— journal of September 2015
physics pp. 483–495
Detector instrumentation for nuclear fission studies
AKHIL JHINGAN1,2
1Inter University Accelerator Centre, P.O. Box 10502, New Delhi 110 067, India
2Department of Physics, Panjab University, Chandigarh 160 014, India E-mail: akhil@iuac.res.in, jhinganakhil@gmail.com
DOI:10.1007/s12043-015-1067-8; ePublication:3 September 2015
Abstract. The study of heavy-ion-induced fusion–fission reactions require nuclear instrumen- tation that include particle detectors such as proportional counters, ionization chambers, silicon detectors, scintillation detectors, etc., and the front-end electronics for these detectors. Using the detectors mentioned above, experimental facilities have been developed for carrying out fusion–
fission experiments. This paper reviews the development of detector instrumentation at IUAC.
Keywords.Multiwire proportional counter; ionization chamber; charge-sensitive preamplifier.
PACS Nos 29.40.Cs; 25.85.−w; 84.30
1. Introduction
The experimental nuclear physics programmes in many accelerator laboratories are focussed at reactions around Coulomb barrier. The experiments that are routinely car- ried out are fission angular and mass distribution, fusion cross-section measurements, pre-scission neutron and charged particle multiplicity experiments, etc. To probe these reactions more deeply, it is required to have detectors with higher detection efficiency, large solid angle coverage, good energy, position and timing resolutions along with high count rate handling capability. At the same time, detectors are required to detect different particles right from protons to heavy evaporation residues, heavy fission fragments, etc.
Using a single detector, it is impossible to carry out all these measurements mentioned above. In recent years, attempts have been made to carry out multiparametric measure- ments with multidetector systems to investigate these phenomena. The commonly used detectors for particle detection are silicon detectors, gas detectors such as ionization chambers and proportional counters, hybrid telescopes (combination of gas and silicon detector), inorganic and organic scintillators, etc.
Fission reactions are investigated by detecting both binary fragments and characteriz- ing them. This is done by either measuring the energies of both or one fragment, or by
2. Multiwire proportional counter (MWPC)
Multiwire proportional counters (MWPC) are perhaps the most important detectors used in fission experiments. They provide good timing and position resolutions, are insensitive to radiation damage, have high count rate handling capability, and can be fabricated with ease in various sizes and geometry to make it compatible with the given experimental requirements. They are being extensively used for carrying out mass distribution of fis- sion fragments, and for detection of evaporation residues. An experimental programme for investigating mass distribution of fission fragments and their total neutron multiplicity is being carried out at IUAC using GPSC and NAND Facilities. Time-of-flight (TOF) systems based on MWPC have been developed for carrying out these experiments. An experimental programme for measuring fusion cross-sections, so as to look for fusion enhancement or fission hindrance [6], is being carried out at IUAC using the spectrome- ters HIRA and HYRA. Both the spectrometers detect fusion products at their focal plane MWPC.
MWPC can be fabricated with a three-electrode geometry [7] having a cathode sand- wiched between two position-sensitive anodes (xandy). The wires inxandyare oriented orthogonal to each other. Cathode is generally made using thin Mylar foil which is aluminized on both the sides. Cathode is kept at negative potential and provides fast timing signal for TOF or velocity measurements. Another option is to have the multistep geometry [8] using four- or five-electrode configuration. All the electrodes are made using wire frames. The detector has two cathodes (one each at the entrance and the exit), one anode sandwiched between two position electrodes (xandy). The region between cathode and position electrodes are made to operate in the drift region, whereas that between positions and anode in the avalanche region. Drift region acts as a preampli- fication region, where the charge produced due to ionization drifts into avalanche region where it subsequently gets multiplied. The last electrode can be removed to provide a four-electrode configuration. This design provides higher gains as compared to the three- electrode design. In comparison, a three-electrode MWPC with aluminized Mylar cathode provides better timing due to more uniform field. In both cases, positions are extracted using the delay line technique. At IUAC, we use the commercially available Rhombus delay chips model TZB 12-5. The chip has an impedance of 50, and each chip has 10 taps with a delay of 2 ns per tap. Wire frames in both cases are fabricated using gold-plated tungsten wires.
2.1 Fission fragment TOF spectrometer for NAND
This TOF spectrometer [9] is based on a pair of MWPC fabricated using three-electrode geometry. A schematic of the detector system along with the read-out electronics is shown in figure 1. The core of MWPC is made of three electrodes: an anode wire plane (x), a central cathode foil and strip plane (y). The cathode is symmetrically placed 3.2 mm apart between the two anodes, and is made of a 2μm-thick Mylar foil aluminized on both surfaces. The x-plane is made of 200 wires, 10μm diameter, stretched on a 2.4 mm thick PCB with an active area of 125×75 mm2. Interwire spacing is 0.63 mm. The Y-plane consists of 30 tin-plated copper strips (perpendicular to the wires of x-plane), 2.2 mm wide with a step of 2.54 mm, made on a printed circuit board. In x-plane four wires are grouped together and connected to one tap of the delay chip. MWPC is isolated from chamber vacuum (low 10−6 Torr) using 1 μm thick Mylar foil at the entrance of the detector. The timing signals from positions and cathode are extracted using the in- house-developed fast timing current-sensitive preamplifiers. Position signals use the non- inverting configuration whereas the cathode uses the inverting configuration. More details about preamplifers are provided in §5. Position signals were subsequently fed to Ortec CF8000 CFD and cathode to Phillips 715 CFD.
The MWPC has been tested offline with241Amαsource at 7 mbar isobutane. A bias of
−600 V was applied to the cathode. The cathode signals, after preamplifier, had an ampli- tude of about 300 mV with rise times close to 3 ns. For position signals the amplitude varied between 30 and 100 mV with rise times around 5 ns. The detector was operated at 3 mbar isobutane and cathode bias of−450 V was applied for in-beam fission experiments [10,11]. For these experiments, one detector is generally placed at 40◦(target-to-detector distance=22 cm) and the other at around 120◦(target-to-detector distance=17 cm) in the spherical scattering chamber of 60 cm diameter as shown in figure 1b. The detector at 40◦ could handle count rates exceeding 100 kHz. Mass information of fission events and its separation from beam- and target-like particles was performed using TOF from cathode with respect to RF (pulsed beam from Pelletron+LINAC). Timing signals (from
Figure 1. (a) Schematic of a three-electrode MWPC. (b) Assembled MWPCs mounted inside the NAND scattering chamber.
Figure 2. (a) TOF spectrum for19F+208Pb. (b) x-position spectrum.
CFD) from both cathodes were fed to Ortec CO4010 logic module where OR of these two signals were AND gated with beam RF. This logic signal provides the master start signal for TOF and position signals. The TOF of other detectors such as neutron detectors are also recorded with respect to this signal. These signals were delayed using Ortec octal gate and delay generator GG8000 and fed to the stop inputs of the TDC (Phillips 7186).
Figure 2a shows the TOF spectra showing clean separation between beam-like particles and fission events as well as target-like particles. A time resolution of about∼300 ps was observed for the position-gated elastic events in 28Si beam. Figure 2b shows the x-position spectra. Each peak corresponds to one delay tap of the delay chip.
2.2 MWPC for GPSC
Two large-area multistep position-sensitive MWPCs [12], with five-electrode geometry, have been developed for experiments involving the study of fission dynamics using GPSC Facility at IUAC. Both detectors have an active area of 200×100 mm2, and provide position signals in both horizontal (x) and vertical (y) planes, timing signals (from anode) for TOF measurements, and energy signals (from cathode) giving differential energy loss in the active volume.
Figure 3a shows a MWPC with four-electrode configuration and figure 3b shows the MWPC set-up inside GPSC for mass distribution experiments. The distance between adjacent wire frames is 3.2 mm. The separation between adjacent wires (20μm diam- eter) is 1.27 mm. The x-frame has 160 wires whereas the remaining frames have 80 wires each. In position electrodes, wires are shorted in pair and connected to one tap of delay chip. The position frames are kept at ground potential by terminating both ends of delay lines through 150 kresistors. Gas detector is isolated from the vacuum cham- ber using 1μm Mylar foil. They have a very poor energy resolution, but is sufficient in many cases to discriminate between heavy fission fragments and lighter projectiles, and its combination with the TOF signals gives a very clean separation of fission fragments from target- and projectile-like background. MWPCs are generally operated with isobutane gas at pressures ranging from 1 to 4 Torr depending upon the nature of the particle (mass and energy). The timing and position signals of the detectors are used for fission coincidence measurements and subsequent extraction of their mass, angular and total kinetic energy
Figure 3. (a) Schematic of a four-electrode MWPC. (b) Assembled MWPCs mounted inside the GPSC.
distributions. Signal processing is similar to that described in §2.1. More details on instrumentation can be found in [12]. The timing signals from the central anode and position signals are amplified by the in-house-developed non-inverting and inverting fast current amplifiers (§5) respectively. Both the cathodes are shorted electrically and signal is processed using a charge-sensitive preamplifier (CSPA) developed in house.
The detector was tested with241Amαsource for determining its position, timing resolu- tion as well as its detection efficiency. Forα-particle detection, the detector was operated using isobutane gas at a pressure of 4 Torr with an operating voltage of+540 and−180 V on the anode and the cathode, respectively. Figure 4a shows the masked 2D plot withαs and figure 4b is the projection ofx-axis. The holes of 1 mm diameter are placed 5.08 mm apart in a rectangular matrix. A position resolution of∼1.1 mm has been observed for alphas. The detector system has been used to study the fission mass distribution [13]
and fission-gated neutron multiplicity [14] for various systems. Figure 5a shows the plot of mass ratio of one fission fragment with the mass of compound nuclei against veloc- ity ratio of the parallel component of fission fragment with that of compound nuclei.
There is clean separation between the transfer channels and the compound nuclei fis- sion for the18O+232Th system. Figure 5b shows the plot between TOF and energy loss showing clean separation between the projectile-like fission fragments and the target-like
Figure 4. (a) Masked 2D plot with alphas. (b) x-projection of the masked spectra.
Figure 5. (a) Mass ratio against the ratio of velocities (parallel to compound nucleus).
(b) Plot of TOF against energy loss.
particles for19F+208Pb system. To extract TOF information, pulsed beams were used from IUAC tandem with a bunchwidth of∼1.1 ns. The detectors in all these experiments were operated at 1.5 Torr isobutane, and the optimum operating voltages were+400 and
−180 V on the anode and the cathode, respectively. At forward angles the count rates exceeded 20 kHz. For beam-like elastics, a timing resolution of∼2.5 ns is observed.
For the fission fragments, time resolutions are expected to be better (∼1 ns) because of higher amplitude pulse.
2.3 Focal plane detector systems for HIRA and HYRA
Electromagnetic separator (HIRA) [3] and gas-filled separator (HYRA) [4] are used for the detection of fusion evaporation products or evaporation residues (ER) by focussing them at the focal plane detectors and suppressing the primary beam-like particles.
MWPCs are used at the focal plane of both the spectrometers to detect ERs. MWPCs have an active area of 150×50 mm and use mutistep geometry as described earlier. The HIRA focal plane detector [15] has a five-electrode geometry. All electrodes are wire frames. The distance between the adjacent wire frames is 3.2 mm. All wire frames are made from gold-plated tungsten wires (diameter 20μm) stretched on a 1.6 mm-thick prin- ted circuit board. The adjacent wires are 1.27 mm apart. The x-position frame has 120 wires and the remaining frames have 40 wires each. As shown in figure 6a, the entire electrode assembly is housed inside a cylindrical housing milled out of a solid aluminum sheet. For HYRA focal plane MWPC [16], a four-electrode geometry is used by remov- ing the last cathode. Design features are identical to that of HIRA focal plane detector except that the anode and cathode wires are placed 0.63 mm apart and the anode uses 10 μm gold-plated tungsten wire. Such a design provides higher gains [17]. The geomet- rical transmission efficiency is 92%. Both MWPCs are operated with isobutane gas at pressures ranging from 1 to 3 Torr. The gas medium is isolated from the vacuum using a 0.5μm Mylar entrance window. The foil is transparent to heavy recoils,A ∼ 200 and above, which are produced in highly asymmetric reactions, and have very low kinetic energies (0.02–0.1 MeV/A) [18,19]. Front-end electronics are identical to that described in §2.2.
Figure 6. (a) Assembled MWPC. (b). Three silicon wafers stacked together at the HYRA focal plane.
The HYRA MWPC is followed by the position-sensitive large-area silicon detector.
This may be a strip detector or resistive anode detectors. Silicon detector gives the energy of the implanted ER. The detectors can be stacked together to provide larger detection area at the focal plane. Figure 6b shows three resistive anode detectors from Eurysis/Canberra, having read-outs from the four corners, stacked together to give an active area of 15× 5 cm2. The detectors belong to the CATE [20] set-up of RISING campaign at GSI.
2.4 Time-zero MWPC
A transmission-type fast timing MWPC [21] has been developed and used in combina- tion with large area position-sensitive MWPCs [12] to obtain absolute timing of the fission fragments, and the subsequent extraction of their mass–energy distributions. The detector can be used as a trigger in multidetector set-ups for measuring neutrons andγs in coinci- dence with the fission fragments. The detector has an active area of 40×40 mm2. It uses a mutistep design with four electrodes made from wire frames that are 1.6 mm apart. The separation between the adjacent wires is 0.63 mm. Cathode and the first ground at the entrance operate in the drift region whereas the anode sandwiched between two grounds operate in avalanche or proportional region. Such a design was preferred to the con- ventional PPAC using two parallel aluminized Mylars to avoid straggling of low-energy heavy ions (0.5 MeV/A in mass 100 region). The detector is operated with isobutane gas at 2–4 mbar pressure. To avoid straggling, the entrance and the exit foils used are of 0.5μm Mylar. Anode is read using fast timing amplifier (§5) whereas cathode is read by charge-sensitive preamplifier.
The detector was tested offline with radioactive sources241Am and252Cf. Forαdetec- tion, detector was operated at 3 mbar gas (isobutane) pressure. A bias voltage of+450 and−180 V was applied at the anode and the cathode, respectively. As shown in figure 7a, signal strengths upto 300 mV with 3.5 ns rise times were observed in contrast to 10 ns rise times and 100 mV strength for MWPC with a wire spacing of 1.27 mm. To evaluate the timing performance, TOF was set up between time-zero and large-area MWPCs [12].
Both the MWPCs were exposed to fission fragments. The large-area MWPC acts as a stop detector. The distance between two MWPCs was 15 cm. A neutron detector was also placed at a distance of 1 m from the source to collect coincident neutrons andγs.
Figure 7. (a) Trace showing comparisons between signals from two MWPCs.
(b) TOF with252Cf between two MWPCs.
Both the detectors were operated at 2 mbar gas pressure. TOF was also generated between the start MWPC and neutron detector. Figure 7b shows the TOF spectrum for fission frag- ments. Figure 8a shows the raw TOF between MWPC and neutron detector depicting the splitting ofγ peaks due to two different groups of fission fragments. On applying the TOF correction from the two MWPCs, a singleγ (figure 8b) peak, having 2.5 ns FWHM was obtained. Subtracting the contributions (time resolution of large MWPC and neutron detector), we can estimate the intrinsic resolution of time-zero MWPC to be∼400 ps.
The detector system was used in one of the experiments to study the mass distribution for the6,7Li+238U system at 30–50 MeV energies. The start MWPC was placed at a distance of 7.5 cm from the target on one of the GPSC arms followed by a large-area position-sensitive MWPC. The second MWPC was placed on the other arm. The time- zero detector provided master trigger (initiates for TDC) for all timing signals such as TOF for two position-sensitive MWPCs and their position signals. The electronic delay between the start and the stop detectors was determined using monoenergeticαs from the241Am source. Absolute TOF was obtained for one of the fragments whereas time
Figure 8. (a) Raw TOF spectrum between start MWPC and neutron detector.
(b) Corrected TOF spectrum showing singleγpeak.
difference between one fragment and its complementary fragment was recorded for the other fragment.
3. Hybrid telescopes for heavy-ion detection
Hybrid telescopes, having a combination of gas and silicon detectors, have been devel- oped [22] for heavy-ion detection and particle identification. The detector telescope has been used for studying the angular distributions of fission fragments. The detector system can also be used to identify projectile-like fragments and thus can be used for studying transfer reactions. As a standard practice, silicon telescopes with very thin (10μm) silicon detectors (E) and thick (100–300 μm) (E) are used for particle identification. The E−E identification technique is threshold-dependent and is governed by the thick- ness of theEdetector. The 10μm-thick detectors are opaque to low-energy heavy ions such as the fission fragments. In such cases a gas detector is extremely useful because its active thickness can be varied by simply adjusting the gas pressure, thus making it transparent for low-energy heavy ions.
The telescope consists of a gas ionization chamber, operating in the axial field geometry mode, followed by a silicon detector. The ionization chamber (IC) is composed of three wire frames of 10 mm active diameter. The wire frames consist of a cathode, a central anode frame and another cathode wire frame. The distance between adjacent wire frames is 10 mm. All wire frames are made from gold-plated tungsten wires of 20μm diameter.
The adjacent wires are 1 mm apart. The two cathodes are grounded whereas the anode operates in the ionization region at 150 V with a gas pressure of 100 mbar isobutane (2.5μm Si equivalent). Entrance foil used is 1μm Mylar (diameter 10 mm). Anode is read using CSPA with a gain of 90 mV/MeV (Si equivalent) and the silicon detector has a CSPA of 20 mV/MeV gain. The preamplifiers are placed next to the detector in vacuum.
Figure 9a shows the detector set-up for investigating the fission anisotropy [23]. Three hybrid telescopes are placed at a distance of 30 cm from the target. Figure 9b shows the scatter plot ofE againstEfrom the 19F +194Pt system. Fission fragments are well
Figure 9. (a) Detector set-up inside GPSC. (b)E−Eplot (FF – fission fragments, PL – projectile-like).
detector telescopes have been used conventionally. Silicon detectors are highly prone to radiation damage. CsI(TI) scintillators coupled to photodiodes offer a very flexible and inexpensive solution for the same. One of the main characteristics of CsI(TI) detectors is its intrinsic ability to discriminate between different light charged particles such as pro- tons,αs, electrons (γ photons), etc. based on their stopping power. This gives rise to different decay time constants in the light output (fast component) for different particles.
They also show reasonably good energy resolution. Currently, the array has 16 detectors.
Each scintillator is of 3 mm thickness with a 20 mm×20 mm active area coupled to a 10 mm×10 mm photodiode. A thickness of 3 mm is sufficient to stop up to 25 MeV of protons. The detectors are mounted in groups of four (2×2) on a common board with preamplifiers on its back as shown in figure 10a.
The photodiodes are read by CSPA. As the charge generated in photodiodes is extremely small, it is desirable for the CSPA to have a high gain, good timing features (ability to distinguish between different decay times from CsI) and low power consump- tion so that it can be placed next to photodiode in vacuum to avoid degradation of signals.
A preamp has been developed in-house with the above requirements. It has a low power consumption of 30 mW, a gain of 2 V/pC (Si equivalent) and exhibits good timing characteristics for particle identification. Ballistic deficit technique has been used for particle discrimination. For each detector, two shaping amplifiers are required: one with
Figure 10. (a) Fission-gated charged particle set-up inside the GPSC. (b) Scatter plot showing separation between different particles.
shorter shaping time (0.5μs) and the other with larger shaping time (3μs – gives total energy).
Offline tests were performed using radioactive sources. Energy resolutions of 200 keV (5.48 MeVα) and 68 keV (1.33 MeVγ) were observed. The preamplifier output was fed to Mesytec STM 16+ amplifier units via a differential driver unit. The out- put of these units were digitized using 7164H ADC (Phillips). The detector system was used to study fission gated pre- and post-scission charged particle multiplicity for the
16O+194Pt system at 100 MeV. As shown in figure 10a, four quads of CsI along with two MWPCs [12] were placed inside the GPSC. The MWPCs detect fission fragments and CsI scintillators detect light charged particles emitted during the reaction. One quad is placed behind one of the MWPC to detect post-scission light charged particles whereas the remaining quads are placed around 90◦with respect to the MWPC to detect the pre- scission particles as well as their angular distributions. The detectors are at a distance of 23 cm from the target. Figure 10b shows the scatter plot between the long and short shaping times, showing clean separation between different particles.
5. Preamplifiers for signal extraction from the detector
Custom-designed preamplifiers have been developed for extracting signals from the detec- tors. Two versions have been developed. First one is charge sensitive for extracting energy information. Second one is fast current sensitive for extracting timing informa- tion. Advantages of both the versions are that they have very low power consumption and thus can be operated in vacuum close to the detector, eliminating the need of long signal transmission cables which deteriorate the quality of signals from the detector. Another advantage of developing the preamplifiers in-house is that there is flexibility of varying the gains and frequency characteristics depending upon the experimental requirements as well as the detectors with which they are being used.
Charge-sensitive preamplifier (CSPA) have been developed using surface mountable devices such as chip resistors, capacitors and transistors. It uses a conventional circuit [25] having a JFET at the input stage followed by a transimpedance amplifier made from BJT. The CSPA has been realized in the form of a eight-pin SIL hybrid (figure 11a) with
Figure 11. (a) CSPA hybrid. (b) Eight-channel CSPA box.
Figure 12. (a) Versions of fast timing preamplifiers developed. (b) Trace showing rise time of 2 ns from the silicon detector.
25×25 mm dimension. Two versions have been developed: one with a power consump- tion of 160 mW showing a higher dynamic range (up to 300 MeV) and the other with a power consumption of 30 mW. Charge sensitivities vary from 7 to 90 mV/MeV (Si equivalent). These preamplifiers are being used for extracting signals from proportional counters, gas ionization chambers, silicon detectors, CsI scintillators coupled to photo- diodes. They have been used as a single unit and stacked together in groups of four/eight on a common mother board (figure 11b) for the read-out of multielement detectors.
The fast current sensitive version [26] utilizes multiple common emitter amplifier stages with an emitter follower at the output stage. Input impedance is 50. Two ver- sions are available: inverting and non-inverting (depending upon the polarity of the input signal to drive the negative signals directly to timing discriminators). Both versions show a voltage gain of 100. Amplifiers have been developed as single-, four- and eight-channel NIM modules (figure 12a). Rise times of 2 ns (figure 12b) and a timing resolution of 130 ps (FWHM) have been experimentally observed with a 40μm thick silicon detector with respect to RF from superbuncher of LINAC.
Acknowledgements
Author acknowledges the support of the group members P Sugathan, N Madhavan, T Varughese, S Nath, K S Golda, J Gehlot, N Saneesh, E T Subramaniam, S Venkataramanan and R Ahuja. The author also acknowledges the encouragement and support from A Roy, S K Datta, R K Bhowmik and G K Mehta. The author thanks Pelletron and LINAC group for providing good quality beams. The author thanks the collaborators B R Behera (Panjab University), S K Mandal (Delhi University), R G Thomas (NPD-BARC), B K Nayak (NPD-BARC), A Saxena (NPD-BARC). He also thanks H J Wollersheim (GSI) and G Pascovici (IKP, Koln) for technical support in detector instrumentation. He thanks and acknowledges the hard work of the students H Singh, P Shidling, S Kalkal, R Sandal, V Singh, S Appanababu, E Prasad, G Mohanto, S Goyal, M Kaur, G Kaur, R Mahajan, M Thakur, P Sharma, K Kapoor, R Dubey and T Banerjee.
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