 0.2 mm, spacing wire-plane 1.2, 1.4 and
1.5 mm). reaches a rather flat maximum and with increasing concentration of water-vapour it slowly decreases. This decrease may be, of course, due to the absorption of alpha particles in the water-vapour, which is condensing on the surface of the source of alpha particles by higher concentrations of vapour. Increase of the vapour concentration changes the formation of negative ions and therefore the number of free electrons decreases. One might expect that this can cause the decrease of the efficiency. However, in fact, when the relative humidity of air is lower than about 70-80%, the efficiency of the counter is increased and the reason for the decrease of the efficiency of the counter above 80% of the relative humidity is also not completely clear at present, presumably because of the absorption of alpha particles o n H 2 0 molecules and on HO ions. The presence of electro-negative molecules of water-vapour leads therefore to the rise of the efficiency. When we were investigating the dependence of the efficiency on the vapour concentration we have observed spark discharges also in the case when the source of alpha particles was screened. These spurious discharges were observed only in a relatively narrow region of the voltage characteristic and in the small region of the relative humidity of the air-filling. They were observed for instance when the diameter of the anode wire was 0.2 mm, the distance of the electrodes was 1.4 mm and the relative humidity of air was 0-35%, for the voltage region from 4000 V to 4100 V. Holding the distance of the electrodes fixed and changing the other parameters
255
we did not observe these spurious discharges. If a resistance, series connected to the anode wire, was gradually increased, the voltage region where the spurious discharges occurred got broader. The place of the observed spurious discharges on the voltage characteristic is the same as that of the beginning of the plateau and corresponding with the beginning of the corona-discharge in the vicinity of the wire. The number of these discharges decreases to zero when the voltage is increased (and also, when the corona current is increased). The spurious discharges described above, are probably caused by the inhomogenities on the surface of the anode wire. As a result of the increase of the corona discharge, the space charge near the wire is increased. The screening effect of the space charge around the wire makes smaller the influence of wire inhomogenities on the discharge processes between the electrodes. If the relative humidity of the gas filling is above 35%, the formation of negative ions with small mobility is probably so intensive that their concentration around the anode wire screens sufficiently inhomogenities of the wire even immediately after the beginning of the corona discharge. This might be also the reason, why by the relative humidity above 35°,/o no spurious discharges occurred. 3. The service life and the stability of the spark counter During the time of a spark discharge the cathode of a spark counter is intensively bombarded by charged ions, what in turn causes a strong local heating of the cathode and its evaporation. Under the microscope small craters can be observed on the surface of the cathode. The borders of these craters protrude sometimes over the plane of the electrode. The electron avalanches and the negative ions falling on the cathode have the same destructive effect as that of the positive ions on the cathode. After a larger number of pulses the surface of the anode wire is very erosed. Microscopical and sometimes also macroscopical inhomogenities of the surface are the source of spurious discharges which are present even when the source of alpha particles is removed or screened. When the counter is in operation, the number of spurious pulses gradually rises, and after some time the counter becomes inadequate for detection. The service life of the spark counter and its longterm stability, were investigated from the point of view of the possibility to use it for a precise measurement of very low alpha activities. The counter of the wire-plane type had the following parameters: the diameter of the molybden anode wire was 0.2 mm, the distance of the
256
g. S,~RO AND V. SRKALOV,~
!it
(A)
After the first spark (as result of the change of Coulomb forces and of the air pressure wave) the anode wire begins to oscillate. If the anode wire is moved away from its equilibrium position, the distance between the electrodes decreases, so that the difference between their potential becomes larger than the breakdown voltage and this in turn causes a new spark. Sometimes the oscillations may turn in a steady resonance state. However, it should be noted that this type of oscillations occurs only in a limited region of the voltage characteristic and therefore this unwanted effect can be avoided by an appropriate change of the working voltage, the time constant of the counter, the length and the diameter of the wire or the distance between the electrodes. The normally working spark counter is shown in fig. 2, where for comparison the counter with the oscillating anode is shown. 4. The sensitive space of the spark counter
(B) Fig. 2. The operating wire-plane spark counter; A - regular operation, B - operation with oscillating wire. wire from the cathode 1.4 mm, the quenching resistor 36 M~. The counter was placed in a climatisation box, where a constant composition of the gas-filling and a constant temperature was held. The long-term stability of the high-voltage supply was 0.1%. Every new spark counter shows rather large changes of its voltage characteristic. The voltage characteristic was stabilized after some thousands of pulses, and therefore a new counter has to be "sparked" before using it and in the case that precise measurements are to be done it is preferable to use the counter after 105 pulses. The effect of "wearing" of the spark counter causes dislocation of the voltage characteristic to higher values of the voltage. We have investigated that the threshold voltages of the counter was dislocated after 107 pulses from 3500 V to 3650 V. At the same time the relative efficiency of the counter decreased for about 25%. For some distances between the electrodes and for some diameters of the anode wire we observed the tendency of the anode wire to mechanical oscillations.
Air-filled spark counters with large inhomogenities of the electric field around one or both electrodes are, in fact, corona-spark counters. However, as follows from the theory of the corona dischargeS), the coronaspark counter can be only made in a limited geometrical region. When the distance between the electrodes decreases, the statical breakdown voltage decreases too. At the same time the corona onset voltage also decreases, but in this case the decrease is slower than is the previous one, and therefore the voltage region where the corona discharge takes place becomes more narrow. For distances between the electrodes below a critical value d~r. the corona discharge does not occur and there will only be a spark discharge. The spark counter without corona discharges around the anode wire either have no plateau, or the plateau is very short and steep. Such counters are of no use for detection of particles. Therefore the spark counter can only be made when the distance between the electrodes is within a certain limited interval. Let us consider a spark counter without corona discharge and with wire-anode of diameter r 0 and with plane cathode. The distance between the axis of the anode and the plane of the cathode is denoted as d. The electrostatic field of such a system of electrodes can be determined by the method of mirror images. This method reduces the present problem to that of two wires with opposite charges of the same diameter ro in a relative distance equal to 2d (fig. 3). The distribution of the electrostatic potential will not be homogenous, due to the Coulomb forces acting between the charges which are on the surfaces of cylindrical conductors (wires). The density of the sur-
THE
MEASUREMENT
OF LOW
face charge will be the largest in the places with the smallest relative distances of electrodes. If both conductors have a circular cross-section it is always possible to shift the electric centre of the conductors so, that their surfaces are equipotential levels. Let us denote the distance between the electric centres of the wire and the surface of the cathode as h. The following relation between h, d and r o has been obtained in 4): h
= (d 2 --
ro2)~.
257
ALPHA-ACTIVITIES
E = hUerf[r/D-rcosot/ln{(d-h)/ro}]
-1,
(2)
where c~ is the angle between the x-axis and r, while r is the radius of the equipotential level on which the field is calculated (fig. 3). The electric centre of the anode wire is displaced relatively to the geometrical centre of d - h towards the cathode, and thus the largest intensity of the electrostatic field will be on the join of both electrodes, where ~ = 0:
(1)
This equation enables us to determine the electric centre of the anode wire if we know its diameter and the distance between its geometrical centre and the plane of cathode. The intensity of the electrostatical field as a function of the voltage Uef f between the wire and the cathode can be expressed by
gmax =
hUerf[r/D-r/ln{(d-h)/ro}] -1.
(3)
In the case of thin conductors, d is approximatively equal to h and therefore we can write Era,x = h Uerr[rlO - r / l n ( 2 d / r o ) ] - I
(4)
If the anode wire is very thin, eq. (4) is valid for every ~Y 1
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258
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TABLE 1
-5
"<
C~
b.
sensitive region
(mm)
(mm)
0.03 0.05 0.08 0.12 0.30 0.40 0.50 0.60 0.70
1.4 1.8 1.6 1.6 2.5 2.1 2.6 2.1 2.8 3.0 0.8 0.0
-4
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o
Max.
Wire dia.
-3 h4 #_
4 2 0
. 2
.
. . . . . 3 4 SUPPLY VOLTAGE,Us, I~V
1.00 1.50
2 5
2.00
Fig. 4. Corona current le and effective voltage Ueff vs supply voltage Us; wire diameter 0.1 mm, spacing wire-plate 1.4 mm, quenching resistor 36 M.Q.
angle and instead of Em~x can be always written E. The operating voltage of the air-filled counter is in the region of the corona discharge. As a result of the corona discharge there appears a space charge around the wire. The potential drop in the corona region narrows that part of the space around the wire, where the intensity of the field reaches the value which is necessary for a growth of electron avalanches into a streamer. The effective voltage of the wire limits the sensitive region around the wire, too. As can be seen from fig. 4 the effective voltage Ueff of the wire is only little changed
by the increase of the supply voltage Us. In practice the value of (Jeff is unchanged and equal to the corona onset voltage U¢o. Thus can be assumed that for all values of Us the sensitive region of a counter will be less or equal to that of a non-coronating spark counter with the voltage of the wire of Uco. The above mentioned equations can be used only for estimating the upper limit of the sensitive region. The effect of the diameter of the anode wire on the magnitude of the sensitive region is very small. Experiments proving this statement have been carried out using the arrangement where the diameter of the wire has been changed from 0.03 m m to 2 m m by a fixed
cpm a
b
e
b
~¢
I
I
114 12
1,0
0,8
0,6 0,4
I
0,2
--"
I
i
ALIA PARTICLES
a
3"
I
0
0,2
0,4
0,6
0,8
1.0
1,2
},4
rnrn
Fig. 5. Variation of efficiency with the distance of the beam of alpha particles from the centre of the anode wire; wire dia. 0.5 mm,
beam direction perpendicular to cathode plane.
259
THE M E A S U R E M E N T OF LOW A L P H A - A C T I V I T I E S cpm
300 -
200-
F.~.~\'~\~\ I
2,0
1,5
1,0
0,5
0
0,5
1,o
1,5
2,0 .,t.
Fig. 6. Variation of efficiency with the distance of the beam of alpha particles from the centre of the anode wire; wire dia 1.5 mm.
distance of the electrodes ( d = 2 mm). The counter was irradiated by 241Am. The collimated beam of alpha particles (width 0.15 mm, divergence 0.6 °) was perpendicular to the cathode plane. The efficiency of the counter was investigated in this experiment as a function of the distance between the incident beam of alpha particles and the centre of the wire. The cathode and the anode were kept in fixed positions and both the collimator and the alpha source were movable with an accuracy of 0.005 mm. The results of the measurements are shown in table 1.
Fig. 7. The wire-plane spark counter. Upper part: movable arm with a collimator; lower part: micrometer screw shifting the cathode.
It has to be mentioned that the change of the diameter of the anode wire from 0.03 to 1.5 mm changes the magnitude of the sensitive region only twice. The sensitive region can be divided into two parts: the region where alpha particles going to the cathode are not screened by the wire (region a in fig. 5) and the region where alpha particles are absorbed in the material of the wire (region b in fig. 5). It may be interesting that the efficiency of the counter does not decrease to zero even if the beam of alpha particles is much narrower than the diameter of the anode wire (the beam is then completly screened by the wire). The streamer can out-grow to the cathode from the opposite side of the wire but this property of the spark discharge for thick wires is limited to the border-region of the wire. When the diameter of the wire increases, the efficiency in the region screened by the wire decreases. A diameter of 1 mm is sufficient to lower the efficiency in this region practically to zero. Further increase of the diameter of the wire makes the dimensions of the inefficient region behind the wire still larger (fig. 6), and when the diameter of the wire is 2 mm, the counter is insensitive even to particles, not screened by the wire. The effect is probably due to the homogenisation of the field between the electrodes and to extinction of the corona discharge around the wire. In an other experiment, the angular dependence of the efficiency of the spark counter of this type was investigated. The measurements were carried out on the counter shown in fig. 7. The 24~Am source of alpha particles was fixed to an movable arm with a collimator. The anode wire was the axis of rotation. The distance between the electrodes was continually changed by a micrometer screw, shifting the cathode. The diameter of the anode and the distance of the electrodes were varied from 0.03 mm to 0.50 mm and from 1 mm to 3 mm respectively. The experiment has shown that it is
260
~. ~AR6 AND V. SRKALOVA
CPM 800
CPM
0"
10"
500
20"
400 300
ZoO°
200 500
2
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/ ......
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J
.
.
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/
./.
ANODE
J
I
-2
mm
CATHODE Fig. 9. Variation of efficiency vs distance of the beam of alpha particles from the centre of the anode wire; beam direction parallel to cathode plane, wire dia. 0.5 ram, spacing wire-plane 2 ram.
/
.
1
.
90"
Fig. 8. Counting rate vs incident angle; curve 1-wire dia. 2r0=0.3 ram, spacing wire-plane d : 1 ram, supply voltage U, = 2.4 kV; curve 2 - 2r0 = 0.03 ram, d = 1.5 mm, U, = 2.7 kV; eurve 3 - 2r0 = 0.03 ram, d = 2 ram, Us = 2.7 kV. p o s s i b l e to m a k e a c o u n t e r with smaller or greater a n g u l a r d e p e n d e n c e , b u t it seems practically impossible to m a k e such a configuration o f electrodes by which the a n g u l a r d e p e n d e n c e will be very small. T h e a n g u l a r d e p e n d e n c e o f the sensitivity is shown in fig. 8 for three different configurations o f the electrodes. In the last e x p e r i m e n t the b e a m o f a l p h a particles (with 0.15 m m , divergence 0.6 °) was shifted parallel with the p l a n e o f the c a t h o d e in the direction from a n o d e wire t o w a r d s the cathode. As can be seen f r o m fig. 9 the efficiency o f the c o u n t e r to a l p h a particles incident p a r a l l e l with the p l a n e o f the c a t h o d e is n o t always zero (as can be expected, on the basis o f the d i a g r a m o f a n g u l a r dependence). T h e efficiency o f the c o u n t e r in the direction f r o m wire to c a t h o d e begins to rise, t h e n it reaches a m a x i m u m a n d decreases to zero a g a i n n e a r the c a t h o d e . T h e m a x i m a l efficiency is n o t n e a r the a n o d e wire, b u t is shifted t o w a r d s the cathode.
W e m u s t confess, t h a t we were n o t able to find a satisfactory e x p l a n a t i o n o f this effect.
5. Conclusion The m e a s u r e m e n t s described a b o v e show t h a t the efficiency o f the s p a r k c o u n t e r with i n h o m o g e n o u s field varies from p o i n t to p o i n t within their sensitive region. The efficiency depends n o t only on the g e o m e t r y o f the counter, b u t also on the a n g u l a r d i s t r i b u t i o n of the detected a l p h a particles a n d on the g e o m e t r y o f the source. These features o f the c o u n t e r o f considered type m a k e impossible to use such a c o u n t e r for a c c u r a t e m e a s u r e m e n t s o f very low a l p h a activities. A n accurate m e a s u r e m e n t can be only d o n e with a p r e p a r a t e having the same energy a n d a n g u l a r d i s t r i b u t i o n o f a l p h a particles and the same g e o m e t r y as the a l p h a s t a n d a r d used for the c a l i b r a t i o n o f the counter.
References 1) R. O. Connor, Proc. Phys. Soc. 64 B (1951) 30. ~) R. M. Payne, J. Sci. Instr. 26 (1949) 321. 3) j. Andrejeshchev and V. M. Isaev, Zh. Eksp. Teor. Fiz. 28 (1955) 335. 4) V. A. Govorkov, Elektricheskie i magnitnyie polja (Moskva, 1951). s) N. A. Kapcov, Elektronika (Moskva, 1953).
THE M E A S U R E M E N T OF L O W A L P H A - A C T I V I T I E S USING T H E SPARK C O U N T E R ~. ~AR6 and V. SRKALOVA Department of Nuclear Physics, Comenius University Brat&lava, Czechoslovakia
Received 24 June 1967 The possibility of using a wire-plane spark counter with air filling at atmospheric pressure for the precise measurement of low alpha activities by presence of other types of radiation has been investigated. For this purpose some important properties of such a counter were examined. A moderate dependence of the efficiencyof the counter on the concentration of the water vapour in air has been found. Also, that the maximal efficiencyis reached about a relative humidity of 50-70%. The width of the sensitive
region of the counter does not depend very much on the anode wire diameter. The change of wire diameter from 0.03 mm to 2 mm changes the width of the sensitive region twice. A strong angular dependence for all the configurations of electrodes has been found. Spark counters can be used for the precise determination of low alpha activities for samples which differ from the alpha standard only by their activity.
1. Introduction The measurement of low alpha-activities requires, like other low-activity measurements, complicated counting apparatus and heavy shielding arrangement. There are two reasons for these additional complications: 1. Every detector of alpha particles is sensitive also to other types of particles causing the ionization; 2. The background of the counter, caused by cosmic radiation and by the natural activity of surrounding materials puts a lower limit on the activities that can be measured by the counter. Although it is possible to lower radically the background it is impossible to get off it completely. Spark counters with inhomogeneous field and with air-filling at atmospheric pressure are sensitive only to particles with large ionization power, like fragments of decayed nuclei and alpha particles. Their sensitivity to other types of radiation can be often neglected. This property of spark counters of the mentioned type enables us to use them for precise measurements of extremely low alpha activities also in the case when other types of radiation with lower ionization power is present. Using a spark counter it is necessary to apply neither a specialized electronic system as anticoincidence system, nor a shielding arrangement for suppressing the background. In many cases, especially by precise construction of the counter, the background tends to zero. In addition to the requirement of low background, the counter proposed for precise measurements of low activities must fulfil also some other conditions. First of all it has to be sufficiently stable. The stability of the counting characteristic of the spark counter in contradistinction to other types of counters, is a rather critical property because of the destructive effects of sparks on the surface of electrodes. In the present paper
the experimental investigation of some properties of the wire-plane spark counter is carried out, especially from the point of view of possibility to use a spark counter for precise measurements of low alpha activities.
2. Gas filling Connor 1) and Payne 2) have shown that the most advantageous filling of spark counters for the detection of alpha particles is air at atmospheric pressure. If the pressure is decreased under the value of atmospheric pressure the properties of the counter get worse and if the pressure is increased above this value, the counter becomes to be more sensitive also to particles with lower ionization power. In addition to the pressure, the efficiency of the counter depends also on the composition of the gas filling. The water vapour concentration has the largest effect on the efficiency of the air-filling spark counter3). In the present work the dependence of the efficiency of the wire-plane spark counter on the concentration of water-vapour in the filling gas has been measured. The diameter of the molybden anode wire was 0.2 mm, the distance between the anode wire and the cathode plane was 1.2, 1.4 and 1.5 mm successively. Plexiglass was used as the insulating material between the anode wire and cathode plane. The counter was placed in a hermetic climatisation box of 80 x 80 x 80 [cm], where the variations of temperature were less than 0.1%, and a constant composition of the gas-filling was held, too. The concentration of watervapour was changed by addition of water-vapour into the climatisation box through the valve. It has been found that the efficiency depends moderately on the concentration of water-vapour in the sensitive volume of the counter. It has to be noted that this dependence is lower than that, found in 3). As can be seen from fig. 1 the efficiency of the counter
254
THE MEASUREMENT OF LOW A L P H A - A C T I V I T I E S
cpm
1800 1,4ram
7600 1400
1200 1,5ram "1000 800 600 400
/
""""-~
"
~
1, 2 ,~m
200 0
RELAtiVE
.u
lolrY,
Fig. 1. Variation of the efficiency vs relative humidity (wire-plane counter, wire dia. 0.2 mm, spacing wire-plane 1.2, 1.4 and
1.5 mm). reaches a rather flat maximum and with increasing concentration of water-vapour it slowly decreases. This decrease may be, of course, due to the absorption of alpha particles in the water-vapour, which is condensing on the surface of the source of alpha particles by higher concentrations of vapour. Increase of the vapour concentration changes the formation of negative ions and therefore the number of free electrons decreases. One might expect that this can cause the decrease of the efficiency. However, in fact, when the relative humidity of air is lower than about 70-80%, the efficiency of the counter is increased and the reason for the decrease of the efficiency of the counter above 80% of the relative humidity is also not completely clear at present, presumably because of the absorption of alpha particles o n H 2 0 molecules and on HO ions. The presence of electro-negative molecules of water-vapour leads therefore to the rise of the efficiency. When we were investigating the dependence of the efficiency on the vapour concentration we have observed spark discharges also in the case when the source of alpha particles was screened. These spurious discharges were observed only in a relatively narrow region of the voltage characteristic and in the small region of the relative humidity of the air-filling. They were observed for instance when the diameter of the anode wire was 0.2 mm, the distance of the electrodes was 1.4 mm and the relative humidity of air was 0-35%, for the voltage region from 4000 V to 4100 V. Holding the distance of the electrodes fixed and changing the other parameters
255
we did not observe these spurious discharges. If a resistance, series connected to the anode wire, was gradually increased, the voltage region where the spurious discharges occurred got broader. The place of the observed spurious discharges on the voltage characteristic is the same as that of the beginning of the plateau and corresponding with the beginning of the corona-discharge in the vicinity of the wire. The number of these discharges decreases to zero when the voltage is increased (and also, when the corona current is increased). The spurious discharges described above, are probably caused by the inhomogenities on the surface of the anode wire. As a result of the increase of the corona discharge, the space charge near the wire is increased. The screening effect of the space charge around the wire makes smaller the influence of wire inhomogenities on the discharge processes between the electrodes. If the relative humidity of the gas filling is above 35%, the formation of negative ions with small mobility is probably so intensive that their concentration around the anode wire screens sufficiently inhomogenities of the wire even immediately after the beginning of the corona discharge. This might be also the reason, why by the relative humidity above 35°,/o no spurious discharges occurred. 3. The service life and the stability of the spark counter During the time of a spark discharge the cathode of a spark counter is intensively bombarded by charged ions, what in turn causes a strong local heating of the cathode and its evaporation. Under the microscope small craters can be observed on the surface of the cathode. The borders of these craters protrude sometimes over the plane of the electrode. The electron avalanches and the negative ions falling on the cathode have the same destructive effect as that of the positive ions on the cathode. After a larger number of pulses the surface of the anode wire is very erosed. Microscopical and sometimes also macroscopical inhomogenities of the surface are the source of spurious discharges which are present even when the source of alpha particles is removed or screened. When the counter is in operation, the number of spurious pulses gradually rises, and after some time the counter becomes inadequate for detection. The service life of the spark counter and its longterm stability, were investigated from the point of view of the possibility to use it for a precise measurement of very low alpha activities. The counter of the wire-plane type had the following parameters: the diameter of the molybden anode wire was 0.2 mm, the distance of the
256
g. S,~RO AND V. SRKALOV,~
!it
(A)
After the first spark (as result of the change of Coulomb forces and of the air pressure wave) the anode wire begins to oscillate. If the anode wire is moved away from its equilibrium position, the distance between the electrodes decreases, so that the difference between their potential becomes larger than the breakdown voltage and this in turn causes a new spark. Sometimes the oscillations may turn in a steady resonance state. However, it should be noted that this type of oscillations occurs only in a limited region of the voltage characteristic and therefore this unwanted effect can be avoided by an appropriate change of the working voltage, the time constant of the counter, the length and the diameter of the wire or the distance between the electrodes. The normally working spark counter is shown in fig. 2, where for comparison the counter with the oscillating anode is shown. 4. The sensitive space of the spark counter
(B) Fig. 2. The operating wire-plane spark counter; A - regular operation, B - operation with oscillating wire. wire from the cathode 1.4 mm, the quenching resistor 36 M~. The counter was placed in a climatisation box, where a constant composition of the gas-filling and a constant temperature was held. The long-term stability of the high-voltage supply was 0.1%. Every new spark counter shows rather large changes of its voltage characteristic. The voltage characteristic was stabilized after some thousands of pulses, and therefore a new counter has to be "sparked" before using it and in the case that precise measurements are to be done it is preferable to use the counter after 105 pulses. The effect of "wearing" of the spark counter causes dislocation of the voltage characteristic to higher values of the voltage. We have investigated that the threshold voltages of the counter was dislocated after 107 pulses from 3500 V to 3650 V. At the same time the relative efficiency of the counter decreased for about 25%. For some distances between the electrodes and for some diameters of the anode wire we observed the tendency of the anode wire to mechanical oscillations.
Air-filled spark counters with large inhomogenities of the electric field around one or both electrodes are, in fact, corona-spark counters. However, as follows from the theory of the corona dischargeS), the coronaspark counter can be only made in a limited geometrical region. When the distance between the electrodes decreases, the statical breakdown voltage decreases too. At the same time the corona onset voltage also decreases, but in this case the decrease is slower than is the previous one, and therefore the voltage region where the corona discharge takes place becomes more narrow. For distances between the electrodes below a critical value d~r. the corona discharge does not occur and there will only be a spark discharge. The spark counter without corona discharges around the anode wire either have no plateau, or the plateau is very short and steep. Such counters are of no use for detection of particles. Therefore the spark counter can only be made when the distance between the electrodes is within a certain limited interval. Let us consider a spark counter without corona discharge and with wire-anode of diameter r 0 and with plane cathode. The distance between the axis of the anode and the plane of the cathode is denoted as d. The electrostatic field of such a system of electrodes can be determined by the method of mirror images. This method reduces the present problem to that of two wires with opposite charges of the same diameter ro in a relative distance equal to 2d (fig. 3). The distribution of the electrostatic potential will not be homogenous, due to the Coulomb forces acting between the charges which are on the surfaces of cylindrical conductors (wires). The density of the sur-
THE
MEASUREMENT
OF LOW
face charge will be the largest in the places with the smallest relative distances of electrodes. If both conductors have a circular cross-section it is always possible to shift the electric centre of the conductors so, that their surfaces are equipotential levels. Let us denote the distance between the electric centres of the wire and the surface of the cathode as h. The following relation between h, d and r o has been obtained in 4): h
= (d 2 --
ro2)~.
257
ALPHA-ACTIVITIES
E = hUerf[r/D-rcosot/ln{(d-h)/ro}]
-1,
(2)
where c~ is the angle between the x-axis and r, while r is the radius of the equipotential level on which the field is calculated (fig. 3). The electric centre of the anode wire is displaced relatively to the geometrical centre of d - h towards the cathode, and thus the largest intensity of the electrostatic field will be on the join of both electrodes, where ~ = 0:
(1)
This equation enables us to determine the electric centre of the anode wire if we know its diameter and the distance between its geometrical centre and the plane of cathode. The intensity of the electrostatical field as a function of the voltage Uef f between the wire and the cathode can be expressed by
gmax =
hUerf[r/D-r/ln{(d-h)/ro}] -1.
(3)
In the case of thin conductors, d is approximatively equal to h and therefore we can write Era,x = h Uerr[rlO - r / l n ( 2 d / r o ) ] - I
(4)
If the anode wire is very thin, eq. (4) is valid for every ~Y 1
i i
I
\\
\\
i
./ /,<
L
J
" / /
//
j
X
/
' i
~,
i
,
1
1¢" ; \
'j
/ L
i
. . . . . .
/
1
):
I I
\\ X%x
\'\ / ////~'~" . . ./
'\ II
~
/
_
•
\ \
1
i i I Fig. 3. P o t e n t i a l d i s t r i b u t i o n for the w i r e - p l a n e s y s t e m a n d its m i r r o r i m a g e s ; M ( x , y ) is the p o i n t in w h i c h w e calculate the field intensity, UM - e q u i p o t e n t i a l level o n w h i c h the p o i n t M ( x , y ) lies, a - surface density o f electric charge.
258
~. g~,R6 AND V. SRKALOVA 20 "
TABLE 1
-5
"<
C~
b.
sensitive region
(mm)
(mm)
0.03 0.05 0.08 0.12 0.30 0.40 0.50 0.60 0.70
1.4 1.8 1.6 1.6 2.5 2.1 2.6 2.1 2.8 3.0 0.8 0.0
-4
10
o
(b
o
Max.
Wire dia.
-3 h4 #_
4 2 0
. 2
.
. . . . . 3 4 SUPPLY VOLTAGE,Us, I~V
1.00 1.50
2 5
2.00
Fig. 4. Corona current le and effective voltage Ueff vs supply voltage Us; wire diameter 0.1 mm, spacing wire-plate 1.4 mm, quenching resistor 36 M.Q.
angle and instead of Em~x can be always written E. The operating voltage of the air-filled counter is in the region of the corona discharge. As a result of the corona discharge there appears a space charge around the wire. The potential drop in the corona region narrows that part of the space around the wire, where the intensity of the field reaches the value which is necessary for a growth of electron avalanches into a streamer. The effective voltage of the wire limits the sensitive region around the wire, too. As can be seen from fig. 4 the effective voltage Ueff of the wire is only little changed
by the increase of the supply voltage Us. In practice the value of (Jeff is unchanged and equal to the corona onset voltage U¢o. Thus can be assumed that for all values of Us the sensitive region of a counter will be less or equal to that of a non-coronating spark counter with the voltage of the wire of Uco. The above mentioned equations can be used only for estimating the upper limit of the sensitive region. The effect of the diameter of the anode wire on the magnitude of the sensitive region is very small. Experiments proving this statement have been carried out using the arrangement where the diameter of the wire has been changed from 0.03 m m to 2 m m by a fixed
cpm a
b
e
b
~¢
I
I
114 12
1,0
0,8
0,6 0,4
I
0,2
--"
I
i
ALIA PARTICLES
a
3"
I
0
0,2
0,4
0,6
0,8
1.0
1,2
},4
rnrn
Fig. 5. Variation of efficiency with the distance of the beam of alpha particles from the centre of the anode wire; wire dia. 0.5 mm,
beam direction perpendicular to cathode plane.
259
THE M E A S U R E M E N T OF LOW A L P H A - A C T I V I T I E S cpm
300 -
200-
F.~.~\'~\~\ I
2,0
1,5
1,0
0,5
0
0,5
1,o
1,5
2,0 .,t.
Fig. 6. Variation of efficiency with the distance of the beam of alpha particles from the centre of the anode wire; wire dia 1.5 mm.
distance of the electrodes ( d = 2 mm). The counter was irradiated by 241Am. The collimated beam of alpha particles (width 0.15 mm, divergence 0.6 °) was perpendicular to the cathode plane. The efficiency of the counter was investigated in this experiment as a function of the distance between the incident beam of alpha particles and the centre of the wire. The cathode and the anode were kept in fixed positions and both the collimator and the alpha source were movable with an accuracy of 0.005 mm. The results of the measurements are shown in table 1.
Fig. 7. The wire-plane spark counter. Upper part: movable arm with a collimator; lower part: micrometer screw shifting the cathode.
It has to be mentioned that the change of the diameter of the anode wire from 0.03 to 1.5 mm changes the magnitude of the sensitive region only twice. The sensitive region can be divided into two parts: the region where alpha particles going to the cathode are not screened by the wire (region a in fig. 5) and the region where alpha particles are absorbed in the material of the wire (region b in fig. 5). It may be interesting that the efficiency of the counter does not decrease to zero even if the beam of alpha particles is much narrower than the diameter of the anode wire (the beam is then completly screened by the wire). The streamer can out-grow to the cathode from the opposite side of the wire but this property of the spark discharge for thick wires is limited to the border-region of the wire. When the diameter of the wire increases, the efficiency in the region screened by the wire decreases. A diameter of 1 mm is sufficient to lower the efficiency in this region practically to zero. Further increase of the diameter of the wire makes the dimensions of the inefficient region behind the wire still larger (fig. 6), and when the diameter of the wire is 2 mm, the counter is insensitive even to particles, not screened by the wire. The effect is probably due to the homogenisation of the field between the electrodes and to extinction of the corona discharge around the wire. In an other experiment, the angular dependence of the efficiency of the spark counter of this type was investigated. The measurements were carried out on the counter shown in fig. 7. The 24~Am source of alpha particles was fixed to an movable arm with a collimator. The anode wire was the axis of rotation. The distance between the electrodes was continually changed by a micrometer screw, shifting the cathode. The diameter of the anode and the distance of the electrodes were varied from 0.03 mm to 0.50 mm and from 1 mm to 3 mm respectively. The experiment has shown that it is
260
~. ~AR6 AND V. SRKALOVA
CPM 800
CPM
0"
10"
500
20"
400 300
ZoO°
200 500
2
1/ '
/ ......
50" / ~
4oo- ~ /I/I//¢ / / / - ~ oo.
0
~/
--""
I / t///],///. / ,v ,,/,g////Z/...;.., " "
J
.
.
100
/
./.
ANODE
J
I
-2
mm
CATHODE Fig. 9. Variation of efficiency vs distance of the beam of alpha particles from the centre of the anode wire; beam direction parallel to cathode plane, wire dia. 0.5 ram, spacing wire-plane 2 ram.
/
.
1
.
90"
Fig. 8. Counting rate vs incident angle; curve 1-wire dia. 2r0=0.3 ram, spacing wire-plane d : 1 ram, supply voltage U, = 2.4 kV; curve 2 - 2r0 = 0.03 ram, d = 1.5 mm, U, = 2.7 kV; eurve 3 - 2r0 = 0.03 ram, d = 2 ram, Us = 2.7 kV. p o s s i b l e to m a k e a c o u n t e r with smaller or greater a n g u l a r d e p e n d e n c e , b u t it seems practically impossible to m a k e such a configuration o f electrodes by which the a n g u l a r d e p e n d e n c e will be very small. T h e a n g u l a r d e p e n d e n c e o f the sensitivity is shown in fig. 8 for three different configurations o f the electrodes. In the last e x p e r i m e n t the b e a m o f a l p h a particles (with 0.15 m m , divergence 0.6 °) was shifted parallel with the p l a n e o f the c a t h o d e in the direction from a n o d e wire t o w a r d s the cathode. As can be seen f r o m fig. 9 the efficiency o f the c o u n t e r to a l p h a particles incident p a r a l l e l with the p l a n e o f the c a t h o d e is n o t always zero (as can be expected, on the basis o f the d i a g r a m o f a n g u l a r dependence). T h e efficiency o f the c o u n t e r in the direction f r o m wire to c a t h o d e begins to rise, t h e n it reaches a m a x i m u m a n d decreases to zero a g a i n n e a r the c a t h o d e . T h e m a x i m a l efficiency is n o t n e a r the a n o d e wire, b u t is shifted t o w a r d s the cathode.
W e m u s t confess, t h a t we were n o t able to find a satisfactory e x p l a n a t i o n o f this effect.
5. Conclusion The m e a s u r e m e n t s described a b o v e show t h a t the efficiency o f the s p a r k c o u n t e r with i n h o m o g e n o u s field varies from p o i n t to p o i n t within their sensitive region. The efficiency depends n o t only on the g e o m e t r y o f the counter, b u t also on the a n g u l a r d i s t r i b u t i o n of the detected a l p h a particles a n d on the g e o m e t r y o f the source. These features o f the c o u n t e r o f considered type m a k e impossible to use such a c o u n t e r for a c c u r a t e m e a s u r e m e n t s o f very low a l p h a activities. A n accurate m e a s u r e m e n t can be only d o n e with a p r e p a r a t e having the same energy a n d a n g u l a r d i s t r i b u t i o n o f a l p h a particles and the same g e o m e t r y as the a l p h a s t a n d a r d used for the c a l i b r a t i o n o f the counter.
References 1) R. O. Connor, Proc. Phys. Soc. 64 B (1951) 30. ~) R. M. Payne, J. Sci. Instr. 26 (1949) 321. 3) j. Andrejeshchev and V. M. Isaev, Zh. Eksp. Teor. Fiz. 28 (1955) 335. 4) V. A. Govorkov, Elektricheskie i magnitnyie polja (Moskva, 1951). s) N. A. Kapcov, Elektronika (Moskva, 1953).