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High resolution low threshold detector telescopes for multifragmentation studies
Nuclear Instntments
and Methods in Physics Research A 368 (1996)
709-718
NUCLEAR INSTRUMENTS P METHODS IN PHYSICS RESEARCH
Section A
EISEWIER
High resolution low threshold detector telescopes for multifragmentation studies D. Fox a**, R.T. de Souzaa**, S.L. Chen a, B. Davin a, T.M. Hamilton a, Y. Lou a J. Dorsett a, J. Ottarson b a Department of h National
Chemistry
and Indiana
Superconducting
Universitj
Cyclotron
Cyclotron
Laboratory,
Faciliry,
Michigan
Indiana
University,
State University,
Bloomington,
East Lansing,
IN 47401,
MI 48824,
USA
USA
Received 15 August 1995
Abstract
A set of compact ionization chamber-Si( IP)-CsI(TI) telescopes for multifragmentation studies are described. These ion chamber telescopes can be used either individually, or to augment the MSU Miniball 47r array. The characteristics of the large-area passivated, ion implanted Si detectors are investigated. Statistics on the overall planarity of these detectors as well as, the thickness of dead layers present at the surface of this type of detectors are examined.
1. Introduction
Nuclei under extreme conditions of temperature and density may decay via disassembly of the nuclear system [ 1,2]. This new decay mode is currently a topic of considerable interest [ 3-61. In order to study the multiparticle final state characteristic of this new decay mode, exclusive measurements with 477 detectors are necessary [ 7,8]. To characterize the multifragmentation of the nuclear system, detection and identification of low energy fragments is required. At present, however, all the arrays suited for studying heavyion induced multifragmentation have relatively high thresholds at backward angles and are difficult to calibrate. To remedy this problem, a set of high resolution, low threshold telescopes, which can be used to augment the MSU Miniball by providing a reference measurement, have been constructed. These telescopes consist of an axial-field ionization chamber-Si ( IP) -CsI (Tl) stack. This paper describes both the characteristics and shortcomings of the individual detector elements, as well as, the performance of the detector telescope. This paper is organized as follows. The design of the ion chamber telescopes is described in Section 2. The performance of the axial-field ionization chamber is explored in Section 3. Measurements of the dead layers of the Si( IP) detectors and their total thicknesses are discussed in Section 4. The radiation hardness of the Si(IP) detectors is also explored in this section. The CsI(T1) portion of the * Corresponding author. Tel. fl 812 855 3767, fax +l 812 855 8300, e-mail [email protected]. ’ Present address: AECL, Chalk River Laboratories, Chalk River. Ontario KOJ 1JO. Canada. Elsevier Science B.V. SSD/O168-9002(95)00803-9
telescope is presented in Section 5. Calibration of the ion chamber telescopes is discussed in Section 6. Channeling effects which limit both particle identification and energy resolution are described in Section 7. The performance of the ion chamber telescopes in heavy ion experiments is described in Section 8.
2. Mechanical design Fig. 1 shows a cross-sectional view of an ion chamber telescope. This design is intended to provide easy access to the silicon detector in a compact geometry. The detector housing is a 154 mm long square based trapezoid, measuring 58 mm on each side at the back and 38 mm on each side at the front. The housing was constructed by welding four pieces of 1 mm thick stainless steel, which comprise the sides of the housing, to stainless steel flanges which form the front and back faces of the detector. After welding, each can was helium leak checked to ensure its vacuum integrity. The entrance window of the telescope is a 1.5 pm aluminized Mylar foil glued to a 25 mm square aluminum window frame which is 3.2 mm thick. The window seals against an ‘O’ring mounted on the front weld flange of the detector. Tests demonstrate that this window design is capable of sustaining a differential pressure of at least 140 Ton without support wires. All the active elements of the telescope are mounted in a self-supporting fashion on an aluminum flange which constitutes the back of the telescope. The stainless steel can slides over the active elements to provide the gas enclosure. Hermetic Lemo connectors mounted on the back aluminum flange of the detector provide easy access to all input and
Light guide
PIN Photodiode
CsI(TI)
Window
Field shaping rings
Photodiode Preamplifier
Si detector 3cm x 3cm 500 pm
154 mm
4
Fig. 1.
Cross-sectional view of aa ion chambertelescope.
output signals. Gas flow through the detector is provided by means of miniature gas feedthroughs mounted on the back flange.
3. Ionization chamber The first active element of the telescope is a 55 mm long axial-field ionization chamber located directly after the window. The design of the axial-field ionization chamber is patterned after earlier designs of axiai-field ion chambers {91. In this ion chamber the axial-field is shaped at its edges by seven square copper rings. The rings are 2 mm thick and am spaced 4 mm apart. The center copper ring serves as the anode of the ion chamber. Due to the inherently weak trans-
-3
5 _...i...,.i 00.00 0.25
0.50 AE in
0.75
CF,
1.00
.
1.25
1.50
(MeV)
F;- Q bn chamber resolutionas a function of depositedenergy.
verse field of this design, a doubly aluminized polyethylene film with an areal density of 50-155 pg/cm2 [lo] is stretched across the anode ring and both sides of the foil are electrically shorted to the anode ring. This geometry results in efficient charge collection with only minor additional energy loss as the particles traverse the foil. Due to the advance preparation required for the stretched polypropylene foil, in some of the telescopes this foil has been replaced by 1.5 pm doubly aluminized Mylar which is co~e~i~ly available. Alternative charge collection methods can be used to lower the telescope’s threshold [ 91. The standard operating pressum range of the ionization chamber is 18-30 Torr of CF4 gas. The charge collection efficiency as a function of reduced electric field strength (E/P) was tested in the range 0.5 to 4.0 V/ (cmTorr) . The ionization chamber telescope operating at a pressure of 30 Ton of CFJ gas was exposed to ty particles originating from a collimated 24’Am source. The reduced electric field strength was varied over a broad range and the total charge collected on the anode was measured. No measurable dependence of the charge collection on the field strength in this range was found. To assess the intrinsic resolution of the ionizadon chamber, the resolution of the collimated “‘Am source was measured at 15, 30, 45, and 60 Torr of CF+ As Fig. 2 clearly shows, the resolution decreases as the energy deposit in the ionization chamber increases. At the higher pressures, the resolution saturates at approximately 11%. Thus, the resolution of this ionization chamber design is approximately 120 keV. Measurements at higher field strengths indicated that re-combination effects at the higher pressures did not provide a limitation. The charge collection efficiency as a function of position was also tested. In this design, the presence of the central anode foil should result in charge collection independent of the position of the ionizing track. To test this hypothesis, a collimated 241Am source was scanned along the face of the ionization chamber, Charge collection at the edges of
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the detector was slightly higher than charge collection at the center of the counter. A maximum variation of 2% was measured.
4. Ion implanted silicon detectors The second element of the detector telescope is a 500 pm ion implanted, SiO2 passivated silicon detector, from Micron Semiconductor [ 111, located directly behind the ion chamber. The active area of this detector is 30 mm x 30 mm. The silicon wafer is mounted on a 37 mm x 40 mm printed circuit board which is 1.28 mm thick. The printed circuit board is a window frame design which allows the silicon wafer to be mounted in transmission mode. The front, n+, side of the detector is oriented towards the entrance window and the detector is positively biased. The preamplifiers for the ionization chamber and silicon detector were designed by Michigan State University (MSU-NSCL). To limit the importance of cable capacitance, the preamplifiers for the ion chamber and the silicon are located external to the can, approximately 25 cm away. To further suppress noise pickup between the detectors and the preamps, doubly shielded 50 R coaxial cable [ 121 were used at the preamp inputs. These preamplifiers are capable of operation in vacuum making the telescopes suitable for experiments in large scattering chambers. In the following sections we discuss the determination of the Si( IP) operating voltage, measurement of the dead layers on the front and back of the Si(IP) detectors, the overall thicknesses of the Si( IP) detectors and their radiation hardness.
4. I. Depletion voltage Previous experience with ion implanted silicon detectors [ 131 indicated that complete charge collection at the back of the Si(IP) detectors is not achieved when operating the detectors at the depletion voltage quoted by the manufacturer. In order to determine the proper operating voltages, an 24’Am (Ysource (E, = 5.485 MeV) was placed facing the back of each Si(lP) detector. The top panel of Fig. 3 shows the resulting (Yenergy spectra from one of the Si(IP) detectors using the manufacturer’s stated depletion voltage. The long, low energy tail is due to incomplete charge collection in the Si( IP) detector. The operating voltage was then raised until the low energy tail disappeared, as shown in the bottom panel, and the peak position remained constant. In Fig. 4 the overbias voltage necessary (with respect to the manufacturer’s quoted full depletion voltage) to achieve full depletion is shown. The percent overbias ranges from 20% to almost 100%. The large variations observed suggest the full depletion bias for individual detectors needs to be carefully measured.
Fig. 3. Energy spectra for alpha particles incident nomal to the front face on S-3 operated at (a) 39 V, the listed depletion voltage, and (b) 62 V.
4.2. Dead layer measurements The dead layers of all the silicon detectors were measured using the “‘Am a source. The measurements were made by comparing the signal obtained with alphas incident normal to the front face of the detector and at an angle of 60” with respect to the normal. Alphas incident at 60’ will lose twice as much energy in the dead layer than alphas at normal incidence. Using this technique both the front and back dead layers were measured. The measured dead layers range from 0.5 to 1.O +rn for the front and 0.5 to 1.2 km for the back of the detector. The distribution of measured dead layers is shown in Fig. 5. Also shown in Fig. 5 are the dead layers measured for the front and back surfaces of segmented 5 cm x 5 cm Si(IP) Si detectors purchased from Micron Semiconductor. The dead layers on these detectors range from 0.4 to 1.1 pm at the front surface and 0.7 to 1.2 ,um for the back surface. 4.3. Si(lPp) detector thickness The overall thickness of the Si( IP) detectors as stated by Micron Semiconductor ranged between 502 and 512 pm. The thickness of each Si(IP) detector was independently measured using proton beams from the tandem accelerator at the University of Notre Dame. Beams of 6, 10 and 12 MeV protons were scattered off a thin Au target (= 60 pg/cm2 Au foil, backed by a 7 pg/cm2 carbon foil). A
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and Me&h. in phys. Res. A 368 (1996)
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10
MeV p t Au
18 MeV F,-i-Au
i 60 t -. i
0
n
I.
88
me
20
I
n
00
-
--L-..
400
Detector
Number
Fig. 4. Percent overbias necessary to achieve full depletion for various
detectors. Solid circles represent 3 cm x 3 cm detectors aad solid squares represent 5 cm x 5 cm detectors.
-
3cm
x 3~x1 back
~
5cm
x 5cm
front
-
5cm
x 5cm
back
3 i
1
0
L_L 0.4
0.6
Si Dead Layer
08
1.0
Thickness
800
1000
Silicon Energy (ADC Channelj
15
15
5
600
12
(pm)
Fig. 5. Distribution of dead layers measured for front aad back faces of the detectors.
Fig,6. Energy spectra for 10 MeV p at @tab= 40’ measured in the back Si detector with (a) no Si(IP) detector between the target and the back Si, (b) a thick Si(IP) detector between the target and the back Si, and (c) the thin Si( IP) detector.
1000 pm surface barrier Si detector, sufficiently thick to stop the scattered protons, was placed behind a collimator at 8 = 40”. For each Si(IP) detector, the energy deposited in the 1000 pm Si detector was measured with and without the Si(IP) detector being in front of it. The thickness of each Si( IP) detector was then determined by the shift in the energy deposited in the 1000 ,um Si detector. The top panel of Fig. 6 shows the energy spectra in the 1000 pm detector when no intervening detectors were placed between it and the target for Ep = 10 and 12 MeV. For each beam energy, the higher and lower energy peaks visible correspond to the elastic sca~e~ng off the “‘Au and MiC respectively. The data shown in the middle panel were taken with a nominally 500 pm Si(IP) detector placed between the target and the back Si with Ep = 10 MeV. Energy loss calculations [ 141 indicate the the shift in the energy spectrum corresponds to a thickness of w 487 pm. The bottom panel shows the results of placing a detector, suspected of being thinner, in front of the back Si, again with Ep = 10 MeV. Clearly this detector is much thinner, % 136 pm based on the shift in the peak. Overall a total of 11 detectors were measured, eight were found to be 494 ,om, two were 487 pm. and one was 136 &urn.The ~lationship between the quoted and measured thicknesses is shown in Fig. 7. A total of four detectors were scanned in order to check the uniformity of the Si(IP) thickness. The scanning was
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Instr. and Meth. in Phys. Res. A 368 (1996)
4.4. Radiation hardness
8
6
-4
Detector
10
1%
Number
Fig. 7. Comparison of actual thicknesswith quoted thickness.
accomplished by moving the position of the ion implanted detector relative to the collimator and back detector. The results of the scans are shown in Fig. 8. For all four detectors the maximum variations are no more than 4 ,um. For the three thick detectors, N 494 I*m, no systematic variations in the thickness were observed. For the thin detector, =: 136 ,um, there is a definite systematic variation in the thickness, one side being thicker, by 4 pm, than the other side.
_
I!16
I
c
‘-’
90
Angle
95
100
-i
i 105
(degrees)
Rg. 8. Thickness ~nifo~ty of 500 and 136 &tn detectors. Different symbols indicate different Si( IP) detectors.
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of Si{lP)
detectors
One of the significant questions in the measurement of heavy-ion reactions with silicon detectors is the radiation damage incurred by the flux of reaction products on the silicon crystal. While this question has been well studied for surface barrier type detectors, for ion implanted silicon detectors quantitative questions regarding radiation damage are still unanswered. To address this question we examined the damage caused to a set of five detectors exposed to a large flux of fission fragments from the reaction ‘$le+2’2Th at El& = 200 MeV. The beam accelerated by the K200 cyclotron at Indiana University ranged in intensity from 10 to 40 e nA. We monitored the leakage current of the detectors as a measure of the radiation damage to the detectors. As can be seen in Fig. 9 the leakage current increases with the total number of incident fission fragments. As the total number of fission fragments incident on the crystal increases from I x I OSto 2 x 1Or4the leakage current increases from approximately 3 to =38 ,u,A. However, the leakage current measured during the experiment does not accurately tetlect the radiation damage caused to the silicon crystals since halting the incident fission since measurement of the leakage current directly after the experiment yielded leakage currents of approximately 10 PA. Perhaps charging of the silicon due to the ionizing radiations [ 61 is mainly responsible for the increasing leakage current during the experiment. In order to maintain a constant temperature for the silicon detectors during the experiment, the CR gas was flowed through a copper coil which was immersed in a constant tem~ra~re circulating bath prior to its ~st~bution to the individual detectors. This method, however, proved an ineffective means to effect the silicon detectors temperature due to the low pressure of the gas. We have also investigated the possibility of re-annealing the damaged detectors following the experiment. In a vacuum vessel we mounted the silicon detector so that it faced a block of copper. The copper block was resistively heated to a temperature of SO’C. The silicon wafer reached a temperature of 8O“C iS°C due to radiative heating. The temperature of the silicon wafer was determined by mounting a the~ocouple on a dummy detector. The silicon detector was maintained at the elevated tempera~re for an extended period. Periodically, the heating process was interrupted and the leakage current of the silicon was measured at room temperature. The dependence of the leakage current as a function of the total re-annealing time is shown in Fig. 10. In the upper panel, the percent change in the leakage currents after re-annealing for as long as -350 h is depicted. The change varied from zero to almost 40%. Two of the five detectors subjected to the procedure showed no decrease in their leakage current. The remaining three detectors exhibited varying degrees of improvement. While the reason two crystals showed no imp~vement is not completely clear, these two detectors were not re-annealed as long as some of the others. As can be seen in the lower panel, the decrease
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in P!z,w. Rex A 368 (1996)709-718
‘-1-‘-,
I’--
30 /
20 I-.
60
0 i-
1
u Rel. 1 C Det. 2 l Det. :3 n Det. 3 'I'net.
0
5
P
/ 20
j
0
: I
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; Cl
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: +.,.+.++++++-I I-++ ! iBElet.
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1
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I
1
A
0
IO" Number
lOL0
lOI2
of Fission
Fragments
lOI Re-- Annealing
Time
(Hours)
Fig, 9. Dependence of detector k&age current on fission fmgment flux. Derectors 2-5 have been displaced by 7, 14, 21, nod 28 pA for clarity.
Fig. 10. Effect of re-annealing radiation damaged detectors
in leakage current does not always follow a strictly linear relationship. Following the re-annealing we tested the resolution of the silicon detectors with an 24’Am a: source. The resolution for the five detectors ranged from 1.5 to 2%. The relationship between the measured leakage current and the measured resolution was found to be essentially linear indicating that re-annealing the silicon detectors does provide a measurable improvement.
The photodiode signal is processed through a charge sensitive preamplifier located directly behind the photodiode. The preamplifier was fabricated at Indiana University from an existing MSU-NSCL design [ 171. To reduce sensitivity of F%Ts in the preamplifier to electrical discharge in the gas volume, the preamplifier was potted in silicon elastomer (Dow Coming Sylgard 184). This elastomer can be readily removed in the eventuality the preamplifier needs to be repaired.
The third element of the telescope is a 30 mm thick, 37 mm square CsI(T1) scintillator crystal. Due to previous experience with the sensitivity of the light output to variations in the thallium doping with a single crystal [ 7 1, the scintillator crystals were purchased from Hilger Analytical [ 151. In order to provide a high degree of diffise reflectivity, the CsI(T1) crystal is wrapped with Teflon tape around the sides. So as to provide reflection of scintillation from the front face and still provide a minimal dead layer for particles entering the crystal, the crystal was wrapped with 1.5 pm aluminized Mylar on the front face. The back face of the Csf( Tl) crystal is optically coupled to a Plexiglas light guide which in turn is optically coupled to a 2 cm x 2 cm Hamamatsu photodiode [ 16 J. The silicon detector and the CsI( Tl) crystal are spaced apart by a Plexiglas spacer which provides precise positioning relative to the back flange.
6.1. SijlP) detector
The crucial element in the energy calibration of the telescope is the energy calibration of the Si( IP) detector. The Si(lP) crystal is calibrated using a a precision electronic pulser at the input of the charge sensitive pre~plifier. The pulser itself is calibrated with reference to the 8.785 MeV a particles from **sTh. In addition, due to our knowledge of the Si( IP) overall thickness we utilize the characteristic energies at which specific ions (eg. ‘%e, ‘Be, ‘*%, “C, “0) punch through the silicon wafer. Finally, primary beams of 4He, i2C, and I60 accelerated to E/A = 22 MeV and degraded to energies as low as E/A = 12 MeV were also used to calibrate the detector. All these techniques resulted in consistent energy calibrations to within l-2%. These crosscalibrations revealed that a either pulser or a calibration
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Instr. and Meth. in Phys. Res. A 368 (1996) 709-718
based on punchthrough particles is sufficient to calibrate the Si (IP) detector at this level. 6.2. ionization chmber The ionization chamber section of the telescope was calibrated using a precision pulser at the input of the charge sensitive pre-amplifier. This technique has been used both as a linearity test, as well as, to provide an absolute calibration. In addition, points have been selected on the characteristic elemental curves in the ionization chamber-Si( IP) AE-E two dimensional plot. By using the energy deposited in the Si (IP) crystal together with the measured 2 of the incident ion and energy loss calculations [ 141, the ionization chamber can be calibrated independently of the pulser. Comparison of these two methods yields agreement of between 5 and 10%. While the electronic pulser is used as the final calibration, the energy loss technique is used as an assessment of the unce~ainty. 6.3. Csi(T1) cryvstal To calibrate the CsI(T1) detector we exposed the telescopes to direct beams of d, 4He, ‘*C, and “0 accelerated by the K 1200 cyclotron at Michigan State University. The measured light output deposited by these beams along with punch through p, d, t, ‘He, and 4He particles are plotted as a function of the energy deposited in the crystal in Fig. 11. While the light charged particles follow essentially a single linear relationship, the oxygen and carbon data deviate from this relation. This deviation is probably due to a “quenching” effect which has been previously studied [ l&-20]. It should be stressed that the deviation does not result simply from a different pulse shape for the heavy-ions due to the charge integration by the preamplifier of the entire signal. The difference in quenching between the C and 0 fragments can be seen in the lower panel of the figure where the thinner Si( IP) detector ( 136 pm) of the telescope provides a lower threshold for particles entering the CsI(T1). These differences clearly indicate that calibration of CsIfTl) detectors requires careful attention to account for the quenching effect. The solid line in both panels of the figure represents a linear fit to the 2 = 1 and Z = 2 data. 6.4. Unijkmity of t~fliMrn concentration Since the light output of the CsI(T1) crystal for a given incident LYenergy is known to depend on the concentration of the thallium dopant, we have investigated the uniformity of the thallium concentration within two CsI(T1) ingots. These ingots are cylindrical and measure 20 cm in diameter by 18-20 cm in height. Both crystals were grown using the Czochralski technique [ 21,223. The ingot was cut into cylindrical slices ~25 mm thick along the growth axis of the crystal. The samples were taken along the perimeter of each cylindrical slice. A total of 18 samples from 5
Fig.
11.Dependence
Solid crystal. MeVlu
squares
of light output on deposited energy for various particles.
represent
3*4He
nuclei
which
punch
through
Open squares represent n beams of 22 MeV/u, which
to deuteron
the SiflP)
detector.
beams of the same energy.
the scintill~or
pass through
response to “C and 22.
and
17, and 12 MeVfu
Open
I60
Open
circles
and
and 12
correspond
and closed diamonds
for 22 MeV/u flower
the Csl(T1)
16 MeV/u,
depict
17 MeVlu
panel).
slices in two ingots were analyzed. The thallium concentration was determined using atomic absorption spectroscopy. The uncertainty in the absolute concentration is estimated to be & 100 ppm while the relative error between measurements is estimated to be 50 ppm. Both crystals were grown to a specification of 2 1600 ppm of thallium. The measured thallium concentrations are shown in Fig. 12. Despite large variations in the Tl concentration, detectors fabricated from this material do not exhibit large variations in light output as a unction of position. This result is in agreement with the flat light response for alpha particles in this concentration range [ 23,241. ~u~e~ore, this high thallium concentration allows the separation of lithium and beryllium isotopes based on pulse shape discrimination [ 131.
7. Channeling effects A known problem with the early use of silicon detectors for the detection of heavy-ions involved the process of channeling [ 251. If the silicon wafers are cut on a Si crystal axis, e.g. ( 1 1 0) axis, an incident heavy-ion can strike a region of reduced electron density and consequently deposit less energy in the silicon crystal. This problem can
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Instr. and Meth. in Phys. Res. A 368 (1996) 709-718
Det. 4
Fig. 13. Channeling effects of 22 MeVIA carbon ions in a nominally 500 pm Si(IP) detector. The channeled ions (estimated by the gated region shown) account for 10% of the incident ions.
6 (degrees) Fig. 12. Position dependence of thallium concentration in two CsI(TI) ingots. (a) Ingot DF-22-6650, (b) Ingot DF-22-66 17. The different symbols correspond to different cylindrical slices in the ingots.
be avoided by cutting the silicon crystal off-axis. To determine whether the silicon detectors purchased from Micron Semiconductor, which are cut on the ( 1 10) axis [ 261, exhibit this problem we exposed the ion chamber telescopes (without gas) to low intensity ‘*C and I60 beams. The ions impinged on the center of the detectors. A two dimensional plot of the energy deposited in the silicon versus the energy deposited in the CsI(T1) crystal is shown in Fig. 13. This spectra of incident 22 MeV/nucl. ‘*C ions shows a long tail extending to higher values of the CsI( Tl) energy. This feature is characteristic of the channeling process. The magnitude of the channeling effect has been estimated with the gate shown in the figure. In the ten detectors measured, the number of carbon ions which channeled ranged from ~1 to N 10%. This difference is not unexpected since channeling depends sensitively on the exact angle at which the crystal is cut. Also shown in Fig. 13 are lines corresponding to the adjacent elements boron and nitrogen. Since channeled ions do not follow the characteristic energy loss curves, channeling results in mis-identification of the particle via the BE-E technique. Off-axis Si( IP) detectors are presently available at a substantially higher cost.
8. Operating characteristics The performance of the ion chamber telescopes is summarized in Figs. 14 and 15. These figures demonstrate the particle identification capabilities of the telescopes. As can be seen in Fig. 14 the gas detector clearly provides elemental identification from Z = 1 to Z = 14. Isotopic identification achieved by the AE-E technique allows separation of oxygen isotopes as seen in Fig. 15. The separation of oxygen isotopes seen in this figure is clearly due to the good planarity of the detectors described previously. Also clearly visible in the spectrum are three lines above the triton line. These lines correspond to pileup within the same detector of particles which originate from different beam pulses. The three lines (in order of increasing Si( IP) energy deposit) correspond to two protons, a proton and a deuteron, and a proton and a Won. These pileup events represent a small fraction of the single proton yield and are due to the high beam current and large solid angle in this experiment. Similarly, above the ‘He line are three bands which correspond to proton-alpha, deuteron-alpha, and t&on-alpha pairs. Such events can be discriminated against by use of a multiple hit time-to-digital converter. Typical energy spectra measured with the ion chamber telescopes are shown in Fig. 16. These backward angle spectra, Blab= 120” are peaked at low energies. The arrows in the figure correspond to thresholds achievable with phoswiches (E/A = 3 MeV) [ 71. The need of the low thresholds of the new telescopes (E/A 5 0.8 MeV) is clearly evident since the reduced barrier for IMF emission is a crucial aspect of the multifragmentation phenomenon.
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111
_lOR i m
448
.z’
h 384 z g 320 8 t& 256 s
192 Y
2
128 64 0
0
64
128 192 256 320 384 448 Es, (AX Channel)
Fig. 14. Two dimensional plot of energy deposited in ion-chamber versus energy depositedin Si( IP) detector.
Fig. 16. Energy spectra of oxygen fragments measured in “N+‘97Au ,$,/A = 156 MeV.
at
9. Conclusions 1800 X00/
-
In summary, we have constructed high resolution low threshold ionization chamber telescopes to study the multifragment decay of highly excited nuclear systems. These telescopes have thresholds which correspond to ~0.8 MeVlu. Element identification is easily achieved for Z = 1 to Z = 14. The detectors are also capable of resolving isotopes for Z 5 10 for particles which punch through the Si(IP) detector (eg, E/A 2 9.5 and 20 MeV for ‘Li and *‘Ne, respectively. Good particle identification coupled with good energy resolution and low thresholds make this type of detector a preferred choice for further multifragmentation studies.
Acknowledgement
EcsI (ADC Channel) Fig. 15. Two dimensionalplot of energy depositedin Si( IP) detectorversus energy depositedin CsI(Tl) crystal.
We would like to thank Dr. J. Kolata. M. Belbot, and K. Lamkin at Notre Dame University for their help during our run at the tandem facility. We are grateful to L. Sexton at Indiana University for the detector fabrication. One of the authors (R.D.) gratefully acknowledges the support of the Sloan Foundation through the A.P. Sloan Fellowship program. This work was supported by the U.S. Depanment of Energy under Grant No. Do-FGOZ-92ER40714 (Indiana University) and the National Science Foundation under Grant Nos. NSF-PHY94-027671 (Notre Dame), NSF-PHY93-NUCRES-14783 (Indiana University) and NSF-PHY-9214992 (Michigan State University).
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References [ I J G. Bertsch and I?J. Siemens, Phys. I_&. 126 ( 1983) 9. [Z] I. Aichelin, G. Peilett, A. Bobnet. A. Rosenhatter, H. Stocker, and W. Greiner, Phys. Rev. C 37 (1988) 2451. (31 R.T. de Sousa et al., Phys. I.&. B 268 (1991) 6. 141 D.R. Bowman et al., Phys. Rev. Lea. 67 (1991) 1527. [S] CA. OgiIvie et al., Phys. Rev. Lett. 67 (1991) 1214. [6] K. Kwiatkowski et al., Phys. Rev. Len. 74 ( 1995) 3754. [7] R.T. de Souza et al., Nucl. Ins& and Meth. A 295 (1990) 109. [ 81 D. Dram et al., Nucl. Instr. and Meth. A 281 ( 1989) 528. [ 91 K. Kwiatkowski et al., Nucl. Instr. and Meth. A 299 (1990) 166. ] IO] D.M. Barrus aad R.L. Blake, Rev. Sci. Instr. 48 (1977) 116. [ 111Micron Se~canduc~r Limited, Lancing, Sussex, England. 1121 MM1 1150 superscreened cable, Permanoid Ltd., Manchester, UK. [ 13] R.T. de Souza, Private Communication. [ 14 J Energy loss calculations used the code Trim90 by J.F. Ziegler et al.: J.I? Biersack and L.G. Haggmark, NucI. Insw. and Meth. 174 (1980) 257: J.F. Ziegler, J.P. Biersack, and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon, New York, 1984).
1151 Hilger Analytical Ltd., Westwood, Margate, Kent, England, CT9 4JL. 1161 Hamama tsu Photonics K.K., Hamamatsu City, Japan. [ 171 W.G. Gong et al., Nucl. Ins& and Meth. A 287 (1990) 639. [IS] D. Horn, G.C. Ball, A. G~ndo-U~b~. E. Hagberg, R.B. Walker, R. Laforest, and J. Pouliot, Nucl. Ins& and Meth. A 320 ( 1992) 273. 1191 K. Michaelian and A. Menchaca-Rocha Phys. Rev. B 49 (1994) 15550; K. Michaelian, A. Menchaca-Rocha, and E. Belmont-Moreno, Nucl. Ins& and Meth. A 356 (1995) 297. 1201 D. Fox, D. R. Bowman, G. C. Ball, A. Galindo-U~b~, E. Hagberg, D. Horn L. Beaulieu, and Y. Larochelle, submitted to Nucl. Insu. and Meth. 1211 J. Czochralski, Z. Phys. Chem. 92 (1918) 219. [ 221 J.C. B&e, The Growth of Crystals fmm the Melt (North Holland, Ams~rd~, 1965). [ 23 j I.A. Tsidin, S.N. Komnik, and L.M. Soifer, Optics and Spectroscopy, VI, 3, March (1959) pp. 265-266. 1241 Proc. 8th Scintillation and Semiconductor Counter Symposium, Washington, DC, March 1-3, 1962. 1251 D.V. Morgan, Channeling Theory, Observations, and Apphcations (Wiley, 1973). [ 261 Cohn Wilburn, Micron Semiconductor, private communication.
and Methods in Physics Research A 368 (1996)
709-718
NUCLEAR INSTRUMENTS P METHODS IN PHYSICS RESEARCH
Section A
EISEWIER
High resolution low threshold detector telescopes for multifragmentation studies D. Fox a**, R.T. de Souzaa**, S.L. Chen a, B. Davin a, T.M. Hamilton a, Y. Lou a J. Dorsett a, J. Ottarson b a Department of h National
Chemistry
and Indiana
Superconducting
Universitj
Cyclotron
Cyclotron
Laboratory,
Faciliry,
Michigan
Indiana
University,
State University,
Bloomington,
East Lansing,
IN 47401,
MI 48824,
USA
USA
Received 15 August 1995
Abstract
A set of compact ionization chamber-Si( IP)-CsI(TI) telescopes for multifragmentation studies are described. These ion chamber telescopes can be used either individually, or to augment the MSU Miniball 47r array. The characteristics of the large-area passivated, ion implanted Si detectors are investigated. Statistics on the overall planarity of these detectors as well as, the thickness of dead layers present at the surface of this type of detectors are examined.
1. Introduction
Nuclei under extreme conditions of temperature and density may decay via disassembly of the nuclear system [ 1,2]. This new decay mode is currently a topic of considerable interest [ 3-61. In order to study the multiparticle final state characteristic of this new decay mode, exclusive measurements with 477 detectors are necessary [ 7,8]. To characterize the multifragmentation of the nuclear system, detection and identification of low energy fragments is required. At present, however, all the arrays suited for studying heavyion induced multifragmentation have relatively high thresholds at backward angles and are difficult to calibrate. To remedy this problem, a set of high resolution, low threshold telescopes, which can be used to augment the MSU Miniball by providing a reference measurement, have been constructed. These telescopes consist of an axial-field ionization chamber-Si ( IP) -CsI (Tl) stack. This paper describes both the characteristics and shortcomings of the individual detector elements, as well as, the performance of the detector telescope. This paper is organized as follows. The design of the ion chamber telescopes is described in Section 2. The performance of the axial-field ionization chamber is explored in Section 3. Measurements of the dead layers of the Si( IP) detectors and their total thicknesses are discussed in Section 4. The radiation hardness of the Si(IP) detectors is also explored in this section. The CsI(T1) portion of the * Corresponding author. Tel. fl 812 855 3767, fax +l 812 855 8300, e-mail [email protected]. ’ Present address: AECL, Chalk River Laboratories, Chalk River. Ontario KOJ 1JO. Canada. Elsevier Science B.V. SSD/O168-9002(95)00803-9
telescope is presented in Section 5. Calibration of the ion chamber telescopes is discussed in Section 6. Channeling effects which limit both particle identification and energy resolution are described in Section 7. The performance of the ion chamber telescopes in heavy ion experiments is described in Section 8.
2. Mechanical design Fig. 1 shows a cross-sectional view of an ion chamber telescope. This design is intended to provide easy access to the silicon detector in a compact geometry. The detector housing is a 154 mm long square based trapezoid, measuring 58 mm on each side at the back and 38 mm on each side at the front. The housing was constructed by welding four pieces of 1 mm thick stainless steel, which comprise the sides of the housing, to stainless steel flanges which form the front and back faces of the detector. After welding, each can was helium leak checked to ensure its vacuum integrity. The entrance window of the telescope is a 1.5 pm aluminized Mylar foil glued to a 25 mm square aluminum window frame which is 3.2 mm thick. The window seals against an ‘O’ring mounted on the front weld flange of the detector. Tests demonstrate that this window design is capable of sustaining a differential pressure of at least 140 Ton without support wires. All the active elements of the telescope are mounted in a self-supporting fashion on an aluminum flange which constitutes the back of the telescope. The stainless steel can slides over the active elements to provide the gas enclosure. Hermetic Lemo connectors mounted on the back aluminum flange of the detector provide easy access to all input and
Light guide
PIN Photodiode
CsI(TI)
Window
Field shaping rings
Photodiode Preamplifier
Si detector 3cm x 3cm 500 pm
154 mm
4
Fig. 1.
Cross-sectional view of aa ion chambertelescope.
output signals. Gas flow through the detector is provided by means of miniature gas feedthroughs mounted on the back flange.
3. Ionization chamber The first active element of the telescope is a 55 mm long axial-field ionization chamber located directly after the window. The design of the axial-field ionization chamber is patterned after earlier designs of axiai-field ion chambers {91. In this ion chamber the axial-field is shaped at its edges by seven square copper rings. The rings are 2 mm thick and am spaced 4 mm apart. The center copper ring serves as the anode of the ion chamber. Due to the inherently weak trans-
-3
5 _...i...,.i 00.00 0.25
0.50 AE in
0.75
CF,
1.00
.
1.25
1.50
(MeV)
F;- Q bn chamber resolutionas a function of depositedenergy.
verse field of this design, a doubly aluminized polyethylene film with an areal density of 50-155 pg/cm2 [lo] is stretched across the anode ring and both sides of the foil are electrically shorted to the anode ring. This geometry results in efficient charge collection with only minor additional energy loss as the particles traverse the foil. Due to the advance preparation required for the stretched polypropylene foil, in some of the telescopes this foil has been replaced by 1.5 pm doubly aluminized Mylar which is co~e~i~ly available. Alternative charge collection methods can be used to lower the telescope’s threshold [ 91. The standard operating pressum range of the ionization chamber is 18-30 Torr of CF4 gas. The charge collection efficiency as a function of reduced electric field strength (E/P) was tested in the range 0.5 to 4.0 V/ (cmTorr) . The ionization chamber telescope operating at a pressure of 30 Ton of CFJ gas was exposed to ty particles originating from a collimated 24’Am source. The reduced electric field strength was varied over a broad range and the total charge collected on the anode was measured. No measurable dependence of the charge collection on the field strength in this range was found. To assess the intrinsic resolution of the ionizadon chamber, the resolution of the collimated “‘Am source was measured at 15, 30, 45, and 60 Torr of CF+ As Fig. 2 clearly shows, the resolution decreases as the energy deposit in the ionization chamber increases. At the higher pressures, the resolution saturates at approximately 11%. Thus, the resolution of this ionization chamber design is approximately 120 keV. Measurements at higher field strengths indicated that re-combination effects at the higher pressures did not provide a limitation. The charge collection efficiency as a function of position was also tested. In this design, the presence of the central anode foil should result in charge collection independent of the position of the ionizing track. To test this hypothesis, a collimated 241Am source was scanned along the face of the ionization chamber, Charge collection at the edges of
D. Fox et al./Nucl.
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Meth. in Phys. Res. A 368 (1996) 709-718
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the detector was slightly higher than charge collection at the center of the counter. A maximum variation of 2% was measured.
4. Ion implanted silicon detectors The second element of the detector telescope is a 500 pm ion implanted, SiO2 passivated silicon detector, from Micron Semiconductor [ 111, located directly behind the ion chamber. The active area of this detector is 30 mm x 30 mm. The silicon wafer is mounted on a 37 mm x 40 mm printed circuit board which is 1.28 mm thick. The printed circuit board is a window frame design which allows the silicon wafer to be mounted in transmission mode. The front, n+, side of the detector is oriented towards the entrance window and the detector is positively biased. The preamplifiers for the ionization chamber and silicon detector were designed by Michigan State University (MSU-NSCL). To limit the importance of cable capacitance, the preamplifiers for the ion chamber and the silicon are located external to the can, approximately 25 cm away. To further suppress noise pickup between the detectors and the preamps, doubly shielded 50 R coaxial cable [ 121 were used at the preamp inputs. These preamplifiers are capable of operation in vacuum making the telescopes suitable for experiments in large scattering chambers. In the following sections we discuss the determination of the Si( IP) operating voltage, measurement of the dead layers on the front and back of the Si(IP) detectors, the overall thicknesses of the Si( IP) detectors and their radiation hardness.
4. I. Depletion voltage Previous experience with ion implanted silicon detectors [ 131 indicated that complete charge collection at the back of the Si(IP) detectors is not achieved when operating the detectors at the depletion voltage quoted by the manufacturer. In order to determine the proper operating voltages, an 24’Am (Ysource (E, = 5.485 MeV) was placed facing the back of each Si(lP) detector. The top panel of Fig. 3 shows the resulting (Yenergy spectra from one of the Si(IP) detectors using the manufacturer’s stated depletion voltage. The long, low energy tail is due to incomplete charge collection in the Si( IP) detector. The operating voltage was then raised until the low energy tail disappeared, as shown in the bottom panel, and the peak position remained constant. In Fig. 4 the overbias voltage necessary (with respect to the manufacturer’s quoted full depletion voltage) to achieve full depletion is shown. The percent overbias ranges from 20% to almost 100%. The large variations observed suggest the full depletion bias for individual detectors needs to be carefully measured.
Fig. 3. Energy spectra for alpha particles incident nomal to the front face on S-3 operated at (a) 39 V, the listed depletion voltage, and (b) 62 V.
4.2. Dead layer measurements The dead layers of all the silicon detectors were measured using the “‘Am a source. The measurements were made by comparing the signal obtained with alphas incident normal to the front face of the detector and at an angle of 60” with respect to the normal. Alphas incident at 60’ will lose twice as much energy in the dead layer than alphas at normal incidence. Using this technique both the front and back dead layers were measured. The measured dead layers range from 0.5 to 1.O +rn for the front and 0.5 to 1.2 km for the back of the detector. The distribution of measured dead layers is shown in Fig. 5. Also shown in Fig. 5 are the dead layers measured for the front and back surfaces of segmented 5 cm x 5 cm Si(IP) Si detectors purchased from Micron Semiconductor. The dead layers on these detectors range from 0.4 to 1.1 pm at the front surface and 0.7 to 1.2 ,um for the back surface. 4.3. Si(lPp) detector thickness The overall thickness of the Si( IP) detectors as stated by Micron Semiconductor ranged between 502 and 512 pm. The thickness of each Si(IP) detector was independently measured using proton beams from the tandem accelerator at the University of Notre Dame. Beams of 6, 10 and 12 MeV protons were scattered off a thin Au target (= 60 pg/cm2 Au foil, backed by a 7 pg/cm2 carbon foil). A
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and Me&h. in phys. Res. A 368 (1996)
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10
MeV p t Au
18 MeV F,-i-Au
i 60 t -. i
0
n
I.
88
me
20
I
n
00
-
--L-..
400
Detector
Number
Fig. 4. Percent overbias necessary to achieve full depletion for various
detectors. Solid circles represent 3 cm x 3 cm detectors aad solid squares represent 5 cm x 5 cm detectors.
-
3cm
x 3~x1 back
~
5cm
x 5cm
front
-
5cm
x 5cm
back
3 i
1
0
L_L 0.4
0.6
Si Dead Layer
08
1.0
Thickness
800
1000
Silicon Energy (ADC Channelj
15
15
5
600
12
(pm)
Fig. 5. Distribution of dead layers measured for front aad back faces of the detectors.
Fig,6. Energy spectra for 10 MeV p at @tab= 40’ measured in the back Si detector with (a) no Si(IP) detector between the target and the back Si, (b) a thick Si(IP) detector between the target and the back Si, and (c) the thin Si( IP) detector.
1000 pm surface barrier Si detector, sufficiently thick to stop the scattered protons, was placed behind a collimator at 8 = 40”. For each Si(IP) detector, the energy deposited in the 1000 pm Si detector was measured with and without the Si(IP) detector being in front of it. The thickness of each Si( IP) detector was then determined by the shift in the energy deposited in the 1000 ,um Si detector. The top panel of Fig. 6 shows the energy spectra in the 1000 pm detector when no intervening detectors were placed between it and the target for Ep = 10 and 12 MeV. For each beam energy, the higher and lower energy peaks visible correspond to the elastic sca~e~ng off the “‘Au and MiC respectively. The data shown in the middle panel were taken with a nominally 500 pm Si(IP) detector placed between the target and the back Si with Ep = 10 MeV. Energy loss calculations [ 141 indicate the the shift in the energy spectrum corresponds to a thickness of w 487 pm. The bottom panel shows the results of placing a detector, suspected of being thinner, in front of the back Si, again with Ep = 10 MeV. Clearly this detector is much thinner, % 136 pm based on the shift in the peak. Overall a total of 11 detectors were measured, eight were found to be 494 ,om, two were 487 pm. and one was 136 &urn.The ~lationship between the quoted and measured thicknesses is shown in Fig. 7. A total of four detectors were scanned in order to check the uniformity of the Si(IP) thickness. The scanning was
D. Fox er al./Nucl.
Instr. and Meth. in Phys. Res. A 368 (1996)
4.4. Radiation hardness
8
6
-4
Detector
10
1%
Number
Fig. 7. Comparison of actual thicknesswith quoted thickness.
accomplished by moving the position of the ion implanted detector relative to the collimator and back detector. The results of the scans are shown in Fig. 8. For all four detectors the maximum variations are no more than 4 ,um. For the three thick detectors, N 494 I*m, no systematic variations in the thickness were observed. For the thin detector, =: 136 ,um, there is a definite systematic variation in the thickness, one side being thicker, by 4 pm, than the other side.
_
I!16
I
c
‘-’
90
Angle
95
100
-i
i 105
(degrees)
Rg. 8. Thickness ~nifo~ty of 500 and 136 &tn detectors. Different symbols indicate different Si( IP) detectors.
713
709-718
of Si{lP)
detectors
One of the significant questions in the measurement of heavy-ion reactions with silicon detectors is the radiation damage incurred by the flux of reaction products on the silicon crystal. While this question has been well studied for surface barrier type detectors, for ion implanted silicon detectors quantitative questions regarding radiation damage are still unanswered. To address this question we examined the damage caused to a set of five detectors exposed to a large flux of fission fragments from the reaction ‘$le+2’2Th at El& = 200 MeV. The beam accelerated by the K200 cyclotron at Indiana University ranged in intensity from 10 to 40 e nA. We monitored the leakage current of the detectors as a measure of the radiation damage to the detectors. As can be seen in Fig. 9 the leakage current increases with the total number of incident fission fragments. As the total number of fission fragments incident on the crystal increases from I x I OSto 2 x 1Or4the leakage current increases from approximately 3 to =38 ,u,A. However, the leakage current measured during the experiment does not accurately tetlect the radiation damage caused to the silicon crystals since halting the incident fission since measurement of the leakage current directly after the experiment yielded leakage currents of approximately 10 PA. Perhaps charging of the silicon due to the ionizing radiations [ 61 is mainly responsible for the increasing leakage current during the experiment. In order to maintain a constant temperature for the silicon detectors during the experiment, the CR gas was flowed through a copper coil which was immersed in a constant tem~ra~re circulating bath prior to its ~st~bution to the individual detectors. This method, however, proved an ineffective means to effect the silicon detectors temperature due to the low pressure of the gas. We have also investigated the possibility of re-annealing the damaged detectors following the experiment. In a vacuum vessel we mounted the silicon detector so that it faced a block of copper. The copper block was resistively heated to a temperature of SO’C. The silicon wafer reached a temperature of 8O“C iS°C due to radiative heating. The temperature of the silicon wafer was determined by mounting a the~ocouple on a dummy detector. The silicon detector was maintained at the elevated tempera~re for an extended period. Periodically, the heating process was interrupted and the leakage current of the silicon was measured at room temperature. The dependence of the leakage current as a function of the total re-annealing time is shown in Fig. 10. In the upper panel, the percent change in the leakage currents after re-annealing for as long as -350 h is depicted. The change varied from zero to almost 40%. Two of the five detectors subjected to the procedure showed no decrease in their leakage current. The remaining three detectors exhibited varying degrees of improvement. While the reason two crystals showed no imp~vement is not completely clear, these two detectors were not re-annealed as long as some of the others. As can be seen in the lower panel, the decrease
714
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in P!z,w. Rex A 368 (1996)709-718
‘-1-‘-,
I’--
30 /
20 I-.
60
0 i-
1
u Rel. 1 C Det. 2 l Det. :3 n Det. 3 'I'net.
0
5
P
/ 20
j
0
: I
40
; Cl
40
:_.
20
: +.,.+.++++++-I I-++ ! iBElet.
*._i_._
4
1
15 20
I
1
A
0
IO" Number
lOL0
lOI2
of Fission
Fragments
lOI Re-- Annealing
Time
(Hours)
Fig, 9. Dependence of detector k&age current on fission fmgment flux. Derectors 2-5 have been displaced by 7, 14, 21, nod 28 pA for clarity.
Fig. 10. Effect of re-annealing radiation damaged detectors
in leakage current does not always follow a strictly linear relationship. Following the re-annealing we tested the resolution of the silicon detectors with an 24’Am a: source. The resolution for the five detectors ranged from 1.5 to 2%. The relationship between the measured leakage current and the measured resolution was found to be essentially linear indicating that re-annealing the silicon detectors does provide a measurable improvement.
The photodiode signal is processed through a charge sensitive preamplifier located directly behind the photodiode. The preamplifier was fabricated at Indiana University from an existing MSU-NSCL design [ 171. To reduce sensitivity of F%Ts in the preamplifier to electrical discharge in the gas volume, the preamplifier was potted in silicon elastomer (Dow Coming Sylgard 184). This elastomer can be readily removed in the eventuality the preamplifier needs to be repaired.
The third element of the telescope is a 30 mm thick, 37 mm square CsI(T1) scintillator crystal. Due to previous experience with the sensitivity of the light output to variations in the thallium doping with a single crystal [ 7 1, the scintillator crystals were purchased from Hilger Analytical [ 151. In order to provide a high degree of diffise reflectivity, the CsI(T1) crystal is wrapped with Teflon tape around the sides. So as to provide reflection of scintillation from the front face and still provide a minimal dead layer for particles entering the crystal, the crystal was wrapped with 1.5 pm aluminized Mylar on the front face. The back face of the Csf( Tl) crystal is optically coupled to a Plexiglas light guide which in turn is optically coupled to a 2 cm x 2 cm Hamamatsu photodiode [ 16 J. The silicon detector and the CsI( Tl) crystal are spaced apart by a Plexiglas spacer which provides precise positioning relative to the back flange.
6.1. SijlP) detector
The crucial element in the energy calibration of the telescope is the energy calibration of the Si( IP) detector. The Si(lP) crystal is calibrated using a a precision electronic pulser at the input of the charge sensitive pre~plifier. The pulser itself is calibrated with reference to the 8.785 MeV a particles from **sTh. In addition, due to our knowledge of the Si( IP) overall thickness we utilize the characteristic energies at which specific ions (eg. ‘%e, ‘Be, ‘*%, “C, “0) punch through the silicon wafer. Finally, primary beams of 4He, i2C, and I60 accelerated to E/A = 22 MeV and degraded to energies as low as E/A = 12 MeV were also used to calibrate the detector. All these techniques resulted in consistent energy calibrations to within l-2%. These crosscalibrations revealed that a either pulser or a calibration
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Instr. and Meth. in Phys. Res. A 368 (1996) 709-718
based on punchthrough particles is sufficient to calibrate the Si (IP) detector at this level. 6.2. ionization chmber The ionization chamber section of the telescope was calibrated using a precision pulser at the input of the charge sensitive pre-amplifier. This technique has been used both as a linearity test, as well as, to provide an absolute calibration. In addition, points have been selected on the characteristic elemental curves in the ionization chamber-Si( IP) AE-E two dimensional plot. By using the energy deposited in the Si (IP) crystal together with the measured 2 of the incident ion and energy loss calculations [ 141, the ionization chamber can be calibrated independently of the pulser. Comparison of these two methods yields agreement of between 5 and 10%. While the electronic pulser is used as the final calibration, the energy loss technique is used as an assessment of the unce~ainty. 6.3. Csi(T1) cryvstal To calibrate the CsI(T1) detector we exposed the telescopes to direct beams of d, 4He, ‘*C, and “0 accelerated by the K 1200 cyclotron at Michigan State University. The measured light output deposited by these beams along with punch through p, d, t, ‘He, and 4He particles are plotted as a function of the energy deposited in the crystal in Fig. 11. While the light charged particles follow essentially a single linear relationship, the oxygen and carbon data deviate from this relation. This deviation is probably due to a “quenching” effect which has been previously studied [ l&-20]. It should be stressed that the deviation does not result simply from a different pulse shape for the heavy-ions due to the charge integration by the preamplifier of the entire signal. The difference in quenching between the C and 0 fragments can be seen in the lower panel of the figure where the thinner Si( IP) detector ( 136 pm) of the telescope provides a lower threshold for particles entering the CsI(T1). These differences clearly indicate that calibration of CsIfTl) detectors requires careful attention to account for the quenching effect. The solid line in both panels of the figure represents a linear fit to the 2 = 1 and Z = 2 data. 6.4. Unijkmity of t~fliMrn concentration Since the light output of the CsI(T1) crystal for a given incident LYenergy is known to depend on the concentration of the thallium dopant, we have investigated the uniformity of the thallium concentration within two CsI(T1) ingots. These ingots are cylindrical and measure 20 cm in diameter by 18-20 cm in height. Both crystals were grown using the Czochralski technique [ 21,223. The ingot was cut into cylindrical slices ~25 mm thick along the growth axis of the crystal. The samples were taken along the perimeter of each cylindrical slice. A total of 18 samples from 5
Fig.
11.Dependence
Solid crystal. MeVlu
squares
of light output on deposited energy for various particles.
represent
3*4He
nuclei
which
punch
through
Open squares represent n beams of 22 MeV/u, which
to deuteron
the SiflP)
detector.
beams of the same energy.
the scintill~or
pass through
response to “C and 22.
and
17, and 12 MeVfu
Open
I60
Open
circles
and
and 12
correspond
and closed diamonds
for 22 MeV/u flower
the Csl(T1)
16 MeV/u,
depict
17 MeVlu
panel).
slices in two ingots were analyzed. The thallium concentration was determined using atomic absorption spectroscopy. The uncertainty in the absolute concentration is estimated to be & 100 ppm while the relative error between measurements is estimated to be 50 ppm. Both crystals were grown to a specification of 2 1600 ppm of thallium. The measured thallium concentrations are shown in Fig. 12. Despite large variations in the Tl concentration, detectors fabricated from this material do not exhibit large variations in light output as a unction of position. This result is in agreement with the flat light response for alpha particles in this concentration range [ 23,241. ~u~e~ore, this high thallium concentration allows the separation of lithium and beryllium isotopes based on pulse shape discrimination [ 131.
7. Channeling effects A known problem with the early use of silicon detectors for the detection of heavy-ions involved the process of channeling [ 251. If the silicon wafers are cut on a Si crystal axis, e.g. ( 1 1 0) axis, an incident heavy-ion can strike a region of reduced electron density and consequently deposit less energy in the silicon crystal. This problem can
D. Fox et al./Nucl.
Instr. and Meth. in Phys. Res. A 368 (1996) 709-718
Det. 4
Fig. 13. Channeling effects of 22 MeVIA carbon ions in a nominally 500 pm Si(IP) detector. The channeled ions (estimated by the gated region shown) account for 10% of the incident ions.
6 (degrees) Fig. 12. Position dependence of thallium concentration in two CsI(TI) ingots. (a) Ingot DF-22-6650, (b) Ingot DF-22-66 17. The different symbols correspond to different cylindrical slices in the ingots.
be avoided by cutting the silicon crystal off-axis. To determine whether the silicon detectors purchased from Micron Semiconductor, which are cut on the ( 1 10) axis [ 261, exhibit this problem we exposed the ion chamber telescopes (without gas) to low intensity ‘*C and I60 beams. The ions impinged on the center of the detectors. A two dimensional plot of the energy deposited in the silicon versus the energy deposited in the CsI(T1) crystal is shown in Fig. 13. This spectra of incident 22 MeV/nucl. ‘*C ions shows a long tail extending to higher values of the CsI( Tl) energy. This feature is characteristic of the channeling process. The magnitude of the channeling effect has been estimated with the gate shown in the figure. In the ten detectors measured, the number of carbon ions which channeled ranged from ~1 to N 10%. This difference is not unexpected since channeling depends sensitively on the exact angle at which the crystal is cut. Also shown in Fig. 13 are lines corresponding to the adjacent elements boron and nitrogen. Since channeled ions do not follow the characteristic energy loss curves, channeling results in mis-identification of the particle via the BE-E technique. Off-axis Si( IP) detectors are presently available at a substantially higher cost.
8. Operating characteristics The performance of the ion chamber telescopes is summarized in Figs. 14 and 15. These figures demonstrate the particle identification capabilities of the telescopes. As can be seen in Fig. 14 the gas detector clearly provides elemental identification from Z = 1 to Z = 14. Isotopic identification achieved by the AE-E technique allows separation of oxygen isotopes as seen in Fig. 15. The separation of oxygen isotopes seen in this figure is clearly due to the good planarity of the detectors described previously. Also clearly visible in the spectrum are three lines above the triton line. These lines correspond to pileup within the same detector of particles which originate from different beam pulses. The three lines (in order of increasing Si( IP) energy deposit) correspond to two protons, a proton and a deuteron, and a proton and a Won. These pileup events represent a small fraction of the single proton yield and are due to the high beam current and large solid angle in this experiment. Similarly, above the ‘He line are three bands which correspond to proton-alpha, deuteron-alpha, and t&on-alpha pairs. Such events can be discriminated against by use of a multiple hit time-to-digital converter. Typical energy spectra measured with the ion chamber telescopes are shown in Fig. 16. These backward angle spectra, Blab= 120” are peaked at low energies. The arrows in the figure correspond to thresholds achievable with phoswiches (E/A = 3 MeV) [ 71. The need of the low thresholds of the new telescopes (E/A 5 0.8 MeV) is clearly evident since the reduced barrier for IMF emission is a crucial aspect of the multifragmentation phenomenon.
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Instr. and Merh. in Phys. Res. A 368 (1996) 709-738
111
_lOR i m
448
.z’
h 384 z g 320 8 t& 256 s
192 Y
2
128 64 0
0
64
128 192 256 320 384 448 Es, (AX Channel)
Fig. 14. Two dimensional plot of energy deposited in ion-chamber versus energy depositedin Si( IP) detector.
Fig. 16. Energy spectra of oxygen fragments measured in “N+‘97Au ,$,/A = 156 MeV.
at
9. Conclusions 1800 X00/
-
In summary, we have constructed high resolution low threshold ionization chamber telescopes to study the multifragment decay of highly excited nuclear systems. These telescopes have thresholds which correspond to ~0.8 MeVlu. Element identification is easily achieved for Z = 1 to Z = 14. The detectors are also capable of resolving isotopes for Z 5 10 for particles which punch through the Si(IP) detector (eg, E/A 2 9.5 and 20 MeV for ‘Li and *‘Ne, respectively. Good particle identification coupled with good energy resolution and low thresholds make this type of detector a preferred choice for further multifragmentation studies.
Acknowledgement
EcsI (ADC Channel) Fig. 15. Two dimensionalplot of energy depositedin Si( IP) detectorversus energy depositedin CsI(Tl) crystal.
We would like to thank Dr. J. Kolata. M. Belbot, and K. Lamkin at Notre Dame University for their help during our run at the tandem facility. We are grateful to L. Sexton at Indiana University for the detector fabrication. One of the authors (R.D.) gratefully acknowledges the support of the Sloan Foundation through the A.P. Sloan Fellowship program. This work was supported by the U.S. Depanment of Energy under Grant No. Do-FGOZ-92ER40714 (Indiana University) and the National Science Foundation under Grant Nos. NSF-PHY94-027671 (Notre Dame), NSF-PHY93-NUCRES-14783 (Indiana University) and NSF-PHY-9214992 (Michigan State University).
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