In order to exploit the high event rates at ISABELLE, it will be necessary to have fast detection with fine spatial resolution. The authors are currently constructing a prototype fine-grained hodoscope, the elements of which are scintillating optical fibers. The fibers have been drawn from commercially available plastic scintillator which has been clad with a thin layer of silicone. So far it has been demonstrated with one mm diameter fibers, that with a photodetector at each end, the fibers are more than 99% efficient for lengths of about 60 cm. The readout will be accomplished either with small diameter photomultiplier tubes or avalanche photodiodes used either in the linear or Geiger mode. The program of fiber development and evaluation is described. The status of the APD as a readout element is discussed, and an optical encoding readout scheme is described for events of low multiplicity

Borenstein, S. R.

Strand, R. C.

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tf <*3KL 29922OG 598FIXE-GRAINED HODOSCOPES BASED ON SCINTILLATING OPTICAL FIBERSS.S. 3orenstein, York College, CUNYNew York, NYR.C. Strand, BNLL'pcon, NYMASTERTalk, presented at theCESRII WorkshopCornell Universitv, Ithaca, NYJune 13, i9SlThe submitted manuscript has been authored under contract 2E-ACO2-:-CKOOOI5with the U.S. (Department of Energy. Accordingly, the U.S. Governmentretains a nonexclusive, royalty-free license to oublish or reproduce thepublished form of this contribution, or allow others to do so, for U.S.Government purposes.STWtnioJi'of THIS K M H T IS mmwiOG 598FINE GRAINED HODOSCOPES BASED ON SCINTILLATING OPTICAL FIBERSS.R. Borenstein, York College, CUNYandR.C. Strand, BNLAbstractIn order to exploit the high event rates at ISABELLE, it will benecessary to have fast detection with fine spatial resolution. The authorsare currently constructing a prototype fine-grained hodoscope, the elements ofwhich are scintillating optical fibers. The fibers have been drawn fromcommercially available plastic scintillator which has been clad with a thinlayer of silicone. So far it has been demonstrated with one ran diameterfibers, that with a photodetector at each end, the fibers are more than 99%efficient for lengths of about 60 en. The readout will be accomplished eitherwith small diameter photomultiplier tubes or avalanche photodiodes used eitherin the linear or Geiger mode. The program of fiber development and evaluationwill be described. The status of the APD as a readout element will bediscussed. Finally, an optical encoding readout scheme will be described forevents of low multiplicity.IntroductionDuring the 1978 summer study'I) it was decided that a research anddevelopment program should be undertaken to develop a fine grained scintil-lating hodoscope to cope with the high rates expected. Early attempts^)to produce long bare scintillating filaments of one nun diameter yieldedmaximum useful lengths of about IS cm, presumably due to cumulative losses atthe scintillator to air interface.(3) j n order to increase the usefullength of the scintillating fiber, the experience and expertise of the fiberoptics industry was used to draw a fiber from a heated preform of polished PVTscintillator, and to coat this with a cladding of lower refractive index. Theprinciple is illustrated in Figure 1 where those rays of light making anangle, relative to the fiber axis, of less than the numerical aperture angleof the fiber are trapped and guided to the end of the fiber where they aredetected by a photodetector. A desirable candidate for the photodetector isthe avalanche photodiode (APD) which has the virtue of small size and immunityfrom magnetic fields. It has a phouon to electron quantum efficiency about 4times greater than that of a photomultiplier tube; however, the probabilitythat the photoelectron will propagate to a usable signal is correspondinglylower. Thus the sensitivity of the APD is comparable to that of the PMT.I. HODOSCOPE DESCRIPTIONA section of a hodoscope constructed from such scintillating fibers isshown schematically as a bilayer of fibers imbedded in a protective isolatingmatrix* The centers of the fibers are offset by one radius from one layer tothe next to assure that there are no cracks through which a particle mightpass without penetrating a reasonable depth of scintillator.II. FIBER EVALUATION PROCEDUREExperimental SetupIn order to establish the feasibility of such a hodoscope, BrookhavenNational Laboratory entered into a contract with Galileo Electro-OpticalCorporation™' to undertake an experimental program to draw and cladscintillating fibers. Various commercially available scintillators, severalcladding materials, curing methods, temperatures, tensions, and draw speedswere tried. The scintillators NE102, NE110 and NE161, were obtained fromNuclear Enterprises. The cladding materials have been a proprietary ultra-violet cured acrylic and a heat cured silicone. The best results to date havebeen obtained with silicone coated NE161. The layout of the test facil-ity^) appears in Figure 2. An electron from Ruthenium 106 penetratestwo small telescope counters Tj and T2 which sandwhich the fiber. Upon acoincidence from T1T2, the signal from SI is detected by an RCA 8850photomultiplier with single photoelectron resolution. From the resultantspectrum in Figure 2 we can calculate the mean number of photoelectrons (4 inthe example shown).A crucial feature of the test equipment is the use of a PMT with singlephotoelectron resolution. In this way all light yield and attenuation dar.aare obtained in terms of absolute number of photoelectrons. The distance frumthe pedestal to the first peak sets the scale in absolute photoelectronsregardless of the gain of the PUT and the associated electronics. Inprinciple, the light yield for a given spectrum is obtained by determining themean of the distribution. In practice, for low light levels it is foundempirically that the light yield is more easily determined by measuring theratio of the heights of the second, third and fourth peaks. For higher lightlevels the mean ts almost the same as the peak of the distribution.Test ResultsThe highlights of the results obtained so far are shown in the curves ofFigure 3 and in the accompanying table where each fiber is characterized by anattenuation length and an effective photoelectron yield at the origin. Thedata were obtained from fibers whose diameter varied from .032" to .090" butthe results have all been scaled to a diameter of .054" or 1.37 mm. Theprogression of curves 1 through 3 show a steady improvement due in part tochoice of material, and in part to gradually improved technique. Curve No. 4underscores the drastic attenuation suffered at the scintillator to airsurface. By using coincidence techniques and setting discriminator thresholdsbelow the single photoelectron level it is possible to construct hodoscopeelements from fibers of about 1 mm diameter in lengths approaching 1 meter.III. READOUT OPTIONSIndividual Fiber ReadoutA useful hodoscope will ultimately consist of hundreds of such scintil-lating fibers a-.d we must now address ourselves to the problem posed by thereadout of so many channels.Two types of photodetectors have been considered for the readoutelements. PMT's and APD's. In the former case, the necessary technology isat hand and the solution consists of devising a scheme to couple the manyfibers individually to small diameter, highly sensitive phototnultipliers.The disadvantages of this solution are:1. The bulkiness of FMT's relative to the fiber diameter.2. The cost of the PMT's which probably will not come down in the nearfuture.3. The inability of PMT's to function in magnetic fields.In the case of the APD, the size of the device is well matched to the size ofthe fiber, and the device itelf is unaffected by magnetic fields. The cost iscomparable to that of a PMT, but as seems to be the case with all silicon, onecan anticipate drastic cost reduction as these devices start to be produced inmuch larger quantities.The main disadvantage of this solution is that one is dealing with a newtechnology, and there is no experience to indicate that the device can besuccessfully operated in the Geiger or saturated mode. In theory, with theAPD cooled to about -70°C and biased above the breakdown voltage, a photo-electron has a small (on the order of 20%) probability of causing a breakdownsignal. This propbability is an increasing function of the overvoltage, butthe signal size is independent of ';he number of photoelectrons that contri-buted to it so that the signal contains no analog information, and it occurswith a probability which depends upon the overvoltage as well as the number ofphotoelectrons. On the other hand, there is a dynamic dark count proportionalto the true counting rate, which also increases with overvoltage. Thus, thereis an optimum overvoltage obtained by trading off required photoelectron inputversus acceptable dark count as shown in Figure 4 which shows required photo-electron input versus overvoltage for a detection efficiency of 99.9%. Theother curve shows the total to input counting ratio as a function of theovervoltage, thus for an acceptable dynamic dark count of 20% the requiredinput is 22 photoelectrons per pulse.Referring back to Figure 3, for fibers of about 1 mm diameter and 50 cmlength, the number of photoelectrons expected with perfect optical couplingbetween fiber and APD is about 30. (8 from Figure 3 multiplied by 4 for thehigher quantum efficiency of the APD). Unfortunately the coupling betweenfiber and APD is far from perfect, and a means of packaging the APD siliconchip to optimize the light coupling is yet to be developed.Optically Encoded ReadoutUnder most circumstances, the high event rates and large multiplicitiesfor which the fiber hodoscope is being developed would preclude any multi-plexing schemes to reduce the number of readout channels. However forspecific applications of low multiplicity it might be possible to effectconsiderable economies in readout electronics by the use of optical encoding.Let us consider one layer of the hodoscope which contains n elements. Atthe upper end, the fibers are grouped in n bundles, each consisting of /nneighboring fibers* At the lower end the fibers are also grouped in /nbundles, but the groupings are such that the first bundle consists of thefirst fiber of each bundle at the upper end, the second bundle consists of thesecond fiber of each bundle at the upper end and so on. Thus a single fibercould be specified by a signal from one bundle at each end of the hodoscope.The number of readout channels has now been reduced from n to 2 /n. Theproblem with this scheme is that if more than one fiber is hit, the encodingis ambiguous. The level of anbiguity can be drastically reduced, by makinguse of the second layer of the hodoscope and arranging the bundles ingroupings which are distinct from those already used. In practice this schemeis of interest when the number of elements is very large; for example 900elements can be read out with just 60 photodetectors. However, for purposesof illustration we will describe a 2 x 25 element bilayer, as shown in Figure5.In this example, the simplest grouping is the ALPHA grouping, A,B,C,D,Eeach containing 5 successive fibers, at the lower end of the front layer. Atthe upper end of the front layer the first of each group of 5 goes to number 1of the numerically grouped photodetectors. So far, a hit in the 9th fiberwould give the signal B4. The rear layer is grouped as indicated by Greekletters at the upper end, and by Hebrew letters at the lower end. The fourgroupings can now be referred to as ALPHA, NUMERIC, GREEK and HEBREW. Thesame hit would now be characterized as B4ss.In the examples shown, 3 hits in fibers, 2,9,20 would give rise to a6-fold ambiguity if only one layer were available. The reso.lution of thisamgiguity is illustrated by listing the expected signals for each ambiguoushypothesis as well as the number of absent signals and the number of extrasignals for each hypothesis.It is worth noting that with ALPHA and NUMERIC signals, the ambiguity is6-fold, the addition of GREEK eliminates 4 of the ambiguities, and the finalambiguity is resolved only with the inclusion of the HEBREW group.We also note that the ALPHA group can be used by itself as a hodoscopewhich is 5 times (/n) as coarse as the physical hodoscope.ConclusionSo far we have demonstrated the feasibility of the scintillating fiber asa hodoscope element. To date the best results have been obtained with NE161as the scintillating core and silicone as the cladding material. Thiscombination is rather fragile and subject to deterioration with handling. Tothis end we are considering the application of a buffer coat, to protect thesilicone cladding.In addition we are studying several alternate cladding materials.With regard to the optical readout, the preferred detector is the APD,and we are actively pursuing methods of both increasing the intrinsicsensitivity of the device, and improving the optical coupling between thefiber and the solid state chip.This research was supported by the U.S. Department of Energy undercontracts No. DE-AC02-76CH00016 and DE-AC02-81ER40035.References1. J. Marx and S. Ozaki, Topic I: Detectors and Experiments, Summary ofActivity, Proc. 1977 ISABELLE Summer Workshop, BNL 50721, p. 4.2. G.T. Reynolds and P.E. Condon, Rev. Sci. Instr. JZ8_, No. 12, Dec. 1957;G.T. Reynolds, Nucleonics J_6, No. 6, June 1958.3. L. Reiffel and N.S. Kapany, Rev. Sci. Instr. 3y 1136-1142 (1960).4. BNL Contract /M63787-S with Galileo Electro-Optical Company.5. S.R. Borenstein, R.B. Palmer and R.C. Strand, Optical Fibers and Ava-lanche Photodiodes for Scintillator Counters, presented at the Inter-national Conference on Experimentation at LEP, Uppsala, Sweden, 16-20June, 1980: Physica Scripta, 23, No. 4:1, pp. 550-555 (April 1981) Ed:Tord Ekelof, Vol. 1-Invited Papers; Pub: Alden Press, Great Britain.FIGURE CAPTIONSFigure la. Scildilating optical fiber, principle of operation;b. Bilayer of fibers, encapsulated in isolating matrix.Figure 2a. Fiber measurement setup: electron from source penetratesscintillators Tj and T2 as well as the fiber whose outputsignal SI is analyzed;b. Charge spectrum of fiber pulse using RCA 8850.Figure 3. Attenuation curves for various fibers.Figure 4. APD operation as a function of overvoltage.Figure 5. Optical encoding and ambiguity resolution.A schematic rendition of a 2 x 25 element bilayer ribbon. Forease of illustration, the fibers are drawn with a square crosssection and the half-diameter shift from one layer to the next isnot shown.PARTICLETRACKESCAPED^CAPTURED.RAYTO PHOTONDETECTOR/ SCINTILLATING FIBER2.1 mm MATRIX2mm SCINTILLATING FIBERHODOSCOPE BI LAYERCHARGE SPECTRA OF SCINTILLATING FIBERUSING RCA 8850 PHOTOMULTI PLIER106RuT,<n> ~ 4SCINTILLATING FIBERSFIBER COMPARISON SUMMARYALL RESULTS NORMALIZED TO cf> = 0.054"<n>\!(cm)FIBER ACTUAL <£1I) NE 110, UV-CLAD 0.054(2) NE 110, Si-CLAD 0.032(3)NEI6l Si-CLAD 0.090(4JNEI6I UNCLAD 0.03216.310.013.56.8«085872006 5<n> AT1=603.15.110.0<3IQ.Acv100Kcm)bJ_JLJeoXGL706050302010nI I 10.8mm APD OPERATED ININPUT PHOTOELECTRONSFOR 99.9% DETECTION- 1 EFFICIENCY— IA/• V^ 1 1 1 11 1SATURATED1MODEOUTPUT PULSESPER INPUT PULSE//1-—-——- 2 . 4- 2 . 2- 2 . 0- 1.8UJCO10 20 30 40 50OVER-VOLTAGE (V)- 1.4- 1.2.01111111i1y?~ "r V 7Elenents hit : 2, 9, 20 - A2B4DSSignal - ABD245BEY & ftHyp2, 9, 202, 10. 194, 7, 20* . 10. 175, 7, 195. 9, 17KequlxedA2B4DSA2B5D4A6B2D5A4BSD2A5B2D4A5MD2SignalMr t 56 Yi a e0 tCt?Absent022A22Extra033323Figure 5. Optical Encoding and AmbiguityResolution.

Fine-Grained Hodoscopes Based on Scintillating Optical Fibers

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Fine-Grained Hodoscopes Based on Scintillating Optical Fibers

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