Automatic gain control circuit for scintillation probe with plastic scintillator

NUCLEAR INSTRUMENTS AND METHODS 118 (1974) 231-235; Q NORTH-HOLLAND PUBLISHING C®_

AUTORIATIC GAIN CONTROL CIRCUIT FOR' SCINTILLATION PROBE WITH PLASTIC SCINTILLATOR B. MACHAJ Institute ofNuclear Research, Division XV, Warsaw, Poland

Received 18 July 1973 An automatic gain control circuit has been described in the paper, allowing stabilization of a radiation spectrum exhibiting no photopeak . Its operating principle is based on comparison of the count rate above two discrimination levels. One of them is usually selected near theupper end of the radiation spectrum

where the relative slope of the integral curve is high, the other one where the relative slopeis low. The AGCdescribed canalso be used for stabilization of a radiation spectrum exhibiting a photopeak.

1. Introduction

- High voltage instabilities resulting in PM tube gain changes according to:

A high counting efficiency and the possibility of analysis of the detected spectrum by a scintillation detector, makes a scintillation probe widely used in

dïclk = amAU/U,

many experiments and measurements where detection of ionizing radiation is involved. Another a0vantage of the scintillation probe is its digital output allowing

accurate digital processing of detected radiation - the presently preferred way of data processing . Cheap plastic scintillators developed in the past years, which can easily be machined into practically any shape, make the scintillation probe still more

attractive . Theadvance of technology in the production of robust and reliable photomultipher tubes also encourages applications of scintillation detectors for

measurements in difficult environmental conditions. On the other hand the scintillation probe is known as very unstable . To cope with stability problems, an automaticgain control circuit must be used if an acceptable stability of measurements is to be achieved . This is especially valid when continuous on-line measurements are to be carried out in difficult environmental conditions, where, e.g., a wide range of temperature changes occur. The most critical component in a scintillation probe is the photomultiplier tube. The PM tube is a very unstable element of any scintillation probe. Its gain stability is poor which is caused by several factors . - The temperature influence on the photomultiplier gain . The temperature gain coefficient can be as much as (-0.2)-(-2)%/°C, depending on the type of PM tube.

- Statistic fluctuations of gain, ±0.7%/d. - Long term drift which can be 10-20% within a few months . The drift direction is changeable and impossible to be foreseen .

with a -proportionality coefficient, m-number of dynodes, U -PM tube high voltage. To cope with these instabilities, especially with instabilities caused by gain drifts, an automatic gain control circuit is required . Many ACC methods and circuits have been developed which can be divided into two main groups according to reference signal: 1) AGC with additional reference signal. a) Additional monoenergetic alpha emitt,:r with its radiation energy out of the range of the

measured spectrum'). b) Additional pulsed light source as reference signa12). c) Additional gamma or beta reference source

with periodically changing radiation intensity by means of rotary shatter. 2) AGC systems employing the same signal for stabilization and measurement by means cf. a) Control of photopeak position of differential 3) . spectrum of detected radiation b) Control of pulse amplitudeof the highest p+dses of the detected radiation spectrum°). These methods which can succesfully be applied to many problems, do have their limitations, however. The AGC method listed under (la) exhibits an error resulting from the difference of the thermal coefficient of the scintillator for reference alpha particles and for detected beta or gamma radiation. The (lb) method

shows an error caused by the thermal coefficient of the light source used as the reference signal. The method

231

232

B. MACHAJ

under (lc) does not introduce additional errors but requires rotating shutters.These shutters are a serious drawback of the AGC system when taking into consideration the reliability of the measuring head, especially in difficult environmental conditions and for large-size detectors . Neither of the two methods under (2) introduce additional errors, as the same signal is used for measurement and for spectrum stabilization. The method under (2a) is limited to cases where the radiation spectrum exhibits a photopeak, and can not be used for ascintillation probe witha plastic scintillator . The amplitude of pulses in the method 'under (2b) is usually sensed by means of a peak detector. It does not require a photopeak in the radiation spectrum, but is dependent on the radiation intensity. To overcome these limitations, a two-discriminator AGC system has been developed . It does not requireany photopeak in the radiation spectrum, although there are no obstacles in applying it also forthe stabilization of such spectra. 2. Two-discriminator AGC circuit The two-discriminator AGC circuit can be constructed in the form shown by the functional diagram in fig. 1 . The principle of operation of this system is as follows. The E, and E2 discrimination levels and Qr, Q2 single pulse charges delivered by current pumps are matched in such a manner, that for a selected PM tube gain, currents I, and 12 are equal and of opposite direction . The current flowing into ratemeter AI is equal to zero. Consequently the voltages d U, and AU2 are equal to zero too, and there is no change of PM tube gain. In case of any drift of the photomultiplier gain, the balance between currents It and 12 deteriorates . A current error d7 is generated resulting in a voltage d U, at the output of ratemeter I, and a change of voltage d U2 at the output of the by power

supply. Thevoltage d U2 controls thePM gain in such a direction as to remove the existing gain drift of the PM tube. The E2 discrimination level is selected near theupper end of the radiation spectrum, where the relative slope `' of the bias curve is high . Level Et is selected at that point of the integral curve where the relative slope is low. When the AGC loop is closed, the variation of PM gain, (dklk)', caused by PM drift, (dklk), is described by') f_dkl _ (dk/k) `k J ' I+A

where

A = A, A, A 3A4 -open loop gain, A, = (dk k)'jd U2 -PM tube gain, A 2 = d7/(dklk)' -gain of discriminators together

with curren. pumps, A3 = AU,jdl -ratemeter gain, A 4 = d U2/AU, - by power supply gain . The change of PM gain with CsSb dynodes against by drifts is given byb)

U,

_dk = 0 .7m A k U

and thus -

= kk,'lA U2

At

=

U n

The currents 1, and 12 at the balance condition are given by Il = 12=Io = niQi = n2Q2 "

(4)

A change of gain of the PM tube due to (dklk) results in a change of currents I, and 12 by (dk'~ AI, = S, n t Qt k CA k' d1 2 = S2 n2Q2 ,

k

where S'

_ ±..In, = 4`111011 dklk AE,/E,

(5)

= dn2/tt2 = An21n2 ,E2 /E 2 dklk

(5a)

is the relative slope of the integral curve at level Et, S2 Fig. 1. Functional diagram of AGC circuit. PM - photomulti plittr tube, Et, E2 - discriminators, Qi, Qz - current pumps, I -ratemeter, It V -by power supply .

is the relative slope of the integral curve at level E2,

233

AUTOMATIC GAIN CONTROL CIRCUIT

,

er  n2 are the count rates above discrimination levels E, and E2, respectively, O_, ; Q2 are the charges delivered in a single pulse by the current pumps in channels 1 and 2, respectively. For small changes of PM` gain, which is valid when the AGC loop is closed, S, and S2 can be considered as constants. For an illustration of S see fig. 2. Taking the difference of dl, and A12' one gets: AI = dlr-dlz = lo(S2-SI)dk/k,

= 1,

for F~oo when condenser C is sfituted by resistor` R . Thus the stabilization coefficient defined as

(8)

T=

RC

C

A, A z Aao Aa + 1

AtAzAa (16)

Aa = d U 2Jd U, = count. A step drift of PM gain, (dklk) 1 (t), results in:

_ d_k 1 Jk d_kl _ dk p ' k ln 1+At A2 Aa(pC)- ' k p+A n IC'

(9)

Because of the stochastic character of the ionization radiation, statistic fluctuations are generated, a(dl) of

where A o = A t AzAa =

(IS)

is in the first case theoretically infinitive, in the second case it is --Ao for Ao> 1 . The time constant of regulation of the gain drift in both cases is the same and equals

dl = lo(S2 - S1) = n2Q2(Sz-St), (Aklk)'

A, = AU, JAI

_dk _ dk 1 Al" l ~ k), k Ao+1 k,.Ao'

(dklk)[(dklk)'] -'

and Az =

AGC loop is closed, caused by the PM drift (Ak1k) tends to zero for t-. coEq.when there it. no parallel resisto . across condenser C : (14) indicates that it tends toi

0.7 in u

nzQz(Sx - Si)Aa+

Is

(10)

C is the integrating condenser of the ratemeter. After transformation of eq . (9) Ao ~dkl' - dk exp(t . k k C )

(11)

In case when in the ratemeter resistor R is connected in parallel to the condenser C, A, and A o are expressed by : A 3 = AU,Jdl = R Ao = AtAzAsoAa =

Aso 1 = 1 +RCp 1+RCp '

0.7 tn nzQz(Sz - St)RAaU

(12) (13)

Performing calculations one gets the change of gain, (dkjk)', when the AGC loop is closed, corresponding to the step change of the PM drift; (dk/k)1(t), as : lkk~' .+' t . L1+Aoexpl_ (14) j` k 1+A ARC /J \o Eq. (9) shows that the gain change (dkJk)', when the

a 3

Experimental characteristics of large plastic scintillator . n - count rate without sample, n' - count rata with sample, S- relative slope of integral curve without sample, S'- relative slope of integral curve with sample. Fig. 2.

234

B . MACHAI

41, which at the input of the ratemeter are equal to a(d!) _ [a2(It)+a2(iz)~} = [Q,(nt-n2)+ 1 1 # . + (Q2_Qón21} = n2 Q2 (n 2 ni)

(17)

The change of PM gain for closed loop and condenser C not shunted by resistor R, caused by statistic fluctuations, is described by ('2k _ a(AI)A 3A4A, faldk = a(AI)A l A4 1+A k) C(p+Ao/C)' a\which after transformation gives: ajkk~"=

Q(dI A,A4 . exp(C Cot)J

(18)

The effective value of these fluctuations is given by :

JA~ _ [0(~k)']2dtl+ a(AI)AlA4 a` k o (2 ki Ao C)~

dk)e St fat

dl = AI,-d12 = Io(a 1 -a 2), a,

ní n,

(22)

count rate with sample count rate without sample

_n2 _ count rate with samp!~ count rate withou= -Ample

a2 _ _42

n2Q2(nz 1- ni `)AI A4 11` S, n t 2C(S2-S,)

(20)

and the relative increase of statistic fluctuations is given by 2 = Q2At A4Si nt I 1_L2 4C(S2-S,) n1/,

change by thesame coefficient wh°.nameasured sample is introduced between : the detector and the radiation source. For such an assumption eq. (4) is always fulfilled, irrespective of radiation intensity.In the case ' when E, and E2 are set far from each other, it may happen that this assumption is not valid. In this case the following relations hold : I, = n1 Q, = to without sample, current 11 , with sample, current I, , h =n, Q, a, = a, Io without sample, current 72, 12 =n2Q2 = Io IZ =n2Q2a2=a2 to with sample, current 12, dl,=I,'-I,=lo (a,--1)-change of current 11 when a sample is introduced . d12=Iz-12 =1o(a2 -1)-change of current 12 whet, a sample is introduced. Thus a current error will be, generated equal to where:

Relative statistic fluctuations of the count rate above discrimination level E1 , caused by the AGC action, equal: aAGC(nt) =° Q(

n1 and n2 above discrimination levels -Et and E2

a GC 2 CQ t J

(21)

Comparing eq. (22) with eq . (6) and transforming one gets _dk _ a1-a2 k Sz-SI'

(23)

where Sz and S', are the relative slopes of the integral curve when a measured sample is introduced between detector and radiation source. The resultingcount rate error of signal n, is equal to

k

dnt _ dk a,-az n - S1 = S1 S'2 -S11 t

(24)

where Ttds means that for at = a2, as expected, there is no a, = na} -statistic fluctuations of signal n, abovedis- erroneous change of PM gain. In the case when crimination level E1 when the AGCloop is a, ~- a2, i .e. fa t and n2 vary by a different factor when a open, sample is introduced between source and detector, a,-statistic fluctuations of signai n1 when the AGC there is an erroneous change of PM gain that is deterloop is closed . mined by eq. (23) and acorresponding count rate error Performing all thecalculations in the above manner, above level E1 given by eq. (24). This phenomenon one gets the same expression for the increase of the requires attention when selecting discrimination levels fluctuations da for the case when condenser C is E, and Ez. shunted by resistor R. An example illustrating the performance of an AGC Time ,two-discriminator method of spectrum stabiliza- system basedon the described principle is ascintillation tion ',is based on the assumption, that the count rates probe with large plastic scintillator. Experimental

AUTOMATIC GAIN CONTROL CIRCUIT

235

The main parameters of this type of AGC system, characteristics of the probe are shown in fig. 2. The curves n and n' represent the count rates without and checked experimentally, showed good agreement with with sample introduced between the detector and calculations. The AGC circuit operated properly for radiationsource ""Cs, respectively. S and S' represent PM gain changes of more than ±50% . When checking the relative slopes of the bias curves calculated accor- the AGC performance, the PM gain changes were ding to eq . (5). The a = n'/n curve shows, that the introduced by PM by variations. It should be mentioned attenuation coefficient is not constant and varies here, that the stability and accuracy of the probe ate' depending on the discrimination level . For discrimi- not limitedby the AGC stabilizatior, coefficient but far nation levels El and EZ selected as in fig . 2 we get: other reasons, e.g. non-linear spectrum attenuation . It' S1 =0.4, S2=5, n, = 36.000 c/s, nZ = 2.000 c/s, was proved that a stabilization coefficient of a few a, =. 0.60, aZ = 0.62. Electronic parameters are assu- hundred can easily be achieved . med to be The author gratefully acknowledges comments of A4 = 40 V/V - by power supply gain, Mr Z. Paryski andthe assistance of Messrs P. Urbafiski m = 11 -number of dynodes, e.g. EMI-6097 and A.Dybkowski of theInstitute of NuclearResearch, PM tube, Warsaw, Poland. He wishes also to express his thanks -by of PM tube, U = 900V to Messrs N. Ellis and J. F. Cameron of Nuclear -single pulse charge, Qz = 5 x 10-7 C Enterprises Ltd., Reading, UK . C = 400pF - integrating condenser, R = 100 kS1 - shunting resistor . References The stabilization coefficient Ao, the AGC response 1) S. A. Scherbakskoy, Rev. Sci . Instr. 12 (1961) . time constant s, the increase of statistic fluctuations 2) W. Marlow, Nucl. Instr. and Meth . 15 (1962) 188. H. De Ward, Nucleonics 13 (1955) 36. Air, andtherelative error caused by thenon-linearity of s) 4) J. Bietin, Pribory i Tekhn. Eksperim . (1961) no. 2. the spectrum attenuation An,/n  calculated according s) :1. Grinberg, B. Sabbah and M. Schuster, Nucl . Insta an.i to eqs (13), (16), (21), and (24) are: Ao = 156, x = 0.3 s, Vieth . 82 (1970) 278. Ats=0.07, and An, /n`, = 0.0018 . s) EMI Electronics Ltd., Photomultiplier tubes (1970) .