US3423586A

US3423586A - Activity counting device in multichannel arrangement

Info

Publication number: US3423586A
Application number: US414249A
Authority: US
Inventors: Israel Pelah; Georges Fraysse; Walter Hage
Original assignee: European Atomic Energy Community Euratom
Current assignee: European Atomic Energy Community Euratom
Priority date: 1963-12-18
Filing date: 1964-11-27
Publication date: 1969-01-21
Anticipated expiration: 1986-01-21
Also published as: CH440472A; DE1267348B; LU47524A1; NL6414363A; GB1091511A; BE655720A

Description

Jan. 21, 1969 l. PELAH ETAL 3,423,586

ACTIVITY COUNTING DEVICE IN MULTICHANNEL ABRANGEMENT Filed Nov. 27, 1964 Sheet of e .22D m ZOTFOmmE A TTORNE YS ACTIVITY COUNTING DEVICE IN MULTICHANNEL ARRANGEMENT Sheet Filed Nov. 27, 1964 lllllllllllllllllllll IIJ 1\, 2

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ATTORNEYS Jan. 21, 1969 3,423,586

ACTIVITY COUNTING DEVICE 1N MULTICHANNEL ARRANGEMENT l. PELAH ET Al- Sheet Filed Nov. 27, 1964 INVENTORS Georges FRAYSSE Walter HAGE Israel FELAH ATTORNEYS Sheet 5 of Jan. 21, 1959 l, PELAH ETAL ACTIVITY COUNTING DEVICE IN MULTICHANNEL ARRANGEMBNT Filed NOV. 27, 1964 A TTORNE YS Jan. 21, 1969 pELAH ETAL ACTIVITY COUNTING DEVICE IN MULTICHANNEL ARRANGEMENT Filed Nov. 27, 1964 Sheet 5:29. t 5:2.; Il o: 2 2 t 2 m. Z n. N. 2

INVENTORS Georges FRAYSSE walter HAGE Israel PELAH A TTORNE YS United States Patent O 50,057/ 63 U.S. Cl. Z50-83.1 2 Claims Int. Cl. G01t 3/00; H013 39/32 ABSTRACT OF THE DISCLOSURE There is provided a foil activity counting device for measuring a neutron ux emitted from radioactive materials. The device comprises a rotating sample carrier unit having samples of radioactive materials deposited thereon, a radiation detection unit mounted in a stationary fashion on the carrier unit, a single measuring channel connected to the radiation detection unit wlhich analyses and amplifies the output signals of the radiation detection unit, a pulse emitter controlled by the rotating sample carrier unit, a single addressing channel connected to the pulse emitter and providing identification pulses in synchronism with the carrier rotation, a multichannel pulse height analyser connected to the outputs respectively of the addressing channel and the measuring channel in order to receive distribution of the pulses from the radiation detection unit. The rotation speed of the rotating sample carrier unit is fixed at such a rate that the 'counting time for one foil is negligible as compared to the drifting time in the measuring channel and the half-time decay time of the foil substance.

The present invention relates to an activity counting device in multichannel arrangement as it is used in connection with neutron flux measurements in a nuclear reactor or in similar nuclear assemblies.

Working with foil-samples a series of foils is temporarily located at different points in the reactor, as well horizontally as vertically, and is activated by the neutrons according to the prevailing neutron flux distribution. The foils exposed are then taken out of the reactor for counting the activity in an appropriate counting device in order to determine by later calculation the neutron flux distribution. Instead of foils, wires or ribbons are in use.

Counting devices for this kind of experiments cornprise necessarily radiation detectors, appropriate counting channels and an addressable Output register with data recorder.

When the samples are counted simultaneously by a corresponding number of detectors, then an equal number of counting channels is generally adopted in order to compensate drifting. It is obvious, however, that in such a multichannel system the number of electronic units for detection, analysing and counting is rather elevated. The other advantage of simultaneous counting, the favorable ratio of effective counting time to the overall operation time for one foil, is partly outweighted by the necessity of submitting results to half-life period corrections.

It is an object of the present invention, to provide a foil activity counting device in which drifting is negligible and half-life period corrections unnecessary, and yet the number of detention and electronic units strongly reduced.

Another object of the invention is to realize the electronic concept of the counting device in a Way that it accepts as `well as multichannel pulse height analyser, a

Mice

multiscaler, as a multiparameter analyser as addressable output register.

Another object of the invention is to arrange and operate the counting device in a manner to perform the Fourier analysis of neutron flux distributions.

Still another object of the invention is to operate the counting device in a way to output values for average spatial neutron flux determination.

Still a further object of the invention is the counting device all transistorized, thus needinlg no heating or cooling.

Another object of the invention is to utilize within the counting device a foil carrier mechanism which permits continuous `charging and decharging of the foils and continuous measuring.

The new counting device in its largest scope is characterized according to the present invention in that it comprises a high speed travelling sample carrier unit with speed control, a radiation detection unit mounted on the carrier unit, a single -measuring channel connected to the radiation detection unit which analyses and amplies the output signals of the radiation detection unit, an internal synchronizing clock device controlled by the travelling sample carrier mechanism, a single addressing channel connected to the clock device and a multichannel analyser as registering unit connected to the outputs of the measuring and addressing channel respectively.

In the addressing channel a continuous series of pulses is generated to assure in the multichannel analyser correct correspondance of the detector pulses fed in the measuring channel, to the samples originating these pulses. In the measuring channel there is provided only a single channel analyser and an univibrator as electronic units.

In the case of a multiscaler as registering unit, the pulse series in the addressing channel is fed immediately to the multiscaler. No further electronic units are involved, unless the multiscaler does not reset automatically to channel 1. In that case, it is necessary to provide a reset pulse at each cycle.

However, when a multichannel pulse height analyser is used as register-inlg output of the counting device, the essential electronic units in the addressing channel are a saw tooth carrier generator and a staircase modulator. Both, the addressing channel and the measuring `channel then are connected to a common output unit, for example, to a signal adder unit, followed by a carrier suppression unit. The latter one is connected to the input of the pulse height analyser. The gate of that analyser is branched to the measuring channel output.

A travelling foil carrier mechanism should be preferably a rotatable disk carrying foils on his upper side `and driven by an electric motor. In the example of a counting device with special modulation of the addressing pulses, a synchronous motor of constant rotation speed of say 1500 t./m. for 50 c./s.-net frequency would incite the synchronizing pulse clock device to generate pulses of a frequency of 25 per sec. in the 'addressing channel. With l0 foils to be measured on the disk-all in circuim.- ferential distribution-and only one radiation detector for measuring the radiation counts, one addressing pulse occurs for one full counting cycle covering l0 foils.

In the case of a multiscaler as registering unit, the pulse frequency of 25 c./sec. is multiplied by the number of the foils on the disk in order to get one pulse for one foil. For the example chosen, the frequency will become 250 c./sec. Simultaneously in a by-pass the pulses of 25 c./sec. are amplified and stretched to obtain reset pulses. In a consecutive mixer the reset pulses are mixed to the addressing pulses and fed to the multiscaler. In the case of a multichannel pulse height analyser as registering unit, the addressing pulses are transformed by the saw tooth generator to a correspondingly shaped pulse series of the same frequency. In order to get correspondance between each foil measured and each group of detected radiation counts-each saw tooth-representing one counting cycleis modulated by a staircase modulator in a way that the step frequency corresponds to the number of foils measured within that cycle, namely 10. Thus for the adopted example with 10 foils on the disk the step width is 4 msec.

In the adder unit (or in the amplifier) measured radiation pulses-positive pulses of constant amplitude are added to (or multiplied with) the said staircase modulated voltage. As there are free distances between the foils, only the first half of each step is utilized for the impression of the radiation pulses.

Thus, originally constant amplitude of the pulses is augmented according to the rising amplitude of each step of the staircase voltage. In this way reidentication of the measured pulses and its originating foils is prepared.

The invention will now be described broader in detail. Other novel and useful characterstics and features of the invention will be apparent from the description and from the drawings referred to.

FIG. 1 shows the principal electronic diagram of the counting device;

FIG. 2 shows a section of the addressing channel, when a multiscaler as analyser is used;

FIG. 3 shows a block diagram of a counting device, when a multichannel pulse height analyser as registering unit is applied;

FIGS. 4a to 4d show the electric signals in the circuitry of the device of FIG. 3 at the points a, b, c and d;

FIG. 5 shows the mechanical parts of clock device involved in the generation of synchronising pulses;

FIG. 6 shows a modified section of the counting device of FIG. 3;

FIG. 7 shows a counting program for Fourier analysis;

FIG. 8 shows a modified version of the foil carrier for variable speed.

According to FIG. 1 the new counting device comprises a high speed travelling foil carrier unit 1 with speed control 2, a radiation detection unit 3 mounted on the carrier unit, a single measuring channel 4 with appropriate electronic units 5 connected to the radiation detection unit 3, an internal synchronising clock device 6 controlled by the travelling foil carrier mechanism 1, a single addressing channel 7 with appropriate electronic units 8 connected to the clock device 6, and a multichannel analyser 9 as registering unit connected to the outputs of the measuring and addressing channel respectively.

The high speed travelling foil carrier unit 1 comprises a motor driven mechanical support on which are xed a number of activated foils or samples to be measured one after the other, as is described later.

The speed of the support can be controlled by different modes, e.g. constant speed, stepwise advance, variable speed. It is of such a rate, that the counting time for one foil is negligible to the drifting time in the measuring channel or to the half-life decay time of the foil substance.

Preferably only one radiation detector is used for the measuring of all foils. Thus the foils are counted successively. Comparing this counting process with multichannel counting, the overall counting time for the samples is smaller. However, this disadvantage is more than outweighted by the enormous reduction of electronic units and by the fact that standard units can be used for all parts of the device.

The internal synchronizing clock of the counting device is a pulse emitter controlled by a moved organ of the travelling carrier device, for instance, by the foil support itself or by a rotating motor part (see further below); in this way synchronizing is assured by a direct mechanical link between the foils and the pulse inciting organ.

Especially in the case when the registering unit 9 is a multiscaler, pulses in the addressing channel are immediately fed into the scaler after appropriate shaping. Charging and clearing of the Scaler memories is carried out automatically. As is shown in FIG. 2, the addressing channel part between the shaper and the multiscaler comprises a frequency multiplier 10, an ampliiier 11 and a mixer 12. Multiplier and amplifier are fed with rectangular addressing pulses of, 'for instance, 25 c/sec., 1 cycle representing one counting cycle. In the multiplier, frequency is multiplied by a factor (of 10) according to the number of samples counted. In the amplifier 25 c./sec.pulses are amplified in order to get reset pulses for the multiscaler at the end of each counting cycle.

In FIG. 3, a counting device for pulse height analyser application is shown, where the measuring and counting channel have a common output and modulation of the addressing pulses is provided.

The foil carrier unit is represented by the rotating disk 13, the synchronous motor 14 and the radiation protection shield 15. The disk is directly coupled to the rotorshaft 16 of the motor and capsuled by the shield of lead 17. The shield is provided with a sluice (not shown) to introduce and to retire samples. Motor windings are connected to a regulated A.C. voltage sources of 50 c./s. over the lines 18.

Mounted on the shield of the foil carrier unit is a single radiation detection unit, composed of a scintillation counter 19 and a photornultiplier 20 of the type 56 AVP. The detection unit is located in a zone covering the periphery of the rotating disk, where the samples 20 are disposed circumferentially. The measuring channel of the device comprises the high voltage supply 21, the negative pole of which is connected to the cathode of the photomultiplier. The anode is circuited to earth over a ohm resistance and simultaneously branched to a coaxial transmission cable 22 of 100 ohms. The cable feeds a tunnel diode single channel analyser 23. The analyser is connected with the univibrator 24, which outputs well shaped radiation count pulses of 2 volts amplitude and 1 lnsec. width.

At point 25 the measuring channel output is branched to a variable delay line 26 and to the coincidence bus of the multichannel pulse height analyser 27. The delay time is variable between zero and 500 nsec. The dead time in the measuring channel must be higher than the dead time of the highest channel of the analyser.

The addressing channel of the new counting device commences with an internal synchronizing clock, which is constituted by the D.C. voltage source 28, the microswitch 29 and the cam 30 on the motor rotor shaft; see also FIG. 5. The electronic units in the addressing channel are the pulse Shaper 31, the sawtooth generator 32 and the staircase modulator 33. Now the output of the channel is branched to the voltage adder 33; see input b. Also the delayed output of the measuring channel is branched to the voltage adder; see input a. As described earlier, the count pulses of the measuring channel are impressed in the adder on the step-modulated synchronizing saw tooth pulses of the addressing channel, in order to prepare reidentification of the signals and foils during and by way of pulse height discrimination.

The adder output, see ref. c, is branched to the carrier voltage suppression unit 34, and this unit is connected to the input bus of the multichannel pulse height analyser 27; see output d. The analyser comprises 100 channels ranging from 0 to 100 volts, and a screen 35 for direct display of the counts in two dimensional array. The ordinate reference is the number of counts, the abscissa reference the foil number. Attached to the pulse height analyser is fast speed recorder 36.

Signals at the points a, b, c and d of the device of FIG. 3 are shown in the FIGURES 4a to 4d. The time axis in FIG. 4a is subdivided in 2 msec. intervals from which alternatively one is filled with pulses from a sample and one is empty. The empty space is due to the necessarily free spaces between the samples on the rotating disk. Pulses in the interval between 0 and 2 ms. are due to sample No. l, pulses in the interval between 4 ms. and 6 ms. are due to sample No. 2, and so on. The pulses amplitude is 2 volts, the pulse width is l msec. A dead time could be efficiently used to suppress eventual noise in the interval between 2 and 4 ms. and following empty parts.

From FIG. 4b can be seen the way in which a tooth is stepmodulated. Of the saw tooth width, which is 400 msec., only the first 8 msec. for the first two steps are designed. The first steps amplitude is 5 volts, the second steps amplitude is 10 volts, and so on. Thus the amplitude of the 10th step is 5 0 volts.

FIGURES 4c and 4d need no further explanation than perhaps this that the overall voltage of each step is 2 volts greater than the step voltage.

It is clear by this, that according to FIG. 4d a radiation pulse series of say l2 volts will enter into channel 12 of the pulse height analyser 27, and that these pulses are due to sample No. 2.

There are some general remarks to be made with respect to the counting device shown in FIG. 3.

The utilized photo multiplier 56 AVP permits direct connecting of the tunnel diode discriminator by a cable of a certain length, if carefully adapted. Because of its elevated gain, it can be fed by a small voltage in order to reduce gain drift. Should a less powerful multiplier be used, a preamplifier with three transistors, one of which wired as emitter follower, of a gain of l0 to 20 must be interconnected between the multiplier and the single channel analyser. As to the microswitch unit, it can be substituted by a system in which pulse generating is incited by magnetic, electric or optical means. But when a cam or a magnet is used, it should be angularly adjustable on the shaft.

The coincidence measurement at the pulse height analyser 27 is made in order to eliminate parasitic pulses due to the step modulator or another noise source.

It is effected in advance to the pulse reception in the adder unit. This is achieved by retarding the signal in the delay line 26.

With only one detection unit no geometric compensation of the measurement due to unequal positioning of the foils is possible. Thus a second detector diametrically opposed to the first one will have to be applied, This detector must be branched in parallel to the first one with a delay circuit between, corresponding to the linear speed of the foil from one to the other. By this means also the relative low counting rate of the device can be ameliorated. With ten detectors in parallel, for instance, the counting rate can be approximated to the half of that of a multi-measuring channel counting device.

At any rate one high voltage source only is necessary for the detector or detectors. The gain of each photomultiplier can be controlled by a potentiometer influencing the voltage difference between the anode and the last dynode of the multiplier when the high voltage is fixed to be constant and equal for all multipliers.

Another possibility of reducing counting time loss is to operate the foil disk driving motor step wise. In this case, the transit time between counting of two successive foils can be 4made extremely small. When in addition ten detectors in parallel are applied as explained further above, then the counting rate reaches nearly the counting rate of a multimeasuring channel device.

As already mentioned above the adder 33 of the circuit arrangement of FIG. 3 can be substituted by an amplilier of high gain. This is shown in FIG. 6, Where the saw tooth generator, the step modulator, the delay line and the pulse height analyser are identical with the corresponding units shown in FIG. 3.

A highly interesting feature of the foil activity counting device according to the present invention is its convenient application in Fourier analysis measurements of the neutron flux of a nuclear reactor for instance. As can be shown by calculation, consecutive measuring with variable speed of a continuous elongated sample of varying activity, for instance a wire or a ribbon, or of discrete samples, as is an array of aligned foils on a rotating disk, primarily activated in a neutron eld along an axis of determined direction, yields counting values, which can serve imediately for computation of the neutron flux distribution. In the case `of the Fourier analyses of the neutron ux in a reactor core for instance, samples activated along the vertical (or a radial) axis of symmetry of the core can be placed one after the other in circumferential order on the rotating disk of the foil carrier unit explained above, and measured with a speed depending upon the function of each term of sum of the Fourier integral respectively. Thus, the constant term and the harmonics of the integral can be obtained by submitting the foils or wires or ribbons to alternative measuring programs providing speed variations according to inverse cosinus or sinus law versions.

Practically, foils exposed in the central zone of the neutron field are measured longer than those exposed above, below or beneath. In FIG. 7 a counting program is shown in which the measuring time t1 to tw for ten foils varies according to a sinus law. The foils are considered to have been exposed in the central vertical axis of a nuclear reactor core, thus foils counted during t1, tw were in positions above and below the bulk ux, While foils -counted t5, t6 were in the centre.

There are several practical methods to perform Fourier analysis with the described counting device. When the drive motor of the foil carrier disk is a synchronous motor, two possibilities exist, either a constructional one or an operational one. In FIG. 8 is shown schematically a constructional version of carrier device, which differs from that in FIG. 3 in that it is composed by two

independent disks

38, 39. Balls 40 on the lower disk support the upper disk permitting relative rotational movement of this disk with respect to the other. Foils 41 to be counted are placed 'on the upper disk in circumferential array. The driving shafts 42, 43 of the device are mounted coaxially and are equipped with gear wheels 44, 45. The gear Wheels mate with corresponding Wheels of a speed transformer 46, driven by the synchronous motor 47. Disk 38 is driven with constant basis speed entraining disk 39. The speed of disk 39 is varied by the speed transformer 46 which adds to the basis speed a supplemental speed amount, modulated according to an inverse sinus or cosinus law. Cams may be involved in this control action.

An operational method without changing the carrier device in FIG. 3 would be to vary the rotation speed of the drive motor according to a sinus or cosinus law over a greater number of rotations. It has been found that speed variation over n rotations yield the same counting results that a unique rotation with a device like that in FIG. 8 or a step-motor device described hereafter.

Speed variation over a greater number of rotations, e.g. rotations, is easy to realise with a voltage controlled commutator motor and even with a synchronous motor, as the frequency of it is to be varied only slowly.

Instead olf using a speed transformer with cams for the reproduction of the motion function, control can also be effected on the basis of a diagram paper sheet, Whereon the motion function is traced in a conductive line palpated by voltage fed contacts. The motion function is then transformed by an appropriate transformer to a corresponding speed variation effective at rotorshaft 39 of the device in FIG. 8.

When instead of foil samples an activated wire (or ribbon) is used, the wire is placed in form of a spire on the rotating disk. Step-wise operation of the disk is then equivalent to the case of n discrete foils. When the disk is driven by a synchronous motor within the device of FIG. 3, the stair-case modulator is no longer necessary.

The counting program shown in FIG. 7 can also be realized by aid of a stepping motor coupled to a sole foil :carrier disk as shown in FIG. 3. In principal each possible motion law can be simulated by the devices described above.

When average spatial neutron flux determination shall be can'ed out, measured pulses merely are summed up in a multiscaler, for instance branched in parallel to the pulse height analyser. This operation is simple Vbecause of the serial mode of function of the counting device.

In the foregoing description, application of either a multichannel pulse height analyzer of a multiscaler as registering unit was explained. There is however also an interest of utilising multidimensional analysers. It arises particularly from neutron eld analysis in fast reactors, where apart from the ux distribution also the neutron energy spectrum is interesting. a

Multidimensional or multiparametric display affords time savings in all analysing work using activated foils, when the spatial distribution `of a physical quantity shall be established. Possible parameters are the spatial axis xJ y, z of a rectangular three coordinates system, and parameters alpha, beta, gamma, etc. for fixed neutron energy ranges. In a thermal nuclear reactor, foils of only one specific substance are used, whereas in fast reactors foils of different substances adapted to xed energy ranges, are applied. For instance In 115, Au 197, I 127, Mn 55, Cu 63, Lu 176 for thermal neutrons, and P 3l, S 32, Al 27, Si 28 for fast neutrons.

From the foregoing it is clear, that in a counting system with multiparametric analysing, identity of the foils is tied to at least two references: -ordonnance number and a parameter. Parameters can be introduced in the counting system by appropriate coders, attached to the analyser, whereas visualisation on the screen of the analyser will always take place in producing the number of counts for one parameter only. The records outputted contain the codes for all parameters involved, thus facilitating later computation `by machines. With multiparametric analysers a convenient means is offered to obtain not only the overall neutron population of a reactor, or the fundamental term or the harmonics in the case of Fourier analysis, but also the populations split up in energy ranges.

We claim:

1. A foil activity counting device for measuring a neutron ux emitted from radioactive materials, which comprises a rotating sample carrier unit having samples of radioactive materials deposited thereon, a radiation detection unit mounted in a stationary fashion over said carrier unit, a single measuring channel connected lo said radiation detection unit which analyses and ampliiies the output signals 'of said radiation detection unit, a pulse emitter controlled by said rotating sample carrier unit, a single addressing channel connected to said pulse emitter and providing identification pulses in synchronism with the carrier rotation, a multi-channel pulse height analyser connected to the outputs respectively of said addressing channel and said measuring channel to receive distribution of pulses from the radiation detection unit, the rotation speed of said rotating sample carrier unit being carried out at such a rate that the counting time for one foil is negligible as compared to the drifting time in the measuring channel and the half-time decay time of the foil substance.

2. A foil activity counting device as defined -in claim 1, wherein the addressing channel contains a sawtooth generator followed by a staircase modulator, and that the measuring channel contains a pulse height analyser followed by a univibrator, that both channels are connected to a signal adder unit, and that between said adder unit and said analyser, a differentiating element followed by an amplier having controllable amplification factor is connected.

References Cited UNITED STATES PATENTS 2,490,298 12/1949 Ghiorso et al. Z50-106 3,141,977 7/1964 Fratantuno 250-106 RALPH G. WILSON, Primary Examiner.

A. B. CROFT, Assistant Examiner.

U.S. Cl. X.R.