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Reduction of `Insensitive Time' in Geiger-Müller Counters

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1948 J. Sci. Instrum. 25 11

(http://iopscience.iop.org/0950-7671/25/1/305)

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Reduction of ‘ Insensitive Time ’ in Geiger-Muller Counters By *A. L. HODSON, B.Sc., T h e Physical Laboratories, T h e University, Manchester

[.MS. $ y s t receiced I Z -March 1947 and in jinal j o m 21 April 19471

JBSTRACT. A simple ‘flip-flop’ circuit has been used to ;everse the voltage of a Geiger counter after each discharge, in order to reduce the insensitive time of the counter by collecting the positive ions on the wire. The new insensitive time is of the order of 3 x IO-5 sec. The circuit has been tested by measuring the :*efficiency of a counter with a new, fast hard-valve anti-coincidence circuit.“

It is well known that, after a discharge in a Geiger counter, ;here is a short period during which the counter is insensi- rive(x). The positive-ion sheath formed in the vicinity of the wire during the discharge reduces the field near the wire below the threshold field necessary to support ionization by collision, and prevents a further discharge taking place. As the positive ions move slowly towards the cathode the field near the wire returns to normal. The time needed for the positive ions to reach the critical distance from the wire corresponding to threshold field defines the ‘ dead-time’ of the counter, during which it is insensitive to further ionizing particles. The further time needed for the ions to reach the cathode is called the ‘recovery time’, and the size of any pulse occurring within this time is determined by the time elapsed since the initial discharge, a pulse at the end of the recovery time being of the same size as the initial pulse.

Stever(1) has shown that the ‘dead-time’ and ‘recovery time’ are each of the order of IO-? sec. or higher (depending on the diameter of the counter, etc.). The ‘insensitive time’ of the counter in a particular recording circuit is determined by the pulse size necessary to operate the circuit, but in the usual recording circuits the ‘insensitive time ’ is appreciable, and at very high counting rates results in a considerable loss of efficiency of the counter. If S is the counting rate per second, and t , the insensitive time, the inefficiency is

[ ~ - e x p ( - K t ~ ) ) x ~ o o ~ ~ = ~ V t ~ x IOOO; (if A\-ti<~). Simpson(z) has reduced the insensitive time of a counter

to approximately z x IO-6 sec. in the following manner. T h e voltage applied to the counter is suddenly reversed after each discharge (so that the wire now becomes negative with respect to the cathode), and the positive ions are quickly collected on the wire. The wire is then returned to its normal operating potential, when the counter is again sensitive to further ionizing particles. Simpson’s circuit is unfortunately rather complicated and requires nine valves for each Geiger counter, although Simpson mentions that it might be simplified slightly. il much simpler circuit (Fig. I ) has been developed in the

laboratories. Valves A and B form a ‘flip-flop’ circuit, valve A normally being conducting and valve B biased beyond cut-off. T h e wire of the counter is connected directly to the anode of valve B, while the cathode is maintained at I jo 1’. above earth potential. X high potential, equal to the normal working voltage of the Geiger counter plus I j o V., is applied to the anode resistance of valve B, s 3 that the normal working voltage is applied to the counter. When an ionizing particle passes through the counter, the negative pulse pro- duced on the wire causes the anode current of valve A to be reduced slightly, and the resulting cumulative action then causes valve B to become fully conducting and valve A non- conducting. The potential of the wire thus drops nearly to

* Since this paper was written, details have been published of a hard-yalve anti-coincidence circuit, similar in principle to the one used here. Du Toit, S. J., Rec. Sri. I?zstrzL??~. 18, p. 31 (1947).

~ ~.

zero so that the cathode is now I jo V-. positive with respect to the wire. (The reversed voltage is kept low to avoid secondary emission at the xire.)

U orhirig pucciiiial

fill\, 6 I iY . + I 5 0 v. 200 v. 1 T l ?

Fig. I . -‘Flip-flop’ circuit used for reducing the insensitive time of Geiger counters. R,=ioo kQ; R2=300 kn; R,=zo kn; R,= jokL2; C,=~oppF . ; C2=zjppF. Valve A, hlullard EF 36; L alve E, CV I 73 (see text)

T h e time during which B is conducting is determined by the time constant C, R,; when the negative pulse received by the grid of A has decreased sufficiently for A to become slightly conducting, the whole process is reversed, and B becomes non-conducting and A fully conducting. T h e potential of the wire therefore returns to its normal value. The time constant C,R, has been adjusted, so that B remains conducting for about IO-5 sec. This is probably slightly longer than the time needed for positive-ion collection. T h e anode resistance of B is small, so that the wire potential returns to its normal value as quickly as possible. Oscillo- graph observations show that the wire returns to normal approximately- z x IO-j sec. after the initial discharge. A Mullard E F j o valve has been used successfully for B with a counter of fairly low working potential (1100 V.); for a counter with a higher 1%-orking potential, a valve giving larger pulses of anode current is necessary, unless R, is increased. c y 1 7 3 and Cossor 807 valves have also been used success- fully. Xny valve which will give pulses of anode current of, say, Zj mA., and which has good anode-cathode insulation, can be used for B.

._

0 10 , 2 0 c 3 .

Fig. 2. Counter arrangement used for measuring the inefficiency of a Geiger counter

T h e circuit has been tested by measuring the inefficiency of a counter. The arrangement consisted of a vertical ‘ tele- scope’ containing five counters in coincidence, the counter under test T being placed between the third and fourth counters, as shown in Fig. 2. I O cm. of lead were placed

VOL. 25, JANUARY 1948 r 1 1 1 1-6

above the ‘telescope’ to select mesons (which usually appear singly), and the telescope counters were shielded on the sides by 15 cm. of lead. The counters were of the copper-in-glass type and were filled Kith 1.5 cm. of ether and 11.5 cm. of argon. Counters I-j were 19 cm. long and 2.5 cm. in diameter, and the counter T was 60 cm. long and 5 cm. in diameter.

The complete circuit used for the measurement of the efficiency of the counter T is given in Fig. 3. Pulses from the Rossi coincidence circuit (valves Vl-s) and from counter T were applied to a new, fast, hard-valve form of the Rossi anti-coincidence circuit ( 3 ) . Fivefold coincidences, and five- fold coincidences unaccompanied by a discharge of counter T (i.e. anti-coincidences), were recorded. Then the inefficiency is given directly by the ratio of anti-coincidences to coinci- dences. Valves Vi-lg form the hard-valve anti-coincidence set. V, and V, constitute a ‘flip-flop’ circuit triggered by the output of the Rossi coincidence circuit, and VI, and VI, a ‘flip-flop’ circuit triggered by the anti-coincidence counter 2“. These two ‘flip-flop’ circuits replace the thyratrons of the Rossi anti-coincidence circuit. V, is normally conducting and VI, non-conducting. When the coincidence ‘flip-flop ’ circuit is triggered, V, becomes non-conducting, and when the anti- coincidence ‘flip-flop’ circuit is triggered, V,, is held con- ducting for about 10-j sec. T h e time constants have been adjusted, so that the negative pulse given to the grid of V , occurs approximately in the middle of the period during which V,, is held conducting (Fig. 4). If the coincidence ‘flip-flop’ circuit alone is triggered, a count is recorded by the thyratron T,, and also an anti-coincidence by T A ; if both ‘flip-flop’ circuits are triggered together no count is recorded by T A .

Anode

[I23

Grid 3f v, 7---

U--- Anode of VI,

Grid

Fig. 4. Wave forms of the circuit of Fig. 3

Test of counter with and without reversing circuit Without reversing circuit With reversing circuit

Counting rate Anti- Inefficiency Insensitive time Anti- Inefficiency Insensitive time )f counter T Coincidences coincidences % x IO-& sec. Coincidences coincidences yo x IO-$ sec.

z3,sec. 3185 54 = '7 7'4 1z4,'sec. 1496 1 3 5 9.0 7'3

The inefficiency of the counter T was first measured without the reversing circuit, the wire being connected directly to the grid of V,, both at the normal counting rate and with the part of T which was unshielded by the lead, exposed to a radioactive source. Then the inefficiencies were measured with the reversing circuit connected, the counter wire being connected to the grid of V, through a z p p F . condenser. The results are given in the table.

S o attempt has been made to estimate the small anti- coincidence rate due to side showers discharging the coinci- dence counters but not counter T. The results indicate a new insensitive time of the order of 3 x IO-^ sec., and this is sufficiently lox for our present purposes. A direct measure of the inefficiency (as described above) is better than observations using a single-sweep time-base, as any secondary reversals caused by secondary emission from the wire would simulate a discharge within the normal 'insensitive time'.

It was found that the reversing circuit appreciably extends the plateau of the characteristic of a Geiger counter, and counters which would normally be useless, count quite steadily when the circuit is used. The plateau has a residual

7024 4 0.06 Z ' j

1167 5 0 ' 4 3 3'5

slope which seems largely due to secondary emission from the wire as it depends on the reversed voltage (increasing rapidly for reversed voltages above zoo V.), although negative-ion formation may also contribute to it. Multivibrator circuits for purely quenching purposes have been described in the literature (4 5 ) .

ACKNO WLEDGEMEXTS

The author wishes to express his thanks to Prof. P. M. S. Blackett for encouragement and for the facilities for carrying out the work, and to D r H. J. J. Braddick, Dr. L. Jinossy and M r B. G. Owen for helpful advice.

REFEREKCES

( I ) STEVER, H. G. Phys. Rev. 61, p. 38 (1942). ( 2 ) SIMPSOK, J. A. Phys. Rev. 66, p. 39 (1944). (3) ROW, B., JINOSSY, L., ROCHESTER, G. D. and BOUND, NI.

(4) GETTIHG, I. A . Phys. Rev. 53, p. 103 (1938). ( j ) MCMASTER, H. and POOL, M. L. Rev. Sci. Instrunt. 11, p. 196

(1940).

Phys. Rev. 58 , p. 761 (1940).

A Micro-indenter for use with a Metahrgicd Microscope By T. A . CRATVSHAW, Stewarts and Lloyds Ltd., Corby, Korthamptonshire

[MS. first receized I 2 May I 947 and in $nul j o m I 8 June I 94 j]

ABSTRACT. -4 simple device is described which can be attached to a metallurgical microscope in order t n make small impressions with a standard Vickers pyramid diamond.

The examination of metallurgical specimens often demands the measurement of the hardness of minute areas, for which purpose the ordinary hardness methods cannot be applied. As an existing device of German origin for making micro- indentations was unobtainable, it was decided to make a simple instrument which would apply a load to a standard Vickers pyramid diamond suitably mounted so as to be capable of replacing the microscope objective. In the first model, the diamond was mounted at the top of a sleeve which could slide easily over a cylinder attached to a lens mount. -4 helical spring inside the cylinder provided the pressure, the sleeve bearing calibrating marks corresponding to different loads. In use, the microscope stage with its specimen was racked down until the requisite load was obtained, as measured by compression of the spring. Difficulties which occurred with this model were that it was easy to overshoot the cali- bration mark accidentally; canting over of the sleeve, giving an uncertain friction effect; and uncertainty of position of the impression due to canting and to the clearances which had to be allowed for easy working.

An improved indenter has recently been completed. In this, the diamond A (Fig. ~ ( a ) ) is mounted at the top of a cylindrical, hardened steel plunger B which is slotted towards the lower end to admit a simple lever C. 4 steel ball fixed to the lever presses against the flat polished upper surface of the slot when a suitable weight D is hung upon the steelyard. In order to obtain precise positioning of the plunger a geometric mounting is employed. A brass pillar E bored out to clear the

plunger, bears six adjusting screws ; those mounted in the upper plane 120' apart (Fig. I (c)), have very slightly convex ends. In the lower plane (Fig. I (b)) two screws have slightly

F \ ?I)

I !I

F

(11) ((1

Fig. I . Diagram of micro-indenter for use with metallurgical microscope

[ '3 1 VOL. 2j, JANUARY 1948