IEEE Twactions on Nuc2eca Science, Vot.NS-25, No.1, Feb4'tuy 1978

ELIMINATION OF IMAGE BLURRING DUE TO DOUBLE SCATTER EVENTS IN

y IMAGING MWPC DETECTORS

D. Ortendahl, K. C. Tam, V. Perez-Mendez and C. B. Lim

Lawrence Berkeley LaboratoryBerkeley, California 94720

and

University of CaliforniaSan Francisco, California 94143

ABSTRACT

In multiwire proportional chmabers used withhoneycomb lead converters for detecting 511 KeV y raysfrom positron annihilation, a source of image blurringis generated by multiple interaction events due to theescape photoelectric x-ray or from the Comptonscattered photon. Using the delay line readout methodthe majority of these double events are eliminatedby using the fact that the sum of the time intervalsfrom the prompt anode signal to the signal arrival ateach end of the delay line is a constant to withinthe timing accuracy for a single interaction. Doubleinteraction events produce a time sum which isshorter. Good improvement in image quality isobtained. The observed number of multiple events islarger than calculations would predict.

INTRODUCT ION

In y imaging devices there are variouscontributions to the background and the degradation ofthe spatial resolution. One such contribution ismultiple interactions where ionization occurs at morethan one point in the detector. This can occur wheneither a photoelectric escape y-ray or a Comptonscattered y-ray converts in the detector producing asecond pulse. In order to retain image quality onemust either reject such multiple events or identifywhich of the interactions is due to the primary y-ray.In NaI crystals for energies below 300 keV and MWPCwhere the y-ray is below 60 keV such events can berejected effectively by using pulse height selection.However, at higher energies pulse height selectionis not as effective since the photoelectric peak isbroadened. Furthermore when MWPC detectors areequipped with honeycomb lead converters or lead-glasstubing assemblies2 (for imaging 511 keV y-rays) thespectrum of conversion electrons is continuous andhence rejection of such events is not feasible by pulseheight selection.

We have studied this problem for the case wherey-rays from positron annihilation are detected byMWPC's with honeycomb lead converters. By using aMWPC with electromagnetic delay line readout wedirectly determine the presence of more than oneevent in the chamber as shown below. We find thatthere are a significant number of such events andtheir removal improves the image quality.

MEASUREMENTS

A typical MWPC fitted with honeycomb leadconverters is shown in Figure 1. For maximumdetection efficiency a converter is used on each sideof the proportional chamber. However, for thepurposes of the measurements described here only the

bottom converter was in place. A photograph of theconverter is also shown in figure 1. The convertercell size was 2.5mm with a height of 15mm. It isdivided into 4 bands in order to provide a driftvoltage of 330 V/cm to extract the electrons. Theefficiency of the single converter has been measuredto be 4.2%. The gas mixture was Ar (70%) C02(30%) atSTP.

A small NaI detector was used in coincidence withthe prompt signal from the anode plane to provide awell collimated beam of 511 keV photons. A pointsource of a few viCi of Ge68 was used. From thegeometry the size of the beam at the converter wasapproximately 3mm.

The position was recorded by electromagnetic delaylines3 with a delay of 2.5 ns/mm. Low noise amplifiersutilizing "electronic cooling" 4 were used at each endof the delay line. Zero-cross discriminators were usedfor timing5. Slewing for these discriminators was lessthan 500 ps over a dynamic range of 30 to 1; this isimportant for converter work since the range insignal amplitude is large.

The well know method of reading the signal fromeach end of the delay line provides a means ofdistinguishing multiple events. The time intervalsbetween the avalanche on the anode wires and thearrival of the pulse at each end of the delay line ismeasured. For single events the sum of the intervalsto each end should simply be the length of the delayline plus some fixed delay in the processingelectronics; a constant to within the accuracy of thetiming measurements. Contributions to the timingjitter are signal to noise, pulse shape and risetime6. An advantage in reading from both ends is thatin the case where the accuracy is limited by timingerrors averaging the positions obtained from each endof the line improves the resolution by a factor of47 . It is also clear that if a second particleinteracts in the chamber before the readout iscompleted, the sum of the two intervals will be somevalue less than the length of the line. Fig. 2 showsthe time sum distribution for an Fe55 source. Asexpected for a single interaction type event, thedistribution is symmetric and narrow with a 5 ns FWHM.

In order to correct for accidental coincidencesbetween the Nal detector and the MWPC, data was takenwith the NaI signal delayed 3us which provided morethan enough time for the delay line to clear.

In Fig. 3 an x-projection of the point responsefunction for the positron source is plotted withaccidental background subtracted. In all casesdiscussed here accidental background is removed byapplying identical cuts to the delayed coincidencedata and subtracting the resultant distributions. TheFWHM of the distribution is 7mm, larger than one wouldexpect from the geometry discussed previously. Thereare also a number of events outside the main peak.

0018-9499/78/0200-0148 $00.75 0 1978 IEEE148

Figure 4 shows the time sum distribution for thisdata. When compared with the distribution for Fe55,it is seen that the distribution is wider (FWHM=20ns)skewed to the low side and there are more events onthe high side of the peak than one would expect. Wehave determined that the events on the right side aredue to saturation of the linear electronics bysignals whose amplitude out of the amplifier isgreater than 3V. Signals as small as 30 mV areaccepted so the dynamic range is high. One caneffectively eliminate this problem by using upperlevel thresholds on the discriminators. These eventsare shown in the plot as a demonstration of oneproblem that is corrected by time sum information.In order to make sure we are not affected by theselarge sums,events are rejected if the sum is largerthan 168ns; this cut is indicated on Figure 3. Figure5 shows the point response function after this cut hasbeen applied; the FWHM is reduced to 6.5 mm.

On the left hand side of the time sum peak thereappear to be a significant number of cases where amultiple interaction has occured. Once away from thepeak, the distribution is relatively flat until itreaches a sum of 150ns at which point it drops off. Asum of 150ns would be expected for the case where aphoton converted at the center of the chamberproducing a secondary photon which converted veryclose to one edge of the chamber giving a sumapproximately equal to half the length of the delayline.

In Figures 6aand 6b the point response functionis plotted for events with a time sum between 141and 168ns and 150 and 168ns respectively. These cutsshould remove many of these multiple events. As canbe seen the number of off-target events (outside themain peak) is reduced and the peak is sharpened.

Table I summarizes the results giving the numberof events contained within different intervals aboutthe mean for various sum cuts. As can be seenimprovement in the in the fraction of events containedwithin a given interval can be obtained with somereduction in the total number of events within theinterval.

A Monte Carlo calculation was performed todetermine the probability of secondary interactionsfrom photoelectric escape photons and Comptonscattering and their contribution to the spatialresolution. The photon is assumed to enter theconverter approximately perpendicular. The materialin the converter is assumed to be distributeduniformly throughout its volume. The point at whichthe 511 keV photon converts is determined and anaverage detection probability is assigned. Aconversion is considered detected if an electronescapes from the wall of the honeycomb into the gas.The average probability for this to occur for 511 keVy's was determined assuming a .1mm wall thickness.This probability is .57 for photoelectric conversionand .12 for Compton scattering. At 511 keV thecross section for photoelectric effect is 1.20 timesthe Compton cross section.

The Compton scattered photon or the 88 keV escapephoton for Pb is followed until it converts or escapesfrom the converter. Figure 7a gives the probabilityof secondary interaction as a function of lateraldistance from the primary conversion. There is a64% chance of producing a secondary interaction.Figure 7b gives the probability of actually detectingthe secondary interaction. Note that there is a 17%chance of detecting a secondary photon. But if werequire that the primary interaction also be detectedthen the probability of having a 2 photon eventdrops to 4%.

Almost all of the secondary events are predictedto occur within 1 cm of the primary interaction. Thereare not enough double events to explain the

experimental measurements. For example the observedtime sum distribution indicates that there should be a5% probability of producing a double event with thespacing between primary and secondary larger than 2cm.In the Monte Carlo Program we have not taken intoaccount the possibility that the secondary photoncould leave the converter and convert in the aluminumlid of the vessel. Since the mean free path in gas islong this could produce a secondary event far from theprimary, however, the probability of producing adetectable electron from the cover would be lower thanfrom the converter. It is probably likely that evenafter this is taken into account that there will besome multiple events that can not be explained. Ourconfidence in the Monte Carlo code itself is bolsteredby the fact that it predicts the detection efficiencyof the converter to within 20%.

CONCLUS IONS

At this point the observed effect is notcompletely understood. Work on the problem willcontinue, both from the point of view of understandingthe physics involved and actually implementing thistime sum technique in the U.C.S.F. MWPC positroncamera. What is clear is that there are multipleinteractions in the chamber which spoil the resolution,and that this method provides a means of correctingthe problem.

TABLE I. The number of events contained in intervals about the mean of

x projection of the point response function is given for varioustime sum cuts. The fraction of those events passing the cut,which fall within the interval is also given.

Interval No Sum Cuts Sum<168ns 141cSum<168ns 150-Sumv<168ns

5 mm 1400349%

10 mm 2160475%

15 mm 2486786%

1249550%

1910577%

2194288%

1148165%

1505786%

1647294%

846472%

1065591%

1134697%

ACKNOWLEDGEMENTS

This work is supported by the U.S. Department ofEnergy under the auspices of the Division of PhysicalResearch.

REFERENCES

1. J. E. Bateman, "The Improvement of the SpatialResolution of a Xenon-Filled X-Ray Imaging MWPC byMeans of Pulse Height Selection", NIM 140, 211-214,(1977).

2. D. Chu, K.C. Tam,V. Perez-Mendez,C.B. Lim,0. Lambert, S. Kaplan, "High Efficiency CollimatorConverters for Neutral Particle Imaging with MWPC"IEEE Trans. Nucl. Sci. 23, 634(1976).

3. R. Grove, I. Ko, B. Leskovar,V. Perez-Mendez,"Phase Compensated Delay Lines for Wire ChamberReadout",NIM 99,381(1972); R. Grove, V. Perez-Mendez, J. Sperinde, "Improved Delay Lines forProportional Wire Chamber Readout.',NIM 106,407 (1973).

4. V. Radeka, "Signals, Noise and Resolution inPosition Sensitive Detectors", IEEE Trans. Nuc.Sci. NS-21, 51 (1973).

5. N. Bhandari, V. Perez-Mendez,F. Kirsten,"Inexpensive Timing Discriminator with Sub-Nanosecond Slewing for Delay Line Readout ofProportional Chambers", Lawrence Berkeley Lab-oratory Report (1976).

6. V. Perez-Mendez, M. Greenstein, D. Ortendahl,"Evaluation of Factors Contributing to PositionAccuracy in Delay Line Readout of MWPC", IEEETrans. Nuc. Sci. NS-24, 209 (1977).

149

t0 10 20 30x(mm)

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Fig. 3. X-projection of point response functionfor the positron source.

Fig. 1. Schematic Diagram of a MWPC with honeycomblead converters. The honeycomb converteris also shown.

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Fig. 7a. Probability of a secondary interaction asa function of distance from the primaryinteraction. The total fraction of photo-electric and Compton interactions is shown.Also given is the probability of only aprimary interaction.

b. Probability of detecting a secondary inter-action. Double refers to the sum of theCompton and photoelectric contributions whenboth secondary and primary interactions aredetected. The scale for the double curve ison the right hand side.

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Fig. 6a. X-projection of point response function forthose events with sums between 141 and 168ns.

Fig. 6b. X-projection of point response functionfor those events with sums between 150 and168 ns.

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