Linder, W.

Loesel, M.E.

Schatz, G.

Schweickert, H.

Loesel, M. E.

KITopen

April 1969Zyk1otron- LaboratoriumKFK 1009Intensity Fluctuations of Cyclotron Microstructure Beam Pulsesw. Linder. M. E. Lösei. G. Schatz. H. SchweickertB. H.NUCLEAR INSTRUMENTS AND METHODS 70 (1969) 151-156; © NORTH-HOLLAND PUBLISHING co.INTENSITY FLUCTUATIONS OF CYCLOTRON MICROSTRUCTURE BEAM PULSESW. LINDER, M. E. LÖSEL, G. SCHATZ and H. SCHWEICKERTZyklotron-Laboratorium, Kernjorschungszentrum KarlsruheReceived 26 November 1968Measurements of the intensity distribution of microstructure beam pulses from an isochronous cycIotron are described. Typical in-tensity distributions resuIt in an increase ofthe chance coincidence rate ofcoincidence experiments by a factor of 2 over the ideal caseof equally sized bursts. Among other factors stray frequencies in the rf may contribute to these intensity fluctuations.151For a normalized intensity distribution h(l) thechance coincidences are given by(5)(4)(6)h(l) = (j(I-Io),N chO = clczI~.1 = J~ h(l)IdI,If all pulSes have the same intensity 10 ,This corresponds to the well known expression ofchance coincidences far a beam without structure inwhich the partieles are spaced randomly in time.For a given mean currentchance coincidences attain a minimum when all pulseshave the same intensity, i.e. when h(I)= (j(I - I). Thisresults in a minimum of machine time required for anexperiment if the mean current is limited by the rate ofaccidental coincidences.For measuring the intensity of single microstructurebursts fast targets for current measurement are re-quired (time constants of the order of a few nsec).Several such targets have been built. Two targets whichmay be used for measurements on the internal beam atdifferent current levels have been described in detail ina previous publicationZ). The Faraday cups normallyused for measurements on the external beam were notsuited for this application as they have a capacity of afew nF with respect to ground. Therefore a specialbeam stop was developed which is shown in fig. 1. Thecapacity is about 20 pF, and with a 50 Q terminator atime constant of 1 nsec is obtained. It should be men-tioned, that there is no secondary electron shield aroundthis probe. This results in an increase of 20% of themeasured current as compared with the incident par-tiele current.3. Measurement of the intensity distribution3.1. TARGETS(3)(1)nN ch = CI·CZ L I~.k= I1. IntroductionIn all experiments involving the measurement oftimecoincident events a knowledge of the rate of accidentIlleVents ls fequiieo. Th6lnherent bunchingof acyclotronbeam by the accelerating rf complicates coincidencemeasurements with resolving times in the range of theburst length. If the assumption is made that all singlemicrostructure beam pulses have the same shape intime and comprise the same number ofparticles therateofchance coincidences can be calculatedI). In the Karls-ruhe Isochronous Cyclotron, however, we observed anappreciable variation of intensity in the single beampulses. The purpose of this paper is to describe mea-suring methods of the intensity spectrum and to esti-mate their influence on chance coincidences and thetime required for an experiment.2. Accidental coincidences in the case of intensityfluctuations of microstructure beam pulsesLet I k be the intensity of the klh pulse arriving at thetarget, and let the coincidence resolving time be largerthan the pulse length and smaller than the pulse distance.The mean number of counts in each of two detectorsdue to a pulse of intensity I k wilI beNlk = cllk ,N Zk = czlk>where Ci and Cz are constants involving parameters ofthe detectors and of the nuclear experiment to bestudied. Then the mean number of chance coincidencesNch can be expressed in the formN chk = Cl czIf. (2)The total number of chance coincidence for n cyclo-tron pulses is152C beamstopmyeroy insulatorCu flange'=iJ;j~~~~~~i~~~~~~w--=a=tereoolingbeam-Al flange joint V'7r--"---''-''---'---''----'-T-'----'-----'---'-'--'''----'-~'-''---'---''-''----'---''--'---'''-''----'---''----''-----''-tyJ'Jto beamtransportsystem10cmFig. 1. A schematicdrawingoJ' the Faradaycup-with a timeconstant oJ' I-nsec.-3.2. ELECTRONIC SYSTEMS AND RESULTSThe fast pulses from the targets occur at a rate of33 MHz. Fig. 2 shows the electronic system used toprocess these pulses with a conventional pulse heightanalyser. First of all the repetition rate of the targetpulses is decreased to 3.3 kHz using a frequency dividerand a fast linear gate. The output of the linear gate isfed through a pulse stretcher to a multichannel pulseheight analyser.In some measurements the signal to noise ratio wasimproved without increasing the thermal load of thetarget by pulsing the ion source. It is then necessary tointroduce an additional gating condition to avoid pro-cessing pulses during the rise and decay times of thecurrent pulse. This is achieved by the circuit shown inthe lower part of fig. 2. This part of the circuit is notused when the ion source is operated in steady statecondition.Measurements were performed at different currentlevels for deuterons and alpha partic1es on the internaland external beams. In all measurements the width ofthe intensity distribution was comparable to the meanpulsesfromfast targets33MHzresonareefit ter.1 sealer500}J sec-.,~II_______________..--J--- --- --- ---- ---_._-_.33MHzfromloop in r.f.fieldFig. 2. Block diagram of electronics for analysing single beam pulses.INTENSITY FL UCTUATIONS 153Fig. 3. Typical distribution of the single microstructure beampulses obtained with the fast Faraday cup and a-particles at apulse current of 30 pA.due to the observed intensity distribution by means ofthe formulae derived in section 2 the measured distribu-tion was approximated by an analytical expression. Thebest fits were obtained by curves which represent atruncated exponential folded with a Gaussian distribu-tionh(I) = {a(2n)tr 1f: 00 (Nil e I)exp {(x-I1)le}··exp[ --t(I -x)2ja2Jdx, I ~.I1'h(l) = {(J(2n)tr 1foo (Nllel)exp{(x-I1)le}'I,·exp[ --t(I -x)2Ia2Jdx, I"?;, 11 •(This type of function",as tri~4 a.s al~a.st-§qJUlresfittingprögram existea. afthis laboratory for the analysis ofGe(Li) y ray spectra). In fig. 4 the measured and fitteddistributions are shown. At the same mean intensity thechance coincidence rate is higher by a factor of 2.04 forthe measured intensity distribution as compared with a(j-function distribution.value. A typical intensity distribution is shown in fig. 3.It was obtained with the external alpha particle beamwith the ion source pulsed at a pulse current of 30 JlA.3.3. INCREASE OF CHANCE COINCIDENCE RATE DUE TOTHE OBSERVED INTENSITY DISTRIBUTIONIn order to calculate the rate of chance coincidences4. Measurements on thetime correlation of the intensityTime correlations of the intensity fiuctuations are im-portant if chance coincidences are measured by delayingone of the pulses by an integral number of rf periods.As will be shown they can also give information on theprocesses which cause the intensity fluctuations.fitted curvechannel350o experimentül püintso100 150 200 250 300Fig. 4. Measured and fitted pulse height distributions.countsjchanne[65006000550050004500400035003000250020001500100050050154 W. LINDER et al.pulses tromtas I Iargel s33MHz tromloop in rttieldde layFig. 5. Block diagram of electronics for time correlation measurements.4.2. MEASUREMENT OF RADIOFREQUENCY DISTRIBUTIONIN THE CENTRAL REGIONWhile the 1.1 MHz modulation of the beam intensitycan be attributed to plasma oscillations in the ion________ , .. __ .. _·__ c . . . ._ .. .._. .._ . __. .'._.. .. __ __. .. _Fig. 5 represents theblock diagram of the electronicsused to measure the distribution of time distances. Thethreshold is~defined by:a:fas(discriminator. The time toamplitude converter has the feature to be internallygated by the start input circuit to accept only stoppulses following a valid start signal. The start pulse isdelayed by a fraction of a rf period such that one pulsecan only trigger either the start or. the· stop input. Inorder to reduce the rate of start signals tue start pulsesare gated at a suitable pulse rate produced by a fre-quency divider from the cyc1otton rf.Some results of these measurements are shown infigs. 6 and 7. As expected the output spectrum ofthe timeto amplitude converter consists of equally spaced lineswith a distance corresponding to 30 nsec. The results offig. 6 were obtained with the plastic target2) on the in-ternal beam at a current of 1 nA. The straight line in thelower part of fig. 6 represents the decrease of themaxima expected from eq. (7) using the independentlymeasured value of P. The excellent agreement of theslopes indicates that the intensities of different micro=structure bursts are uncorrelated.An entirely different situation is displayed by themeasurements of fig. 7. Here the fast high currenttarget2) \vas used at an internal deuteron current of10 JiA. The spectrum shows periodic intensity fluctua-tions at approximately 11 and 1.1 MHz. Similar timecorrelations habe been measured at quite different levelsof current. It was unfortunately not possible to corre1atethe presence or absence of these correlations with theoperating conditions of the ion source or other compo-nents of the machine.nnel numberthreshold 600 mV~ P=0,036r::;:: threshold 100 mVP=O,454-............~~~ehaI eounts/channelWn =P(l-Pt-\ (n~1). (7)Any time correlation, as rnight be caused, e.g., by aperiodic change of the intensity, would result in adeviation from this dependence.4.L TIME INTERVALS BETWEEN SUCCESSlVE BEAMPULSESABOVE A GIVEN THRESHOLDOne method of investigating time correlations of theintensity of single pulses is to measure the distance intime of pulses which are large enough to overcome acertain threshold. This distance is, of course, always aninteger multiple of the rf period '"C. Let P be the fractionof all pulses which exceed the threshold. If the intensi-ties of different rfbursts are uncorrelated the probabili-ty of measuring a distance of n- '"C is given by10°o 50 100 150 200 250Fig. 6. Spectrum of time intervals between two successive pulsesabove a given threshold. The data were taken at a mean deuter-on current of I nA.INTENSITY FLUCTUATIONS 155counts I channe!120100I80604020I P= 0,3093084075066057048039030021012030 t[nsec]Fig. 7. Spectrum of time intervals between two successive pulses above a given threshold. The data were taken at a mean deuteroncurrent of 10 pA.source the observed frequency of 11 MHz appears to betoo high for such oscillations. On the other hand, in-tensity modulations at this frequency were observedfrequently. As a possib1e explanation it was proposedthat tbis might be due to a 11 MHz component super-imposed on the accelerating 33 MHz rf. As the first oneand a half orbits of the Karlsruhe Isochronous Cyclo-tron are defined by a large number of slits3) the inten-sity of the beam depends very sensitivelyon the rfamplitude. This might result in an intensity modulationof the observed structure if a 11 MHz component werepresent in the rf field. To test this conjecture the radio-frequency distribution near machine center was mea-sured. In order to avoid any change ofthe centre geom-etrya dummy ion source was built with a small pick upe1ectrode at the position of the centre of the ion sourceextraction slit. Fig. 8 shows a photograph of this dum-my source. The voltage from the pick up electrode wasanalysed using a selective microvoltmeter (Rhode undSchwarz type USVH-BN-1521) in the frequency rangefrom 10 kHz to 10 MHz and a spectrum ana1yser(Tektronix type 1 L 20) in the frequency range from10 MHz to 200 MHz. In order to avoid sparking it wasnecessary to reduce the rf amplitude by a factor of ap-proximate1y 2.5 with respect to the va1ue in normalmachine operation. The results of these measurementsare shown in fig. 9. Besides the basic frequency of33 MHz and its harmonics at least 9 different fre-quencies can be detected, including a small contributionat 11 MHz. It should be pointed out, though, that thecapacitive type of coup1ing employed tends to enhancethe higher freqencies. It is known that the rf system ofthe cyclotron has a second characteristic frequency at38 MHz which might also explain the presence of the71 MHz oscillation. The origin ofthe other frequencies. RF powero [db] 33-20-40-60385066I100I I150I RF frequency200 [MHz]Fig. 8. Dummy ion source with pick up electrode. Fig. 9. Measured harmonics in the central region.156 w. LIND ER et al.(excepting the harmonics) is unknown. In spite of thelow level of the 11 MHz rf field present it is proposed asa tentative explanation of the intensity modulation ob-served at this frequency. Of course, the infIuence ofthese stray frequencies upon the acceleration processdoes not only depend on the amplitude, but also on thephase with respect to the main frequency which wasnot measured.5. DiscussionThe measurements reported in this paper have shownthat the intensity of single microstructure bursts mayfIuctuate considerably more than the average currentfrom a.cyclotron. These fIuctuations may have an ap-preciable infIuence on chance coincidences in coinci--dence- experiments~ Amongthecausesknowntobe pres~ent are jnstabilities of the ion source power suppliesand oscillations in the ion source plasma. An additionalfactor is given by stray frequencies in the accelerating rf.The latter which may be a special feature of the Karls-ruhe Isochronous Cyclotron due to its special centredesign results in time correlations in the intensity fIuc"tuations. These correlations become important ifchancecoincidences are measured simultaneously with realcoincidences by delaying one of the signals by a fixednumber of rf periods.We are indebted to H. Müller for his assistance insetting up and preparing the measurements.Thanks are also due to the operating staff under F.Schulz for their efficient cooperation.References1) A. Z. Hrynkiewicz, S. Kopta, S. Szymczyk and P. Wolszak,Nuct-In:str:an:d Meth: <19 (1967) 229:2) D. Hartwig, W. Linder, M. E. Lösei, G. Schatz and H.Schweickert, KFK 754 (1968).3) M. Reiser, Nucl. Instr. and Meth. 13 (1961) 55.

Intensity fluctuations of cyclotron microstructure beam pulses

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Intensity fluctuations of cyclotron microstructure beam pulses

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