Ortendahl, D.

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Lawrence Berkeley National LaboratoryRecent WorkTitleELIMINATION OF IMAGE BLURRING DUE TO DOUBLE SCATTER EVENTS IN Y IMAGING MWPC DETECTORSPermalinkhttps://escholarship.org/uc/item/8c4279q9AuthorOrtendahl, D.Publication Date1977-10-01eScholarship.org Powered by the California Digital LibraryUniversity of Californiau v ' • 'J ..• \) v l{e..-"7 Presented at the IEEE Nuclear Science Sympositun, San Francisco, CA, LBL-6436 r: I October 20, 1977 ELIMINATION OF IMAGE BLURRING DUE TO DOUBLE SCATI'ER EVENTS IN Y IMAGING MWPC DETECTORS RECEIVED D Or dahl K C T LAVJRENCE • ten ' • • am, . BH'Kfl~V LABORATOR V. Perez-Mendez, and C. B. 11m Y October 20, 1977 DEC 1 5 1977 LIBRARY AND DOCUMENTS SECTION Prepared for the U. S. Department of Energy under Contract W-7405-ENG-48 For Reference Not to be taken from this room DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. 0 8 0 8 ELIMINATION OF IMAGE BLURRING DUE TO DOUBLE SCATTER EVENTS IN x IMAGING MWPC DETECTORS D. Ortendahl, K. C. Tam, V. Perez-Mendez and C. B. Lim lawrence Berkeley laboratory Berkeley, California 94720 and University of California San Francisco, California 94143 ABSTRACT In multiwire proportional chmabers used with honeycomb lead converters for detecting 511 KeV x rays from positron annihilation, a source of image blurring is generated by multiple interaction events due to the escape photoelectric x-ray or from the Compton scattered photon. Using the delay line readout method the majority of these double events are eliminated by using the fact that the sum of the time intervals from the prompt anode signal to the signal arrival at each end of the delay line is a constant to within the timing accuracy for a single interaction. Double interaction events produce a time sum which is shorter. Good .improvement in image quality is obtained. The observed number of multiple events is larger than calculations· would predict. INTRODUCTION In y imaging devices there are various contributions ·to the background and the degradation of the spatial resolution. One such contribution is multiple interactions where ionization occurs at more than one point in the detector. This can occur when either a photoelectric escape x-ray or a Compton scattered x-ray converts in the detector producing a second pulse. In order to retain image quality one· must either reject such multiple events or identify which of the interactions is due to the primary x-ray. In Nal crystals for energies below 300 keV and MWPC where the x-ray is below 60 keV such events can be rejected effectively by using pulse height selection.l However, at higher energies pulse height selection is not as effective since the photoelectric peak is broadened. Furthermore when MWPC detectors are equipped with honeycomb lead converters or lead-glass tubing assemblies2 (for imaging 511 keV x-rays) the spectrum of conversion electrons is continuous and hence rejection of such events is not feasible by pulse height selection. We have studied this problem for the case where ~-rays from positron annihilation are detected by MWPC's with honeycomb lead converters. By using a MWPC with electromagnetic delay line readout we directly determine the presence of more than one event in the chamber as shown below. We find that there are a significant number of such events and their removal improves the image quality. MEASUREMENTS A typical MWPC fitted with honeycomb lead converters is shown in Figure 1. For maximum detection efficiency a converter is used on each side o.f the proportional chamber. However, for the purposes·of the measurements described here only the bottom converter was in place. A photograph of the converter is also shown in figure 1. The converter cell size was 2.5mm with a height of 15mm. It is divided into 4 bands in order to provide a drift voltage of 330 V/cm to extract the electrons. The efficiency of the single converter has been measured to be 4.2%. The gas mixture was Ar (70%) C02(30%) at STP. , A small Nal detector was used in coincidence with the prompt signal from the anode plane to provide a well collimated beam of 511 keV photons. A point source of a few ~Ci of Ge68 was used. From the geometry the size of the beam at the converter was approximately 3mm. The position was recorded by electromagnetic delay lines3 with a delay of 2.5 ns/mm. Low noise amplifiers utilizing "electronic cooling" 4 were used at each end of the delay line. Zero-cross discriminators were used for timingS. Slewing for these discriminators ~1as less than 500 ps over a dynamic range of 30 to 1; this i~ important for converter work since the range in signal amplitude is large. The well know method of reading the signal from each end of the delay line provides a means of distinguishing multiple events. The time intervals between the avalanche on the anode wires and the arrival of the pulse at each end of the delay line is measured. For single events the sum of the intervals to each end should simply be the length of the delay line plus some fixed delay in the processing electronics; a constant to within the accuracy of the timing measurements. Contributions to the timing jitter are signal to noise, pulse shape and rise time6. An advantage in reading from both ends is that in the case where the accuracy is limited by timing errors averaging the positions obtained from each end of the line improves the resolution by a factor of f.r. It is also clear that if a second particle interacts in the chamber before the readout is completed, the sum of the two intervals will be some value less than ihe length of the line. Fig. 2 shows the time sum distribution for an Fe55 source. As expected for a single interaction type event, the distribution is s~nmetric and narrow with a 5 ns FWHM. In order to correct for accidental coincidences between the Nai detector and the MWPC, data was taken with the Nai signal delayed 3~s which provided more than enough time for the delay line to clear. In Fig. 3 an x-projection of the point response function for the positron source is plotted with accidental background subtracted. In all cases discussed here accidental background is removed by applying identical cuts to the delayed coincidence data and subtracting the resultant distributions. The FWHM of the distribution is 7mm, larger than one would expect from the geometry discussed previously. There are also a number of events outside the main peak. ' -2-Figure 4 shows the time sum distribution for this experimental measurements. For example the observed data. When compared with the distribution for Fe55, time sum distribution indicates that there should be a it is seE.'n that the dis tri but ion is wider ( FWHf1=20ns) 5% probability of producing a doub 1 e event with the skewed to the low side and· there are more events on spacing between primary and secondary larger than 2cm. the high side of the peak than one would expect. We In the Monte Carlo program we have not taken into have determined that the events on the right side are . account the possibility that the secondary photon due to saturation of the linear electronics by could leave the converter and convert in the aluminum signals \·/hose amplitude out of the amplifier is lid of the vessel. Since the mean free path in gas is greater than 3V. Signals as s~all as 30 mV are long this could produce a secondary event far from the accepted so the dynamic range is high. One can primary, however, the probability of producing a effectively eliminate this problem by using upper detectable electron from the cover would be lower than level thresholds on the discriminators. These events from the converter. It is probably likely that even are shown in the plot as a demonstration of one after this is taken into account that there will be problem that is corre~ted by time sum information. some multiple events that can not be expiained. Our In order to make sure we are not affected by these confidence in the Monte Carlo code itself is bolstered large sums,events are rejected if the sum is larger by the fact that it predicts the detection efficiency than 168ns; this cut is indicated on Figure 3. Figure of the converter to within 20%. 5 shows the point response function after this cut has been applied: the FWHM is reduced to 6.5 mm. On the left hand side of the time sum peak there appear to be a significant number of cases where a multiple interaction has occured. Once away from the peak, the distribution is relatively flat until it reaches a ·sum of 150ns at which point it drops off. A sum of 150ns would be expected for the case where a photon eonverted at the center of the chamber · producing a secondary photon which converted very close to one edge of the chamber giving a sum approximately equal to half the length of the delay line. In Figures 6a and 6.b the point response function is plotted for events with a time sum between 141 and 168ns and 150 and 168ns respectively. These cuts should remove many of these multiple events. As can be seen the number of off-target events (outside the main peak) is reduced and the peak is sharpened. Table I summarizes the results giving the number of events contained within diff~rent intervals about the mean for various sum cuts. As can be seen improvement in the in the fraction of events contained within a given interval can be obtained with some reduction in the total number of events within the interval. A Monte Carlo calculation was performed to determine the probability of secondary interactions from photoelectric escape photons and Compton scattering and their contribution to the spatial resolution. The photon is assumed to enter the converter approximately perpendicular. The material in the converter is assumed to be distributed uniformly throughout its volume. The point at which the 511 keV photon converts is determined and an average detection probability is assigned. A conversion is considered detected if an electron escapes from the wall of the honeycomb into the gas. The average probability for this to occur for 511 keV y's was determined assuming a .lmm wall thickness. This probability is .57 for photoelectric conversion and .12 for Compton scattering. At 511 keV the cross section for photoelectric effect is 1.20 times . the Compton cross section. The Compton scattered photon or the 88 keV escape photon for Pb is followed until it converts or escapes from the converter. Figure 7a gives the probability of secondary interaction as a function of lateral distance from the primary conversion. There is a 64% chance of producing a secondary interaction. Figure 7b gives the probability of actually detecting the secondary interaction. Note that there is a 17% chance of detecting a secondary photon. But if we require that the primary interacti.on also be detected then the probability of having a 2 photon event drops to 4%. Almost all of the secondary events are predicted to occur within 1 em of the primary interaction. There are not enough double events to explain the CONCLUSIONS At this point the observed effect is not completely understood. Work on the problem will continue, both from the point of view of understanding the physics involved and actually implementing this time sum technique in the U.C.S.F. MWPC positron ·Camera. What is clear is that there are multiple interactions in the chamber which spoil the resolution, and that this method provides a means of correcting the problem. ACKNOWLEDGEMENTS This work is supported by the U.S. Department of Energy under the auspices of the Division of Physical Research. REFERENCES 1. J. E. Bateman, "The Improvement of the Spatial Resolution of a Xenon-Filled X-Ray Imaging MWPC by Means of Pulse Height Selection", NIM liQ_,·211-214, (1977). D. Chu, K.C. Tam,V. Perez-Mendez,C.B. Lim, 2. 3. 4. 5. 6. D. Lambert, S. Kaplan, "High Efficiency Collimator Converters for Neutral Particle Imaging with MWPC" IEEE Trans. Nucl . Sci. 23, 634 ( 1976). R. Grove, I. Ko, B. Leskovar,V. Perez-Mendez, "Phase Compensated Delay Lines for Wire Chamber Readout",NIM 99,381(1972}; R. Grove, V. Perez-Mendez, J. Sperinde, "Improved Delay Lines for Proportional Wire Chamber Readout'.' ,N IM .!2.§_, 407 (1973). V. Radeka, "Signals, Noise and Resolution in Position Sensitive Detectors", IEEE Trans: Nuc. Sci. NS-21, 51 (1973). N. Bhandari, V. Perez-Mendez,F. Kirsten, "Inexpensive Timing Discriminator with Sub-Nanosecond Slewing for Delay Line Readout of Proportional Chambers", Lawrence Berkeley Lab-oratory Report (1976) . V. Perez-Mendez, M. Greenstein, D. Ortendahl, "Evaluatlon of Factors Contributing to Posit·ion Accuracy in Delay Line Readout of MWPC", IEEE Trans. Nuc. Sci. NS-24, 209 (1977). -' Fig. 1. F;g. 2. Fig. 3. Fig. 4. Fig. 5. 0 0 '.I 0 CAPTIONS Schematic Diagram of a MWPC with honeycomb lead converters. The honeycomb converter is also shown. Time sum distribution for an Fe55 source. X-projection of point response function for the positron source. Time sum distribution for the positron source. The cut on the high side of the peak is shown. · X-projection of point response function for those events with time sums less than 168 ns. Fig. 6a. X-projection of point response function for those events with sums between 141 and 168 ns. Fig. 6b. X-projection of point response function for those events with sums between 150 and 168 ns. Fig. 7a. Probability of a secondary interaction as a function of distance from the primary interaction. The total fraction of photo-electric and Compton interactions is shown. Also given isthe probability of only a primary interaction. b. Probability of detecting a secondary inter-action. Double refers to the sum of the Compton and photoelectric cC•itributions when both secondary and primary interactions are detected. The scale for the double curve is on the right hand side. 0 -3--4-TABLE I. The number of events contained in intervals about the mean of x projection of the point response function is given for various time sum cuts. The fraction of those events passing the cu~ which fall within the interval is also given. Interval No Sum Cuts Sum<168ns 141 <Sum <168ns 150 <Sum 468ns 5mm 14003 12495 11481 8464 49% 50%~ 65% 72% 10 mm 21604 19105 15057 10655 75% 77% 86% 91% 15 mm 24867 21942 16472 11346 86% 88% 94% 97% p 0 . ; u -5-Converters Wire Planes \ . \ . , . . XBB 751-0782 Fig. 1 -6-I I I 1500 f.- -cniOOO - -c C\J .......... en I-z ::J 0 u 500 - -OL-----~~-------~~~~--~~~~~--~ 100 120 140 160 180 TIME SUM (ns) Fig. 2 3000 E2000 E ........ en 1-z ::::> 0 (.) 1000 .;; 0 2 -7-FWHM 7mm Fig. 3 6000 rn ~4000 ......... en ..... z ~ 0 (.) 2000 -8-I RT CUT 0 50 100 150 200 TIME SUM (ns) Fig. 4 0 0 3000 E2000 E .......... en 1--z :::> 0 (.) 1000 0 0 9 0 3 -9-SUM <168ns FWHM 6.5mm 10 20 30 40 x(mm) Fig. 5 50 60 3000 E 2000 E ......... (J) ....._ z :::> 0 u 1000 0 0 -10-.. 141 <SUM <168ns FWHM 5mm 10 20 30 40 50 60 x(mm) Fig. 6a 0 0 E2000 E .......... Cf) r-z ::::> 0 01000 0 0 -11-150 <SUM <168ns FWHM 4mm 10 20 30 40 50 60 x(mm) Fig. 6b -12-% ~~--~----~---r----~---.----,-~-, ~ HONEYCOMB LEAD CONVERTER(2.5 •• cell) 100a• wall 40 r I NlERACTION I ll ~1-- SINGLE 7 INTERACTION 20 . I SECONDARY 7 INTERACTION /PHOTOELECTRIC 144 %) --COMPTON 120 %) 500 if-L----t---·--t---+--1 7 INTERACTION INCLUDING ELECTRON EMISSION 40 SINGLE 7 INTERACTION 30 / SECONDA~Y 7 INTERACTION 20 DOUBLE 14 %) I COMPTON 113%) PHOTOELECTRIC 14% J 10 , , /TOTAL SECOIDAIY (11%) b I% j ~10 I I i I ,7.5 5.0 2.5 .~~~~~~~~r--n--~ Fig. 7 u ...; J ') ,j ..) This report was done with support from the Department of Energy. Any conclusions or opinions expressed in this report represent solely those of the author(s) and not necessarily those of The Regents of the University of California, the Lawrence Berkeley Laboratory or the Department of Energy. . . ~ TECHNICAL INFORMATION DEPARTMENT LAWRENCE BERKELEY LAB ORA TORY UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA 94720 

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ELIMINATION OF IMAGE BLURRING DUE TO DOUBLE SCATTER EVENTS IN Y IMAGING MWPC DETECTORS

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ELIMINATION OF IMAGE BLURRING DUE TO DOUBLE SCATTER EVENTS IN Y IMAGING MWPC DETECTORS

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