Nuclear Instruments and Methods 187 (1981) 423-433 North-Holland Publishing Company
APPLICATION OF A MULTIWIRE PROPORTIONAL CHAMBER FOR THE MEASUREMENT OF DIFFERENTIAL (n, CHARGED PARTICLE) CROSS SECTIONS C. DERNDORFER, R. FISCHER, P. HILLE, G. STENGL and H. VONACH
lnstitu t ffir Radiumforschungund Kernphysik der OsterreichischenAkademie der Wissenschaften,Boltzmanngasse3, A-1090 Vienna, Austria Recieved 24 October 1980 and in revised form 25 February 1981
A new method for the measurement of energy and angular distributions o f charged particles from neutron induced nuclear reactions is described. A small cylindrical multiwire proportional chamber in connection with a central CsI(T1) scintillator allows simultaneously the measurement o f the energy o f charged particles at all reaction angles, the particle identification via the dE/dX-E detector system and via pulse shape discrimination, and the measurement o f the background, thus reducing accelerator and measuring time. Data acquisition is made on-line via CAMAC to a PDP-11 computer. The design o f the system and its performance under realistic measuring conditions are described in detail.
1. Introduction Investigation of angular and energy distributions of charged particles from neutron induced reactions has so far been done mostly by means of counter telescopes [1-11 ] and has required very long measuring times due to the extremely low event rates and background problems. In order to improve this situation, which so far has restricted most of such investigations to a neutron energy around 14 MeV, multiple telescope sy&tems have been developed [12-14] which increase the count-rate by about an order of magnitude compared to conventional telescopes. As a further step in this direction a measuring system consisting of a small multiwire proportional chamber and a Csl scintillator has been developed which is equivalent to a system of 32 counter telescopes and permits simultaneous measurement of the full angular and energy distribution of charged particles from neutron induced reactions. In the following the design and performance of this system is described in detail. An even more detailed description including results of (n, a) measurements on 5°Cr and 93Nb at E n = 14.1 MeV is given in refs. 15 and 16.
semicircle around the inner wall of the cylindrical counter chamber. Charged particles from neutron induced reactions - if emitted in the right direction
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~'gra#hiterl~ covered with O,06rnrngold
~counfing wire a r i d ~ graphitering
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2. General description of the system The whole measuring system is shown schematically in fig. 1. The target to be investigated is laid in a 0029-554X/81/0000-0000/$02.50 © North-Holland
Fig. 1. The Vienna multi-telescope system (schematic); (a) top view, (b) side view.
C. Derndorfer et al. / Application o f a MWPC
- traverse first the ring-shaped MWPC consisting of
32 separate proportional counters and are then stopped in a CsI scintillator shielded against direct source neutrons by means of a tungsten shadow bar. Each of the proportional counters in conjunction with the central scintillator acts as a normal counter telescope and allows measurements of particle energy and particle identification. The different telescopes correspond to different reaction angles and allow simultaneous measurement of charged particle spectra at 16 reaction angles. The second half of the chamber not covered by the target is used for simultaneous measurement of the background.
3. Detailed design of the system
3.1. Proportional counter
The cylindrical proportional counter (see fig. 2) consists of the ground-plate (part A), the top-plate (part B)-both made of stainless steel-, a disk made o f polypropylen (part C) to hold the housings for the springs, a cover (part D) made of PVC, a print (part F) and an insulator (part E). Steel pins covered with gold (thickness 0.2 mm) carry parts B-D. These pins are riveted with parts A and B and therefore separate the sense wires from each other. The gold-coated tungsten wires (sense wires: 20 /~m ~b, grid wires: 50/~m ~) are soldered at the print plate. They each are separately tightened with a tension of 0.5 N by small springs which are mounted in housings to prevent electric discharges. This construcPART D'-..~ ~
3.2. Central detector
A CsI crystal of 1" diameter and 1 mm height coupled to an AVP 56 photomultiplier was chosen as the central energy detector. For this thickness, protons emitted from the target foil with an energy of up to 20 MeV are stopped. The crystal is shielded against the neutron source by 15 cm of tungsten and 7 cm of brass, resulting theoretically in an attenuation factor of 22. In practice, as measured by foil activation a shielding factor of 9.7 could be obtained because of the contribution from scattered neutrons. 3.3. Target holder
The target holder (fig. 3) was designed for achieving minimum background. A graphite ring covered with a thin gold foil 120 mg/cm 2) is used as target backing. In the graphite there is practically no proton production because of the large negative Q-value for the 12C(n, p) reaction and only emission of relatively low energy a-particles [Qn,a(12C) = -5.7 MeV] which are absorbed in the gold foil. For stability this target backing is enclosed in a stainless steel housing and the target area is defined in height by two stainless steel rings covered with gold thick enough to absorb all charged particles produced in the stainless steel.
P A R T C --..
I ' L\l
PART A PART F ~
tion proved very successful during the experimental runs because of the constant mechanical tension of the wires. In order to achieve a smaller gradient of the electrical field at the end of the wires field tubes (1 mm ¢ , 5 mm length) are mounted in the insulators.
PART E "
Fig. 2. Detailed design of the proportional counters.
C. Derndorfer et aL / Application of a MWPC
The angular aperture functions of the telescopes were calculated according to the following procedure: The part of the target foil corresponding to a certain telescope and the crystal surface are both decomposed into a large number of elements and the reaction angle is calculated for each combination (ca. 3 × 106 ) of elements according to the relation (see fig. 4)
GRAPH/T-RING AU-FOIL (.063MM) TARGET-FOIL
X.Y a = arc cos IXI • I Y I '
Fig. 3. Cross-section of target holder.
X and Y being vectors with components X = (A + R cos/3, R sin/3, h ) ,
Y = (R cos/3 - r cos 7 , R sin/3 - r sin 7 , h ) .
The geometry of the multitelescope system is displayed in fig. 4 showing the relative positions of neutron source, target foil and charged particle detector. For application of this system to measure absolute double-differential charged particle production crosssections the solid angle per telescope, the angular aperture function for each telescope and the neutron flux at the position of each telescope must be known. The quantities were determined in the following way, The solid angle per telescope was calculated numerically to be (13.3 + 0.6) msr by averaging over the solid angle of the CsI crystal seen from different points of the target foil. This value includes the loss of solid angle caused by the steel support pins of the proportional counter construction (see fig. 2). Similarly the average path length in the gas volume needed for the energy-loss corrections was determined as ~ = 93.4 mm.
Each combination of elements is given its proper weight according to the solid angle subtended by the crystal element to the foil element and the distance of the foil element to the neutron source and the aperture function IV(a) is finally calculated as the distribution of the weighted element combinations in the reaction. As shown in fig. 5, the average reaction angle varies from 22.06 ° for the first to 165.65 ° for the last telescope, the angular resolution (fwhm of the aperture functions) amounts to 11.5°-16 ° except for the first telescope which has a fwhm of 22.5 ° . The neutron flux distribution for the telescopes can in principle be calculated from the known variation of the distance from the neutron source along the target foil. This procedure however, is rather inaccurate because of attenuation and scattering effects and therefore an experimental determination of the neutron flux as function of position on the
Fig. 4. Geometry of the multi-telescope system.
C Derndorfer et aL / Appfication o f a M W P C
REACTIANGLE ON ~(DEGREES)
Fig. 5. Angular acceptance functions of the various telescopes.
target foil was done in the following way: (1) The neutron monitor (LiI crystal within the shield of the target room) was calibrated in terms of absolute neutron source strength via the well-known 27Al(n, c024Na cross-section . (2) The flux distribution along the target foil was determined relative to the mentioned monitor by activation of an Al foil in the position of the target foils. This foil was then cut into parts corresponding to the different telescopes and the absolute 24Na activities were measured for those parts.
x 3.-4.6"'-,6.7. ~colc
lO 11 12 13 14 15 16
Fig. 6. Neutron flux distribution along the inner wall o f the detector chamber for a n e u t r o n strength of 101 o n/s. Scales refer to absolute n e u t r o n flux and n e u t r o n flux relative to value at position o f telescope no. 1.
The resulting flux distribution is given in fig. 6. The errors given for the experimental points refer to the relative flux distribution only. For the absolute neutron flux there exists an additional normalization error of 3% . Also shown in the figure is the flux distribution calculated from neutron source strength and distance taking into account neutron attenuation in the TiT target assembly and the telescope chamber (assuming perpendicular incidence). The agreement between the two flux determinations is reasonable if one considers the large attenuation effects around 80 ° (see fig. 1) resulting in a corresponding dip in the observed flux distribution. The somewhat too high intensity at forward angles is probably due to inscattering from the nearby tungsten shield; as the figure shows contributions from scattered neutrons to the flux do not exceed 10% for all parts of the target foil inspite of the somewhat bulky chamber and shield construction. 4. Electronics and data handling Fig. 7 shows the block diagram of the associated electronics. Each wire produces both an analog (DE) and a logical signal (Time-out). The logical signals are fed to an address logic which transforms them to a 5-bit address characterizing the different counting wires, simultaneously they are used as timing signals in a fast coincidence with the photomultiplier anode signals to identify the coincidences between the proportional counter and the scintillator pulses. The analog pulses from the proportional counters are at first combined in four summing amplifiers summing eight preamplifier outputs each. The outputs of these summing amplifiers are fed into linear gates, which are opened in a case of coincidence with the scintillator and eventually all proportional counter signals are combined in a final summing amplifier. By means of this arrangement 4 times higher counting rates can be admitted to the proportional counters than in case of direct summing of all proportional counter outputs (DE signals) in one summing amplifier. The CsI scintillator is used to produce a timing signal, an energy (E) signal and a pulse shape (PS) signal. The latter is derived from the timing and energy signals by the method of zero-crossing with RC shaping [18,19] using integrating and differentiating time constants of 0.5 and 1.6/2s, respectively. Finally, for each event five parameters, the E, PS, DE signals, the wire address and the time difference between the proportional counter (time-out)
C Derndorfer et al./Application ofa MWPC
example at low particle energies and high count rates. In a second step chance coincidences are subtracted both in the target and the background counter and the net time spectra derived by subtraction of the true coincidence background from the corresponding foreground spectra. These net spectra are then converted into absolute c.m. double differential cross-sections as function o f channel energy by means of the program ESA .
5. System performance 5.1. Test with a-sources
Fig. 7. Block diagram of system electronics (schematic) S summing amplifier, DC - address decoder, GL - gate logic, KK - coincidence anticoincidence circuit, AN - anode of photomultiplier, D11 - dynode 11 of photomultiplier, CO crossover pickoff, SK - slow coincidence, TFA - timing filter amplifier, GGA - gate generator for ADC's, DE energy loss signal from proportional counters, P S - pulse shape signal, E - energy signal, M - monitor.
and the timing-signal of the E-detector are stored sequentially on disc in an on-line computer (see fig. 7). In this way the angular distribution of all kinds o f charged particles emitted b y the investigated target and the background are measured simultaneously within and besides the time peak allowing optimum off-line choice o f coincidence width and subtraction o f chance coincidences. A number o f computer programs was developed to analyze the above 5-parameter data and derive absolute double differntial particle production cross-sections d2a/dE d~2 for protons and a-particles. In a first step the particles in question are identified b y searching for events located b o t h in the appropriate regions of the E - D E and E - P S planes. In this way excellent particle identification is obtained even if the particle identification b y either method is not ideal as for
After completion the system was first thoroughly tested by means of an extended 241Am a-source (3.2 × 56.6 cm 2) mounted in the target-holder in the same way as the actual (n, charged particle) targets. Ar + 5% CO2 gas at about 100 mbar was used in these tests. The main results o f this test are: 1) The energy resolution o f the CsI detector for the a-particle arriving at the detector with an energy o f 4.72 MeV (energy loss in the gas = 0.785 MeV) is about 400 keV. An additional measurement with a ThC" a-source showed an energy resolution o f 630 keV at 8.78 MeV. 2) The base width o f the time distribution is about 500 ns most o f the time jitter being probably due to differences in the electron drift times in the proportional counters. 3) The energy resolution (fwhm) of all proportional counters was in the range of 20% as expected from the differences in path length (10%) due to the extension o f source and central scintillator. 4) The gas gain of all proportional counters showed variations o f at most -+10% from the average. This could easily be compensated b y gain adjustment in the electronics as is demonstrated in fig. 8 which shows the pulse height spectra of all proportional counters measured simultaneously in coincidence with the E-detector with the described extended a-source. 5) The efficiency o f all proportional counters was checked b y comparing the number o f pulses in the a-peak in the E-detector with the number o f coincidences between E and dE/dx detector and found to be larger than 99%.
C. Derndorfer et aL / Application of a MWPC
PUL 5E HEIGH T
Fig. 8. Pulse height spectra (in coincidence with E-detector) for all proportional counters for 241 Am measured with an extended h-source simulating the (n, charged particle) target.
5.2. Tests with neutron induced reactions The described system was tested under actual measuring conditions with nitrogen (Melamin) and hydrogen (CH2) targets and actually used for the measurement of the energy and angular distributions of the a-particles in the S°Cr(n, a) and 9aNb(n, a) reactions. [15,16]. From these measurements the performance of the measuring system can be summarized as follows. 5.2.1. Energy resolution The energy resolution achievable in practice is. demonstrated in fig. 9, which shows the a-spectra from a Melamin target at 22 °. The half-width of the 14N(n, ao) peak at 12 MeV is 1.1 MeV. Target thickness and kinematic broadening due to finite angular resolution contribute about 200 keV and 700 keV respectively leaving about 800 keV as inherent resolution of the E-detector. 5.2.2. Time resolution The time resolution (base width) was found to be about 500 ns for the whole investigated energy range Ea = 5 - 2 0 MeV and Ep = 2 - 1 4 MeV indicating that it is limited at present by the drift time differences in the proportional counters.
5.2.3. Particle identification in the E - d E p l a n e As already reported the half-width of the dE peak was found to be 25% for 8 MeV a-particles. For protons of 6 MeV considerably larger half-widths of 40% are observed, as the energy deposition of protons is less than 1/10 of that of a-particles of the same energy and thus both energy loss straggling and preamplifier noise are relatively more important for protons. Under these conditions a-particles can be clearly separated from protons and deuterons in the E - d E plane, but no clear p - d separation is possible. Fig. 10 shows the particle separation achieved under actual measuring conditions at an average neutron flux of 4 × l0 s n/cm 2 s at the target foil. The large pulses above the a-peak are probably due to Ar recoil nuclei from Ar(n, a) reactions in the counter gas. 5.2.4. Particle identification by pulse shape discrimination in the CsI detector Although the CsI crystal was chosen as thin (1 mm) as to just stop all protons originating in the target foil up to 20 MeV and shielded as effectively as possible against the source neutrons, 7-interactions with the crystal produce a high count rate up to pulse heights corresponding to 4 MeV protons resp. 8 MeV a-particles. Thus the main task of particle identifica-
C. Derndorfer et al. / Application o f a MWPC
170 160 1~0 14o 13o 120 11o 100 So 80
70 60 50 40 30 ZO 10 0
Fig. 9. a-energy spectrum of telescope no. 1 (22 °) from a 0.93 mg/cm 2 Melamin target, counter gas Ar + 5% CH 4 at 100 mbar, tantalum backing, no background subtracted, measuring time 40 h at source strength of about 1.3 × 101 o n/s 1SO 1't.0 130 120
110 100 ~10 80 70 60 ~0 C,<.- P A R TICLES
't0 30 ZO tO 0
Fig. 10. Particle identification by means of the DE-pulse spectra. Energy loss spectrum of proportional counter in coincidence with scintillator pulses equivalent to 4 - 5 MeV a-particles or 2 - 3 MeV protons.
C. Derndorfer et al. / Application of a MWPC
tion by PSD in the scintillator is the rejection of "),-induced events especially at low energies in order to reduce chance coincidences. This aim could be reached approximately down to s-energies of 4 MeV and proton energies of 2 MeV, respectively (see fig. 11).
iments is about a factor 2 - 3 larger than in (n,c 0 experiments. Thus investigation of (n, p) and (n,c0 spectra in separate runs optimized each with respect to the conditions discussed before will still give better results in a given total measuring time than the attempt to do simultaneous measurements of the (n, p) and (n,c0 data in one experiment. This situation will however probably change after the improvements in the system electronics discussed in sect. 5.2.9.
5.2.5. Optimum measuring condition for proton and s-spectra It has been one of the design aims that the system should enable simultaneous measurement of proton and c~-spectra. Experience has shown, however, that it is very difficult to optimize simultaneously measuring conditions for both problems. In order to get enough pulse-height for the most energetic protons a gas pressure of about 200 mbar has to be used (instead of 100 mbar for c~-particles) which produces a rather large energy loss for low energy c~-particles (1.8 MeV for E = 4 MeV). In addition in (n, p) exper- ' iments the maximum permissible neutron flux is only about 20% of that allowed in (n,c 0 experiments, whereas the tolerable target thickness in (n, p) exper-
5.2.6. Background Typical o~-background spectra as observed in the investigation of the S°Cr(n,a) reaction with 14.1 MeV neutrons  are given in figs. 12 and 13. Fig. 12 shows the true background and fig. 13 the additional background contributions from chance coincidences for the source strength of 1.2 × 101° n/s used in that experiment. The absolute cross-section scale refers to a target thickness of 3 X 1019 atoms/cm 2 typical for (n,c 0 experiments. Proton background was investigated in a separate test experiment. Results both for the time and chance coincidence-induced
PSD- Pulse- Height
Fig. 11. Pulse shape spectrum of protons and c~-particlesin the CsI scintillator for pulse heights equivalent to 2-3 MeV protons and 4-5 MeV a-particles, respectively. (Abscissa: pulse height of TAC meas. the scintillation pulse shape see fig. 7).
C. Derndorfer et al. / Application of a MWPC
lO LO J 5 ~. - BACKGROU~ o6-
Xa ob 1
02 02 01 Ol .005
Fig. 12. Genuine (chance coincidence corrected) e-particle background for assumed target thickness of 3 X 1019 atoms/ cm2 (a) angular range (10-40) ° (b) 90-180 ° .
background are given in fig. 14. In these proton background investigations the bare graphite-ring (without the 0.063 mm Au foil shown in fig. 3) was used as proton background from graphite should be almost zero. Most of the true c~-particle and proton background especially for high energies is probably due to charged particles from neutron interactions in the gold sheet covering the stainless steel rings defining the target height (see fig. 3) and (in case of c~-particles) also from the gold foil covering the graphite ring. Therefore the background decreases strongly with reaction angle as obvious in fig. 12. The large increase in true background intensity below Ee = 8 MeV is not completely understood. Part of it may be due to c~-particles from Ar(n, c0 reactions in the counter gas but estimates of this contribution showed that it cannot account quantitatively for the observed background at low c~-energies. Concerning the practical application of the multi-telescope system figs. 1 2 - 1 4 show that investigations of
.001 -~ 4
Fig. 13. e-background from chance coincidences for an assumed target thickness of 3 X 1019 atoms/cmz and average neutron flux of 4 X 10s n/cm 2 s at the target foil.
(n,c~) and ( n , p ) reactions with tolerable effects to background ratios are possible down to (n,c 0 crosssections in the 2 - 5 mb and. (n,p) cross-sections in the 1 0 - 2 0 mb range for 14 MeV incident neutron energy. 5.2. 7. Maximum admissible neutron flux
The maximum admissible averaged neutron flux at the target foil was found to be about 4 X 10 s n/cm 2 s in ( n , a ) and 8 X 104 n/cm2 s in ( n , p ) experiments corresponding to a neutron source strength 1 × 10 9 n / s - s r and 2 × 10 ~ n / s . s r for the chosen average source-target distance of 50 cm. The neutron flux in case of (n, a) reactions is limited to the quoted value because of 1) deterioration of pulse shape discrimination in the CsI detector due to pulse pile-up effects (the observed single count rate above 1 MeV equivalent proton energy being 6 X 103/s); 2) dead time o f about 12% due to a total count rate of 3 X 104/s (if the trigger threshold is chosen as
C. Derndorfer et al. / Application of a MWPC
high as possible compatible with safe detection of a-particles only) in the MWPC (time-out) and a dead time of 4/~s in the address-logic. The much lower admissible neutron flux in (n, p) experiments is due to the much smaller energy loss of the protons in the MWPC requiring a much lower trigger threshold for safe detection of protons up to 14 MeV. Accordingly the neutron flux limitation in (n, p) experiments to the relatively low quoted value is due to effects (1) and (2) discussed before and in addition pulse pile-up in the summing amplifiers which combine the analog pulse of eight counting wires, begins to become a limiting factor. Also in both cases at the stated maximum flux levels background from chance coincidences begins to become comparable to the true background (see section 5.2.6).
5.2.8. Low-energy limits o f the system Due to the energy loss of the charged particles in the counting gas at the pressure of 100 and 200 mbar necessary for detection of a-particles and protons, respectively the system is limited to detection of particles with E~ > 3 MeV and Ep > 1.5 MeV, if we require a pulse height equivalent to 1 MeV proton energy. As however already discussed in section 5.2.6 there is a large increase in background and chance coincidences at low energies making meaurements below E~ = 5 MeV and Ep = 3 MeV rather difficult.
20 p - BACKGROUNO
Fig. 14. Proton background for the angular range 90-180 ° for an assumed target thickness of 3 X 1019 atoms/cm2: (a) genuine background; (b) background from chance coincidences for average neutron flux of 8 X 104 n/cm 2 s at the target foil.
5.2.9. Possible improvements o f the system The main drawbacks of the system present are the rather low admissible neutron flux in (n, p) reactions, the relatively large intensity of random coincidences especially below Ea = 5 MeV and Ep = 3 MeV and the relatively high a-background at energies below 8 MeV. These disadvantages will be eliminated to a large extent by means of the following improvements. 1) Addition of a second ring-shaped MWPC inside the present one and requiring triple coincidences for accepted events. 2) Replacement of the present preamplifiers by the recently developed low-noise charge-integrating preamplifier TRA 510 with corresponding improvement in the signal-to-noise ratio by about a factor 4. 3) Reduction of the dead-time of the address-logic. Improvement (1) will drastically reduce the rate of chance coincidences. Improvement (2) will allow the system to operate at sufficiently low pressure (100 mb) both for a- and proton detection and to drasti-
C Derndorfer et aL/Application ofa MWPC cally reduce the pulse length and the corresponding pile-up effects in the summing amplifiers. Therefore by means o f these improvements it should become possible to increase the maximum admissible neutron flux in (n, p) experiments nearly to the values already achieved in (n, a) measurements and to improve the background conditions especially in the low energy region.
6. Summary and conclusions The advantages and disadvantages of the various methods used to investigate (n, p) and (n, a) spectra and angular distributions (including the system described here) have recently been discussed in detail b y one o f the authors . With respect to the system described in this paper that discussion can be summarized as follows. The multitelescope system described in this paper allows one to measure double differential p - a n d a-production cross-sections in neutron induced reactions much more efficiently than most systems designed so far. Only the Geel 5-fold telescope  system has a somewhat better measuring efficiency. Compared to the Geel system our system has the advantage o f allowing a much more detailed investigation of angular distribution (16 instead o f 5 angles) and o f allowing simultaneous measurements of foreground and background but the disadvantages of needing much larger target foils (100 cm 2 compared to 5 cm 2 in ref. 13) and somewhat larger background because o f the use o f triple coincidences and partial shielding of the proprotional counters in the Geel design. At low charged particle energies there are lYackground problems as in all systems not using magnetic transport of the charged particles. This domain of low energy probably can be investigated better b y means of spectrometers of the quadrupole type  especially if the measuring efficiency o f these systems is improved by use o f large-solid angle achromatic magnetic transport systems. Thus the main application o f the described system will be the investigation o f (n, p) and (n, a) reactions in the mass range A = 5 0 - 2 0 0 especially studies for
A > 80 where to date very little is known about double-differential production cross-sections for charged particles in neutron induced reactions. The authors gratefully acknowledge the support from Dr. Rytz, BIPM, who supplied us with the extended 241Am a-source, Dr. Mayer, Universitht Miinchen, who produced for us the Melamine target and Dr. P. Maier-Komor, Technische Hochschule Mtinchen. This work was supported by the Austrian Fonds zur F6rderung der wissenschaftlichen Forschung.
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