Tu; ?F/75 a Zerctctme Detector for Nuclear
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TU; ?F/75 A ZERCTCTME DETECTOR FOR NUCLEAR FRAO1ENTS US I! IG CHANNEL FLECTRnN MULTIPLIER PLATES Bo Sundqvist Tanden Accelerator Laboratory, Uppsala, Sweden •Hr '.vACT: . *-. literature on zerotime detectors which use the emission of ". ocndary electrons from a thin foil is reviewed. The construction • a zerotiine detector using multiplication of the secondary •,'2Ctrons with two Mallard channel electron multiplier-plates (CEMP) •.' tandem is described. Results of tests of such a detector with a particles from a natural a source are given. Tstal time resolutions of about 200 ps (FVHM) with a Si(Sb) detector as the stop detector has been achieved. The contribution from the zerotiine detector is estimated to be less than 150 ps (FVJW). The application of this detector technique to the construction of a heavy-ion spectrometer o and a Be detector is discussed. This work was supported by the 9wedish Atomic Research Council CONTENTS 1 INTRODUCTION 3 2 A REVIEW OF THE LITERATURE ON ZEROTIME DETECTORS USING 5 THIN ELECTRON EMITTING FOILS 3 THE CHANNEL ELECTRON HiLTTPLTER PLATE (CEHP), AND ITS 8 USE IN ZEROTIME DETECTORS i* THE CONSTRUCTION OF A ZEROTIME DETECTOR USING CEMP 10 MULTIPLICATION OF SECONDARY EMITTED ELECTRONS FROM A THEN FOIL 5 TESTS OF THE SEEZT-DETECTOF. AND DISCUSSION OF RESULTS lu 6 FUTURE APPLICATIONS OF THIS DETECTOR TECHNIQUE 17 ACKNOWLEDGEMENTS 19 REFERENCES 20 TABLE CAPTIONS 22 TÄBIZS 23 FIGURE CAPTIONS 25 FIGURES 25 1 INTRODUCTION The interest has recently been growing rapidly in the corrlex nuclear reactions in which the ingoing particles induce neny different reactions. To be able to study a particular reaction the fragments produced must be identified, i e the ness (M) the nuclear charge (Z) and the energy (E) of the different fragRjents have to be determined. For light particles (A<6) the AE-E telescope method is often very powerful for identification purposes. Different algorithms are used to give a function proportional to MZ . However, for heavier ions an MZ determination alone does not uniquely identify the fragment as already Be and B isotopes overlap. Additional information is needed in such cases and a time-of-flight measurement gives the nass of the particle if the energy is known. The most natural extension of the telescope technique is to separate the AE and E detectors and irtcsure the time-of-flight between them, but in nany cases difficulties arise. For example, multiple scattering of the heavily ionizing particle in the AE detector in combination with the long flight distance decreases the efficiency of the detector system. Another problem is that for a long time practical considerations like making a thin and homogeneous enough Si-AE-detector have made it difficult to apply the telescope technique to heavily ionizing particles, such as low- energy light ions and heavy ions in general. Therefore considerable effort has been directed towards developing thin (less than a few 2 tens of ug/cm ) start (zerotime) detectors for tijne-of-flight measurements of heavy ions. Besides thin Si-detectors, thin scintillator foils and electron emitting foils have been studied and used as zerotime detectors. The first two kinds of detectors are discussed by Goulding and Harvey and the literature on zerotime detectors usiiig secondary electron emission in thin foils is reviewed in some detail in section 2 of this report. The zerotime detector c-vi, of course, be used in combination with a AE-E telescope placed at a certain distance to pet enou#* mass resolution. Recently, it has turned out that in the case of he^vy ions the most powerful AE-E detector telescope is ar. ionizatior. 2 3) chamber as a AE detector ' with an extremely thin entrance window and a silicon surface barrier E detector inside. The gas pressure is changed from experiment to experiment tc eet a AE detector with suitable thickness. With such a heavy-ion spectrometer ions with A<40 have been uniquely identified . However, a drawback with the use of an ionization chamber as the AE detector is its poor timing properties. Consequently, large flight distances have to be used. The use of two "zero"time detectors of xiu: kind described in this report in combination with a AE-E telescope of the type mentioned would be a powerful tool to study low ererey heavy ion reactions. This work is part of a larger project which aims at the construction of a heavy-ion spectrometer of the proposed type. In the third section of this report the features and characteristics of channel electron multiplier plates used for the multiplication of secondary electrons will be described. In the fourth section the construction of a zerotime detector using the secondary electrons produced when a heavy ion passes a thin foil will be discussed. In the following such a detector will be referred to as a SELZT detector (Secondary Electron Emission Zero Time). The fifth section deals with the tests of the detector and finally in the last section applications of this detector technique will be discussed. 2 A REVIEW OF THE LITERATURE ON ZEROTTME DETECTCD" USI>JC THTN ELECTRON EMOTING FOILS For A general discussion of identification problems in nuclear reactions the reader is referred to review articles by Goulding and Harvey and England and references therein. Here the production and detection of secondary electrons produced when a charged particle passes a thin foil and the use jf this technique for zerotime detectors will be discussed. The principles were worked out already in 1957 by Stein and Leachman and the idea has later been used by several people to investigate fission fragments 6>7>8»9). in recent years, the Marburg group lo«n>12»lj> (Schneider, Kbhlraeyer, Pfeffer, PQnlhofer, Kbde and Back) and also Dietz et al 1U>, Clerc et al 15'16), Gaber 17), Berndt 18) and Ändrade et al 19) have applied this technique to nuclear reaction product analysis. Vhen a charged particle passes a thin foil of number of low energy electrons are produced. Tijese electrons can be detected and the signal used as the start signal in a time-of-flight measurement. V M Agramovich et al 20) have measured the energy distribution of these secondary electrons induced in an Al-foil by 5.5 MeV a particles and fission fragments. The mean energy in the two cases was measured to be 1.8 eV and 0.3 eV, respectively. Measurements by 21) Seiler have shown that for carbon foils electrons of such 2 energi.es ccoe from the cuter 100 A of the foil, i e 2.5 ug/an of carbon. Schneider et al and Clerc et al have shown that 2 for lovf-energy a particles on carbon foil (10-20 pg/cm ) on the average 5-10 electrons are emitted and for fission fragments the corresponding figures are 150-20C. Due to the short range of the secondary electrons it is of no use to make the carbon foil thicker 2 than abxit 10-20 yg/cm . So far carbon foils have turned out to be best suited in this »rclicition but S~ "- ~~J '—' 1iO Kit al have shown th-it higher yields ran >_> achieved with Li C ind MgO (in the case of ^,fO up to a factor 8). Also the orient it inn of the foil relative the dirvcti-Ti of the hor.y icn is of ~_mportance 1°). Ir. principle, xt is best to rave tht ncrnf.l to the foil forming an angle of finest 9T to th^ bearr. direction. However, the knowled;*? about these problems is rather poor and more work is needed. As can be seen from the figure of seeemdary electron yield for a particles the detector is of limited use for light ions. The efficiency of the detector will be very low and the timing; properties degrade with smaller yield of secondary electrons. Therefore, it is of great interest to try to find new ^eonetries and fcil rraterials to inprove the yield of secondary electrons. An experrnent is being started at this laboratory to try to increase the induced charged-particle yield of secondary electrons from foils by investigating different materials and preparation techniques. So far the discussion has concerned eV electrons. Besides these lnw energy electrons also a nurrtoer (but considerably smaller) of 5-electrons (knock on electrons) in the keV rango are produced when a charged particle pseses a thin foil. These electrons should if possible be avoided in the detection of secondary electrons as they will give wrong time signals. Several different techniques have been used to detect the secondary 5 6 7 8 9) electrons. In the first experiments ' » ' ' the electrons were preaccelerated tc some 10-20 keV and detected with scintillation counters. Later Schneider et al used an open dynode structure as multiplier. Also solid state detectors have been used instead of scintillation counters . Recently, the channel electron multiplier (OEM) 11>19) and the channel electron multxDlier plate (CIMP) fl '18:* have been used. These two devices have given the 18) best results so far as far as timing is concerned. Berndt has used a conbinatirrn of CO1P and Si(Sb) detector. This caribination has the nice feature ;f spacx information if the SKS1;) iot-i-ctcr is position sensitive. In table I the results reported are shvn. The most promising technique seems, partly as a result ^f this work, to be the use rf two channol electron multiplier plates in tandem. Besides the use of seendary cloctron emission for zerotime detectors, Clerc et al have proposed an array <->f foils to be used as a Z detector. Due to the 6 electrons produced in the first foil the yield of SE electrons from two foils in series is more than twice the yield from one foil and the difference is highly 22) Z-dependent.