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Ionization chamber telescopes for position determination and nuclear charge identification of fission fragments
N U C L E A R INSTRUMENTS AND METHODS
159 ( 1 9 7 9 )
125-134 ; ©
N O R T H - H O L L A N D PUBLISHING CO.
IONIZATION C H A M B E R TELESCOPES FOR POSITION DETERMINATION AND NUCLEAR CHARGE IDENTIFICATION OF FISSION FRAGMENTS Y. Y O S H I D A ,
K. T S U J 1 , F. TOYOFUKU and A. KATASE
Department o/ Nuclear Engineering, Kyushu University, Fukuoka 812, Japan Received 10 July 1978 Ionization chambers for the position determination and nuclear charge identification of fission fragments are described with which are measured the energy E, the specific energy loss 3E, the arrival time t and the two space coordinates x and y (or 0) of the detected fragments. Position resolutions (fwhm) of Ax<<,O.5mm and Ay<~0.4 mm (30<~0.5 °) were obtained for fission fragments from a 252Cf source. A nuclear charge resolution (fwhm) of 3Z~2.0 was also obtained by coincidence measurements of the light ones with the gamma rays. The atomic number Z could be assigned to the respective fragments emitting each of the many prompt gamma rays in 252Cf spontaneous fission.
1. I n t r o d u c t i o n
For measurements of heavy ions which have energy higher than 10MeV and energy per mass unit lower than several M e V / a m u , gas ionization chambers have recently been shown to be probably more suitable than semiconductor detectors for (a) identification of particle or nuclear charge, (b) detection of position (two space coordinates), (c) variety of geometry and dimension, (d) freedom from radiation damage and (e) easiness of construction, and to have similar performances in (f) energy resolution and (g) time resolutionl-a~). (h) the possibility of large dimension (c) together with (f) and (g) mentioned above is considered to mean, moreover, that ionization chambers are effective in mass assignment of heavy ions by using the timeol:flight method12). Fission fragments have usually a kinetic energy of 50-120MeV and the energy per mass unit is 0.3-1.5 M e V / a m u . Then, ionization chambers are expected to be very suitable in m a n y cases for fission fragment spectrometry• The results tested recently with the mass and energy separated fission fragments from the spectrograph " L O H E N G R I N ''s-8) showed that the energy straggling turned out to be much larger than predicted by Tschal~ir ~3) due to the charge changing effects ~4•15) and to limit the resolution of the AE counter or the resolving power of nuclear charge Z. Also, the ionization chamber was shown to be inferior to the semiconductor detector as a AE counter in the case of the L O H E N G R I N beam of heavy ions which was very narrow (1 or 2 m m 2) and parallel, because the ratios of the experimental st:raggling value 6) to the theoretical one ~3) in the gas (Ar+CH4 or Ar+CO2) was 2.3 and larger than 1.'7 in the solid (Si or C)6). In most measurements,
however, the heavy ion beam is not so narrow and parallel and detectors are usually necessary to have a sensitive area larger than 1 cm 2 . Then semiconductor detectors are considered to be not so suitable for nuclear charge identification because it is quite difficult to fabricate a semiconductor detector with a thickness of several/2m with an area larger than 1 cm 2 whose uniformity of thickness is better than a few percent. On the other hand, the thickness of a 4 E gas ionization chamber can be freely and accurately controlled. Even in the case of a radial beam the thickness can be corrected according to the incident angles by measuring also the incident positions simultaneously. Therefore, it can be concluded in the usual spectrometry that
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126
Y. Y O S H I D A et al.
the ionization chamber is more suitable for charge identification of heavy ions 3'9'1°) as mentioned in (a) at the beginning, if the entrance window and the shapes of the electrodes are properly made. For fission fragment spectrometry with ionization chambers, the following facts are obtained from fig. 1~6). The value of energy loss per path length (dE/dx) is almost maximum for the incident energy of the light fragments and is a little smaller than the maximum for that of the heavy ones. The difference of dE/dx values caused by the difference of nuclear charge Z, [A(dE/dx)/AZ] is almost largest at the energy corresponding to the difference of nuclear charge Z, [A (dE/dx)/zlZ] rapidly as the energy of the ions decreases. From these facts, the following are also mentioned for ionization chamber spectrometry. 1) Position detection of the incident fragments can be made rather easily and two-dimensionally (x and y or 0). 2) The entrance window should be very thin and the charge collection near the window should be made completely. 3) The identification of the nuclear charge Z of the fragments will be possible for the light ones with the AE-E telescope but will be very difficult for the heavy ones. Three types of ionization chamber were constructed on reference to that used in ref. 1 for the coincidence measurements of the fission flag-
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@ Fig. 2. Cross section of the system consisting of a 252Cf source chamber and three ionization chambers, which is installed in the source chamber of a B-spectrometer for the coincidence measurements of the fragments with internal conversion electrons.
ments with gamma rays [Ge(Li)] or internal conversion electrons (//-spectrometer) following the spontaneous fission of 252Cf. In ref. 1 the investigations were made mainly with 5.49 MeV u-particles from an 24~Am source, while in the present paper the measurements have been done with the fission fragments to prove the performances (a)-(g) mentioned at the beginning, especially (a) identification of nuclear charge Z and (b) detection of position (two space coordinates). The performance (a) was investigated by making the coincidence measurements of the fragments with the gamma rays already assigned ~9) and the performance (b) by putting the collimators just behind the entrance windows. In the following section each geometry and operation of the three types of ionization chamber is described, while in section 3 the experimental results obtained with the present chambers are given for the fission fragments from a 252Cf source.
2. Geometry and operation of the ionization chambers Fig. 2 shows a cross section of the system consisting of a 2s2Cf source chamber and three ionization chambers for the coincidence measurements of the fragments with the internal conversion electrons. This is installed by the flange (F) in the source chamber (D) of the iron-free 7r~/2 beta spectrometer of INS Univ. of TokyolT). (E) shows the 252Cf source chamber. (A), (B) and (C) are the ionization chambers, where (B) and (C) are named "90°-detector '' because they are put at a right angle to the flight direction of the electrons analyzed by the //-spectrometer, and (A) is named "0°-detector '' because it is put opposite to this direction. The details of the 0°-detector [(A)] and the 90°-detector [(B) or (C)] are shown in figs. 3 and 4, respectively. The Cf source chamber and the frames and supporting plates of the electrodes of the ionization chambers are made of polyacetal or acrylates and t3artly of teflon. The anodes (AE and E) and the cathodes are made by sticking copper plates on the acrylic plates. The dotted lines parallel to the anode and the cathode of each detector are the Frisch-grids, which are made of copper or tungsten mesh (50 mesh). The solid lines parallel to them are the 0-grids, which are used for the measurement of the incident angle (0 or y) of the fragments on the detectors on the plane parallel to the electrodes. The ratios of the strength of the electric field to the gas pressure were 1, 2, and
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3 V/cm.torr in the spaces between the grounded Frisch-grid and the cathode (EFc/P), between the 0-grid and the Frisch-grid (Eov/P) and between the anode and the 0-grid (EAo/P), respectively. The dotted line near the entrance window of the 0°-de tector [(A)] shows the entrance grid and was used to correct the distortion of the parallelity of the lines of electric force near the window. The window is a VYNS film of thickness 50-200 ~g/cm 2, and the seal was made with a binding agent together with a supporting mesh of tungsten (diameter 30 ~m, spacing 0.5 mm and transparency 88%) or of stainless steel (width 50 ~m, spacing 0.5 mm and transparancy 81%). Evacuation and gas flowing of (D), (E) and the cylindrical vessel which contained three ionization chambers were carefully made not to tear the window films by using the needle valves and the valves of (I), (J), (K), (L) and (NI). Methane gas of 99% purity flowed, in common to the three ionization chambers through the valves of (I) and (J). The pressure was 200 tort and was stabilized within 1%. Five charge-sensitive pre-amplifiers are contained in (G) nd (H), respectively. As shown in fig. 3, the anode of the 0°-detector [(A)] is separated into two parts, AE and E, at
100 mm from the source, and the gap is l mm. The 0-grid is composed of 30 wires stretched radially at 1° intervals centering around the source between the grooves of two circular frames of teflon which have been cut beforehand with the pitches proportional to their radii. The two 90°-detectors (B) and (C) are identical. The maximum angle of 0 of this detector subtended by the source is large (90°). The 0-grid is composed of 37 wires stretched radially at about 2.7 ° intervals centering around the source. These detectors are used to measure only the specific energy loss z/E and the space coordinate 0 of the fission fragments because they are small owing to the limit of dimension of the source chamber of the fl-spectrometer and because the anodes are not separated. Fig. 5 shows a schematic view of the read-out of x- or 0- (or y-) position used in the present ionization chambers. The space coordinate x of the incident fragments on the detector is measured by the time difference between the signal from the z/E-anode and that from the cathode because the cathode signal starts at the time of incidence of the fragments on the detector while the AE-anode signal starts at the time when the electrons creat-
128
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presently used for the 5-dimensional coincidence measurements by list mode. Fig. 7 shows cross sections of the ionization chamber presently used for the identification of atomic number Z of the fission fragments by the zlE-E telescope in the coincidence measurements of the fragments with the gamma rays. This chamber is similar to the 0°-detector [(A)] and is about twice the size of (A) so that it may be operated at rather low pressure with a thin entrance window. The window used is a VYNS film of about 50 Hg/cm 2 thickness, and the energy loss of the fission fragments in this window is about 3 MeV. The energy straggling is estimated to be about 200 keV (fwhm) and small compared with the resolution of the chamber. The separation of
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for the 5-dimensional coincidence measurements. ed around the path of the fragments in the chamber drift through the Frisch-grid. The 0- (or y-) position is measured by the time difference between the signal from the dE-anode and that induced on the wires of the 0-grid adjacent to the drift path of the electrons. The inductor L and the capacitor C are connected to each terminal of the wires of the &grid, and the LC delay line is constructed as shown in the figure. To avoid deterioration of the waveform of the 0-signal by the reflection cf the signal at the opposite terminal of the LC delay line, a charge sensitive preamplifier was connected also to the opposite terminal and impedance matching by Radeka's "cooled damping" method IS) was done by-adjusting the value of Co in the relation shown in the lower-right part of the figure. For the present measurements of positions, L and Cr were 200 #H and 4 pF, respectively, and C as well as Co was varied in each measurement as will be mentioned in the next section. Fig. 6 shows a block diagram of the electronics system
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IONIZATION
129
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the anode into AE and E and the shape of the entrance window are made to draw concentric circles with the source for their center. The distance from the source to the window is 20 m m and the radial effective lengths of AE-anode and E-anode are 80 m m and 90 m m , respectively. The 0-grid was not used in this chamber, and the position of only the x-coordinate was read out. Methane gas of 99% purity flowed through the chamber at a pressure of 90 torr which was controlled within an accuracy of 1 tort. The relations under the above conditions between the average energies which were used in the present 3E-E telescope are shown in fig. 1 by the arrows for the light and heavy fragments, respectively, in 252Cf spontaneous fission. The average energy losses in the AEcounter are 2/3 and 3/4 of the incident energies of light and heavy fragments, respectively. The pulse height values of the AE-signal were corrected after the measurements by the respective values of x measured simultaneously. Nuclear charge Z of the fragments was assigned on the two-dimensional AE. E~-plane, where AE is the corrected value and E~ is AE plus E. The Frisch-grid was constructed by stretching the wires parallel at intervals of l mm. To the anode and the cathode were applied + 2 5 0 V and - 1 2 5 0 V, respectively, and the ratios of the electric field to the pressure were 2.32 and 1 . 1 6 V / c m . t o r r , respectively. The circumference of the space between the cathode and the Frisch-grid was surrounded by 23 field wires, to each of which was applied the proper dc bias to make the lines of electric force in any position in the space (especially near the window) normal to the electrodes. The whole detector system was put in a small thermostatic chamber and its temperature was controlled at 26°C within an accuracy of 0.3 °C.
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Fig. g. x-position spectrum for fission fragments measured with the "0°-detector" (fig. 3). The schematic view of the arrangement of the source, the source collimator and the detector collimator is also shown in the upper part 'of the figure. The measured width is equal to the geometrical one within the experimental uncertainty. The intrinsic resolution (fwhm) of vposition of the "o°-detector" is smaller than 0.5 ram.
AE-anode signals were amplified by a timing filter amplifier (r~,~ = 1/is, zdm.= 120 ns), and a constant fraction discriminator (ARC timing, f = 0.55, delay 13 ns) was used to derive the start signals to a time-to-amplitude converter. The time constants of the T F A for the cathode signals were 0.9/~s (z~n~) and 50 ns (rd~rf), and a fast leading-edge discriminator was used to derive the stop signals to the TAC. The time range in the measurement of the spectrum in fig. 8 was about 1/~s which corresponded to about 500 channels. As shown in fig. 9, the shape of a peak in the position spectrum is elliptical from the geometrical Z
81
3. Measurements and results 3.1. MEAStmEMENTOV THV X-COORDINATE Fig. 8 shows the x-position spectrum for the fis.,;ion fragments from a 252Cf source measured with lhe 0°-detector (fig. 3 or (A) in fig. 2). The measurement was done with the source and detector collimators in an arrangement as shown in the upper part of fig. 8. The source collimator o f 2 m m diameter was used because the source area was large and the diameter was 8 ram. The detector collimafor had five holes of 2.0 m m diameter at intervals of 4.5 ram, and was put just behind the entrance window of the ionization chamber, as shown. The
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130
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consideration of the circular hole of the collimator, if we neglect the width caused by the intrinsic character of the detector ([fwhm]m~,~) and the width caused by the dimension of the source. Its full width at half-maximum [fwhm]g~o is equal to that of the circular hole of the collimator immediately after particles have passed through the hole. Then, [fwhm]g~o - x/3 D ,-~ 1.73 mm ('." D=2.0 mm). 2 As shown in fig. 8, the present result obtained by the measurement I1fwhm]me,~) is 1.72mm and agrees very well with the geometrical one ([fwhm]~eo). If we consider the experimental uncertainty to be 4%, then [ f w h m (x)]i.~t~ ~ ~ / ( [ f w h m ] 2. . . . - [ f w h m ] g 2e o )
< 1.72x/(1.042 - 12) = 0.49 mm. Therefore, the intrinsic x-position resolution of the 0°-detector for the fission fragments ([fwhm(x)L~,~) is estimated to be smaller than 0.5 ram. 3.2. MEASUREMENT OF THE 0-COORDINATE Fig. 10 shows the 0-position spectrum for the fission fragments from a 2s2Cf source measured with the 90°-detector (fig. 4 or (B) or (C) in fig. 2). i
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As shown in fig. 10, the present result obtained by the measurement ([fwhm(0 or y)l. . . . ) is 1.20 mm or 1.50 ° and agrees well with the geometrical one within the experimental uncertainty. If we consider the experimental uncertainty to be 6%, then
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The measurement was done by using the source collimator and the detector collimator which had circular holes of 1.5 mm diameter as shown in fig. 10. The detector collimator just behind the entrance window of the ionization chamber has eleven holes at intervals of about 8.7 ° or 7 m m as shown. The values of C and Co shown in fig. 5 were 0.4pF and 150pF, respectively. The time constants of the timing filter amplifiers for the AE-anode and 0-grid signals were equal and also equal to those for the AE-anode signals in the measurement of the x-coordinate mentioned in section 3.1. A constant fraction discriminator (ARC timing, f = 0.55, delay 50 ns) was used for the 0-grid signals to derive the stop signals to a time-to-amplitude converter. The time range in the measurement of the spectrum in fig. 10 was about 800 ns which corresponded to 500 channels. The inequality of each peak area in fig. 10 was caused by the inequality of the solid angle and of the source density subtended by each hole of the detector collimator, and the detection efficiency is considered to be independent of the incident angle 0 of the fragments. In the same way as in the case of x-position in section 3.1, the geometrical width is calculated from the relation shown in fig. 9, and
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Fig. 10. 0-(or y-) position spectrum for fission fragments measured with the "'90°-detector '' (fig. 4). The schematic view of the arrangement of the source, the source collimator and the detector collimator is also shown in the upper-left part of the figure. The inequality of each peak area is caused by the inequality of the solid angle and of the source density subtended by each hole of the detector collimator. The measured width is equal to the geometrical one within the experimental uncertainty. The intrinsic resolution of 0-(or y-) position o f the "90°-detector '' is smaller than 0.5 ° (or 0.4 mm).
Therefore, the intrinsic 0- or y-position resolution of the 90°-detector for the fission fragments is estimated to be equal to or smaller than about 0.5 ° or 0.4 m m , respectively. The intrinsic 0- or y-position resolution of the 0°-detector [(A)] obtained in the simultaneous measurement with the x-position spectrum shown in fig. 8 w a s about 0.7 ° or 0.8 m m and larger than that of the 90°-detector [(B) or (C)]. In this case, the values of C and Co shown in fig. 5 were 22 pF and 27 pF, respectively.
IONIZATION
131
C H A M B E R TELESCOPES
ments, they were corrected by the respective values of x after the measurements. Correction by the values of 0 was not necessary because the shape of the entrance window and the separation of the anode into AE and E were made to draw concentric circles with the source for their center as mentioned in section 2. Each value of LIE was empirically corrected by the respective values of x using the pulse height spectra of AE obtained experimentally as a function of x. The values of the correction agreed rather well with those obtained from the geometrical consideration. The long-term components of the variations of the values of AE and E due to the variations of the pressure and temperature of the flowing gas of the counter were also corrected in common to each magnetic tape by making equal to each other the peak positions of the spectra of AE or E collected and summed for each tape. The residual error (fwhm) of each value of A E or E due to those variations is estimated to be about 0.4% after the above corrections. Fig. 11 shows a contour map of the A E - E t spectrum and the projected spectra for the fission fragments which were in coincidence with the gamma rays, where Et is A E plus E. This figure was obtained after the above corrections from the exper-
3.3. MEASUREMENT OF THE ATOMIC NUMBER Z OF THE FISSION FRAGMENTS
As stated in section 2, the resolving power of the ionization chamber to identify the atomic number Z of fission fragments was investigated by coincidence measurements of the fragments with the already assigned gamma rayst9), which were emitted from the specific fragments of the atomic and mass numbers of Z and A. The fission fragments were measured with the ionization chamber shown in fig. 7 which was almost twice the size of (A) and had a thin entrance window of 50 # g / c m 2. The gamma rays were measured with a Ge(Li) detector having a sensitive volume of about 60 cm 3, which was put adjacent to and on the right of the ionization chamber shown in the lower-right part of fig. 7. The five-dimensional coincidence measurements by list mode of E, AE, x, T and Ey were made by using an electronics system similar to that shown in fig. 6, where E, AE and x are the corresponding values for the fission fragments, T is the time difference between the fragments and the gamma rays from them, and E~ is the energy of the gamma rays. The resuits were recorded on 51 magnetic tapes 2400 feet long. As the values of d E were measured different according to the incident positions of the frag-
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Fig. 11. Contour map of A E - E t spectrum and projected spectra for the fission fragments which were in coincidence with the gamma-rays. A E is the corrected value as explained in the text, and E t is xlE plus E. Expanded view of the light fragment region is also shown in the right figure, and the number of N (nuclear charge parameter) is assigned as shown to each set obtained by linking together the divisions.
132
Y. YOSHIDA et al.
imental results collected and summed for only one magnetic tape. The upper-right part of the contour map corresponds to the light fragment region and the lower-left part to the heavy one. The energy values are written in the figure as an approximate standard, which was obtained by the calculation from the output pulse heights when pulses from the pulser were fed to the ionization chamber. Each energy value of the peaks in the projected spectrum of E, is considerably smaller than the respective incident energies of light and heavy fragments. This is considered to be mainly due to the pulse height defect of the detector 2°,2]) and the energy loss of fragments in the entrance window. The right figure shown in fig. 11 shows the expanded view of the main part of the light fragment region which was divided into 8 by 15. The respective intensities of the already assigned m) twelve gamma rays from the fragments of Xe, Ba and Ce were obtained for each of the 120 gamma spectra in total which was in coincidence with the fragments in each division obtained above. The "nuclear charge parameter" N was assigned as shown lor the identification of the atomic number Z of the fragments on the basis of each set obtained by linking together the divisions in which the coincidence intensities of the individual gamma rays mentioned above were the largest among the neighboring divisions. The nineteen coincidence gamma spectra, each of which was distinguished by N, were obtained by summing up all the gamma spectra that had been in coincidence with the fragments in each division on the corresponding N~ and recorded on the 51 magnetic tapes. For simplicity, fig. 12 shows only the six gamma spectra in coincidence with the light fission fragments, each of which was obtained by adding up three of the eighteen coincidence spectra distinguished by N in the order of N. These spectra were collected and summed from the 51 magnetic tapes in total. The energy range of the gamma rays measured was about 70-890 keV, and fig. 12 shows only a part of the range (262-400 keV). When the light fragments are measured with the ionization chamber, :the heavy ones fly in the opposite direction and are stopped in the source backing of platinum plate. Then, most gamma rays in the above energy range from the heavy fragments could be measured without Doppler shift or broadening because the flight time is smaller than 1 ps. However, the gamma rays from
the light ones were measured at the smaller energy with a little broadening due to the Doppler effect because they were emitted in a direction opposite to the flight direction of the fragments. Therefore, each spectrum sorted by N contains gamma rays with the smaller energy by about 5% which are emitted from the light fragments of the atomic numer Z L corresponding to the number N, and also the unshifted gamma rays from the heavy ones of the atomic number Z , equal to 98Z L, where 98 is the atomic number of Cf. For example, in the spectrum sorted by the N numbers of 7 to 9 in fig. 12 are contained the gamma rays from s~Ba isotopes and those from 42Mo isotopes. For the latter are written in the figure the energy values Doppler shifted and lowered by about 5%. The Z and A numbers and the transitions of the fragments emitting the respective gamma rays are ....
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Fig. 12. A part of the coincidence gamma spectra sorted by nuclear charge parameter N. Each spectrum contains the gamma rays with the energy smaller by about 5% emitted from the moving light fragments of-the atomic n u m b e r Z L corresponding to N, and also the gamma rays not Doppler-shifted from the stopped heavy ones of the atomic n u m b e r Z H equal to 98 Z k. The g a m m a transitions assigned by ref. 19 and by the present work are shown in the spectra.
I O N I Z A T I O N CHAMBER TELESCOPES
also written in the figure, which were assigned by ref. 19 and the present work. Each intensity of gamma rays was obtained for each of the 19 coincidence spectra distinguished by N. From the intensities each gamma transition intensity was calculated by using the internal conversion coefficients of the transitions. Some exampies of the transition intensities and their values converted to the yields per fission (%) are shown in fig. 13 against the nuclear charge parameter N. ]'he assignment of each transition shown in the figure is according to ref. 19. It is clear from the figure that each Z number of the fragments emitting the respective gamma rays can be determined irrespective of the mass number from the N values of the corresponding peak positions, if the fragments are known to be light or heavy• The rmclear charge resolution of the present ionization chamber for the light fragments [fwhm(Z)] is obtained to be about 2.0 from the figure, but it is about 1.6 if only two or three divisions on each N are used to obtain the gamma transition intensities.
133
Similar procedures were followed also in the heavy fission fragment region, but the Z numbers of the fragments could not be determined on the AE.Ecplane. Therefore, the heavy fragment region was divided into only four divisions, and each coincidence gamma spectrum with the fragments in the respective divisions was obtained and compared with those obtained from the light fragment region. By confirming for each gamma ray the energy difference due to Doppler shift and the consistency of intensities between the peak in the coincidence spectrum with the heavy fragments and the peak with the light ones, it was determined whether each gamma ray was emitted from the light fragments or the heavy ones. From the above results and the previously mentioned results with the light fragments, the respective Z numbers of the light and heavy fragments which emit each gamma ray were determined. The gamma yields per fission and the Z numbers of the fragments emitting each gamma ray could be determined for about one hundred prompt gamma transitions following the spontaneous fission of 252Cf 22).
•5
4. Conclusion It has been shown for fission fragments that ionization chambers are very suitable for the measurent of incident position (two space coordinates) v and the identification of atomic number. When we Z o considzr their excellent properties in variety of .0 ~ geometry and dimension, in freedom from radia,7 tion damage and in easiness of construction, and ~also their rather good properties in energy and lad a. time resolution, ionization chambers are useful in '-'.~ many applications for the spectrometry of fission ~.5 Wfragments or other heavy ions.
tt) I.--
- ix,o 0
t_) ~..
UJ 1,..--
4
O 1.-.
1...
t.9
5
I0
15
NUCLEAR CHARGE PARAMETER
N
Fig. 13. Gamma transition intensities against N, which were obtained from the present coincidence measurement of the fragments with the gammas from 252Cf fission. The assignment of each transition is according to re['. 19. It is clear from the figure that the atomic number Z of the fragments can be determined by the N value of the corresponding peak. The nuclear charge resolution of the present ionization chamber telescope ([fwhm(z)]) is obtained from the figure to be about 2.0 for the light fragments.
We are very much indebted to Mr. Y. Matsumote for the fabrication of the electronics•
References 1) H. Sann, H. Damjantschitsch, D. Hebbard, J. Junge, D. Pelte, B. Povh, D. Schwalm and D. B. Tranthoai, Nucl. Instr. and Meth. 124 (1975) 509. 2) M. M. Fowler and R. C. Jared, Nucl. Instr. and Meth. 124 (1975) 341. 3) H. R6sler, J. K. Millard and N. W. Hill, Nucl. Instr. and Meth. 99 (1972) 477. 4) j. R. Erskine, T. H. Braid and J. C. Stoltzfus, Nucl. Instr. and Meth. 135 (1976) 67. 5) K. Sistemich et al., Nucl. Instr. and Meth. 133 (1976) 163.
134
Y. YOSHIDA et al.
6) p. Armbruster et al., Nucl. Instr. and Meth. 132 (1976) 129. 7) H. G. Clerc et al., Nucl. Instr. and Meth. 124 (1975) 607. 8) G. Siegert et al., Phys. Lett. 53B (1974) 45. 9) j. Barrette, P. Braun-Munizinger and C. K. Gelbke, Nucl. Instr. and Meth. 126 (1975) 181. 10) p. Gl~issel, R. C. Jared and L. G. Morretto, Nucl. Instr. and Meth. 142 (1977) 569. I1) D. Shapira, R. M. Devries, H. W. Fulbright, J. Toke and M. R. Clover, Nucl. Instr. and Meth. 129 (1975) 123. 12) A. Gamp, W. Bohne, P. Braun-Munizinger and C. K. Gelbke, Nucl. Instr. and Meth. 120 (1974) 281. 13) C. Tschal~ir, Nucl. Instr. and Meth. 61 (1968) 141. 14) O. Vollmer, Nucl. Instr. and Meth. 121 (1974) 373.
15) K. Tsuji, Y. Yoshida, F. Toyofuku and A. Katase, to be submitted. 16) L. C. Northcliffe and R. F. Shilling, Nucl. Data Tables A7 (1970) 233. 17) M. Fujioka and K. Hisatake, INS TL-89 (1966). 18) V. Radeka, IEEE Trans. Nucl. Sci. NS-21, no. 1 (1974) 51. 19) E. Cheiftez, J. B. Wilhelmy, R. C. Jared and S. G. Thomson, Phys. Rev. C4 (1971) 1913. 20) E. C. Finch, Nucl. Instr. and Meth. 113 (1973) 43. 21) H. W. Schmidt and R. B. Leachman, Phys. Rev. 102 (1956) 183. 22) F. Toyofuku, Y. Yoshida, K. Tsuji and A. Katase, to be submitted to Nucl. Phys.
159 ( 1 9 7 9 )
125-134 ; ©
N O R T H - H O L L A N D PUBLISHING CO.
IONIZATION C H A M B E R TELESCOPES FOR POSITION DETERMINATION AND NUCLEAR CHARGE IDENTIFICATION OF FISSION FRAGMENTS Y. Y O S H I D A ,
K. T S U J 1 , F. TOYOFUKU and A. KATASE
Department o/ Nuclear Engineering, Kyushu University, Fukuoka 812, Japan Received 10 July 1978 Ionization chambers for the position determination and nuclear charge identification of fission fragments are described with which are measured the energy E, the specific energy loss 3E, the arrival time t and the two space coordinates x and y (or 0) of the detected fragments. Position resolutions (fwhm) of Ax<<,O.5mm and Ay<~0.4 mm (30<~0.5 °) were obtained for fission fragments from a 252Cf source. A nuclear charge resolution (fwhm) of 3Z~2.0 was also obtained by coincidence measurements of the light ones with the gamma rays. The atomic number Z could be assigned to the respective fragments emitting each of the many prompt gamma rays in 252Cf spontaneous fission.
1. I n t r o d u c t i o n
For measurements of heavy ions which have energy higher than 10MeV and energy per mass unit lower than several M e V / a m u , gas ionization chambers have recently been shown to be probably more suitable than semiconductor detectors for (a) identification of particle or nuclear charge, (b) detection of position (two space coordinates), (c) variety of geometry and dimension, (d) freedom from radiation damage and (e) easiness of construction, and to have similar performances in (f) energy resolution and (g) time resolutionl-a~). (h) the possibility of large dimension (c) together with (f) and (g) mentioned above is considered to mean, moreover, that ionization chambers are effective in mass assignment of heavy ions by using the timeol:flight method12). Fission fragments have usually a kinetic energy of 50-120MeV and the energy per mass unit is 0.3-1.5 M e V / a m u . Then, ionization chambers are expected to be very suitable in m a n y cases for fission fragment spectrometry• The results tested recently with the mass and energy separated fission fragments from the spectrograph " L O H E N G R I N ''s-8) showed that the energy straggling turned out to be much larger than predicted by Tschal~ir ~3) due to the charge changing effects ~4•15) and to limit the resolution of the AE counter or the resolving power of nuclear charge Z. Also, the ionization chamber was shown to be inferior to the semiconductor detector as a AE counter in the case of the L O H E N G R I N beam of heavy ions which was very narrow (1 or 2 m m 2) and parallel, because the ratios of the experimental st:raggling value 6) to the theoretical one ~3) in the gas (Ar+CH4 or Ar+CO2) was 2.3 and larger than 1.'7 in the solid (Si or C)6). In most measurements,
however, the heavy ion beam is not so narrow and parallel and detectors are usually necessary to have a sensitive area larger than 1 cm 2 . Then semiconductor detectors are considered to be not so suitable for nuclear charge identification because it is quite difficult to fabricate a semiconductor detector with a thickness of several/2m with an area larger than 1 cm 2 whose uniformity of thickness is better than a few percent. On the other hand, the thickness of a 4 E gas ionization chamber can be freely and accurately controlled. Even in the case of a radial beam the thickness can be corrected according to the incident angles by measuring also the incident positions simultaneously. Therefore, it can be concluded in the usual spectrometry that
1,ot
n
[HeclW Fragment] II n (AEH) .III I ~
(C H4)
\\
II II II II
so •
',I / / / II . //11 ,.~L, II ! ! / / I~k=~=~
(EH)
\\ \\
W ' ~ °Ce -56~'-~
\~
,oz-
1o 40 2o I
o.o, olo2 oio5 o., &
,
o5
I
~
i
;
I
,
lo 20
E/A (MeV/AMU) Fig. I. Electronic stopping power of heavy ions in methane gas calculated from the tables of ref. 16. The ranges of the average energies which were used in the present 3E-E ionization chamber telescope (fig. 7 and section 3.3) for the light and heavy fragments in 252Cf spontaneous fission are also shown by the arrows.
126
Y. Y O S H I D A et al.
the ionization chamber is more suitable for charge identification of heavy ions 3'9'1°) as mentioned in (a) at the beginning, if the entrance window and the shapes of the electrodes are properly made. For fission fragment spectrometry with ionization chambers, the following facts are obtained from fig. 1~6). The value of energy loss per path length (dE/dx) is almost maximum for the incident energy of the light fragments and is a little smaller than the maximum for that of the heavy ones. The difference of dE/dx values caused by the difference of nuclear charge Z, [A(dE/dx)/AZ] is almost largest at the energy corresponding to the difference of nuclear charge Z, [A (dE/dx)/zlZ] rapidly as the energy of the ions decreases. From these facts, the following are also mentioned for ionization chamber spectrometry. 1) Position detection of the incident fragments can be made rather easily and two-dimensionally (x and y or 0). 2) The entrance window should be very thin and the charge collection near the window should be made completely. 3) The identification of the nuclear charge Z of the fragments will be possible for the light ones with the AE-E telescope but will be very difficult for the heavy ones. Three types of ionization chamber were constructed on reference to that used in ref. 1 for the coincidence measurements of the fission flag-
(9
@
@
@ Fig. 2. Cross section of the system consisting of a 252Cf source chamber and three ionization chambers, which is installed in the source chamber of a B-spectrometer for the coincidence measurements of the fragments with internal conversion electrons.
ments with gamma rays [Ge(Li)] or internal conversion electrons (//-spectrometer) following the spontaneous fission of 252Cf. In ref. 1 the investigations were made mainly with 5.49 MeV u-particles from an 24~Am source, while in the present paper the measurements have been done with the fission fragments to prove the performances (a)-(g) mentioned at the beginning, especially (a) identification of nuclear charge Z and (b) detection of position (two space coordinates). The performance (a) was investigated by making the coincidence measurements of the fragments with the gamma rays already assigned ~9) and the performance (b) by putting the collimators just behind the entrance windows. In the following section each geometry and operation of the three types of ionization chamber is described, while in section 3 the experimental results obtained with the present chambers are given for the fission fragments from a 252Cf source.
2. Geometry and operation of the ionization chambers Fig. 2 shows a cross section of the system consisting of a 2s2Cf source chamber and three ionization chambers for the coincidence measurements of the fragments with the internal conversion electrons. This is installed by the flange (F) in the source chamber (D) of the iron-free 7r~/2 beta spectrometer of INS Univ. of TokyolT). (E) shows the 252Cf source chamber. (A), (B) and (C) are the ionization chambers, where (B) and (C) are named "90°-detector '' because they are put at a right angle to the flight direction of the electrons analyzed by the //-spectrometer, and (A) is named "0°-detector '' because it is put opposite to this direction. The details of the 0°-detector [(A)] and the 90°-detector [(B) or (C)] are shown in figs. 3 and 4, respectively. The Cf source chamber and the frames and supporting plates of the electrodes of the ionization chambers are made of polyacetal or acrylates and t3artly of teflon. The anodes (AE and E) and the cathodes are made by sticking copper plates on the acrylic plates. The dotted lines parallel to the anode and the cathode of each detector are the Frisch-grids, which are made of copper or tungsten mesh (50 mesh). The solid lines parallel to them are the 0-grids, which are used for the measurement of the incident angle (0 or y) of the fragments on the detectors on the plane parallel to the electrodes. The ratios of the strength of the electric field to the gas pressure were 1, 2, and
IONIZATION
CHAMBER
/0£DETECTOR (®)1 &E-ANODE
[90°-DETECTOR(®o, © )1
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Si
/-MESH+VYNS-WINDOW ~r..:..:-:..~:-,.,v:.-;. :. ::..-:::-~-,:.-::.::~..:::':.:l ~ / I" ~CATHODE "----'--I" ~ .~120ram / ~ FRISC..-GRID
SOURCE
F~B-¥"8"~'~" ....................[ ]
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0-GRID I:-~:.::V.'W:..:::,:. ~;:~;-~i::'.'.~: :,:~':~
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127
TELESCOPES
~
CATHODE
"~ MESH+VYNS-WINDOW &E-ANODE ~-- ENTRANCE-GRID
\
48mm i S% o',~ oI~ o~
~']~
7~irm
~'1 -0-GRID (37WIRES)
Fig. 3. Cross sections of the "0°-detector '', which is shown as the ionization chamber A in fig. 2.
Fig. 4. Cross sections of the "90°-detector '', which is shown as the ionization chamber B or C in fig. 2.
3 V/cm.torr in the spaces between the grounded Frisch-grid and the cathode (EFc/P), between the 0-grid and the Frisch-grid (Eov/P) and between the anode and the 0-grid (EAo/P), respectively. The dotted line near the entrance window of the 0°-de tector [(A)] shows the entrance grid and was used to correct the distortion of the parallelity of the lines of electric force near the window. The window is a VYNS film of thickness 50-200 ~g/cm 2, and the seal was made with a binding agent together with a supporting mesh of tungsten (diameter 30 ~m, spacing 0.5 mm and transparency 88%) or of stainless steel (width 50 ~m, spacing 0.5 mm and transparancy 81%). Evacuation and gas flowing of (D), (E) and the cylindrical vessel which contained three ionization chambers were carefully made not to tear the window films by using the needle valves and the valves of (I), (J), (K), (L) and (NI). Methane gas of 99% purity flowed, in common to the three ionization chambers through the valves of (I) and (J). The pressure was 200 tort and was stabilized within 1%. Five charge-sensitive pre-amplifiers are contained in (G) nd (H), respectively. As shown in fig. 3, the anode of the 0°-detector [(A)] is separated into two parts, AE and E, at
100 mm from the source, and the gap is l mm. The 0-grid is composed of 30 wires stretched radially at 1° intervals centering around the source between the grooves of two circular frames of teflon which have been cut beforehand with the pitches proportional to their radii. The two 90°-detectors (B) and (C) are identical. The maximum angle of 0 of this detector subtended by the source is large (90°). The 0-grid is composed of 37 wires stretched radially at about 2.7 ° intervals centering around the source. These detectors are used to measure only the specific energy loss z/E and the space coordinate 0 of the fission fragments because they are small owing to the limit of dimension of the source chamber of the fl-spectrometer and because the anodes are not separated. Fig. 5 shows a schematic view of the read-out of x- or 0- (or y-) position used in the present ionization chambers. The space coordinate x of the incident fragments on the detector is measured by the time difference between the signal from the z/E-anode and that from the cathode because the cathode signal starts at the time of incidence of the fragments on the detector while the AE-anode signal starts at the time when the electrons creat-
128
v. YOSHIDA et al.
I READ-OUT OF X-POSITION] &E-ANODE
\ E-ANODE\
[1"
............
, + , o
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4t-~ ZO: , / " ~ 4H~ Z~td =./"~-'11"-'~ 41--# C'I
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presently used for the 5-dimensional coincidence measurements by list mode. Fig. 7 shows cross sections of the ionization chamber presently used for the identification of atomic number Z of the fission fragments by the zlE-E telescope in the coincidence measurements of the fragments with the gamma rays. This chamber is similar to the 0°-detector [(A)] and is about twice the size of (A) so that it may be operated at rather low pressure with a thin entrance window. The window used is a VYNS film of about 50 Hg/cm 2 thickness, and the energy loss of the fission fragments in this window is about 3 MeV. The energy straggling is estimated to be about 200 keV (fwhm) and small compared with the resolution of the chamber. The separation of
+;+4f'
'
Io -I
cL,%BRo ,c,-
°pF
,
t-- m c,J
Fig. 5. Schematic view of the read-out of .x- or 0-(or 3") position used in the present ionization chambers. ~ is the drift
velocity of the electrons generated in the chamber. The values of C and CO were varied in each detector.
L+i~il~ ~o,ho~,+--I-~l L
+,o+
Aoc ~c'
~ A o c l
It
1
Fig. 6. Block diagram of the electronics system presently used
for the 5-dimensional coincidence measurements. ed around the path of the fragments in the chamber drift through the Frisch-grid. The 0- (or y-) position is measured by the time difference between the signal from the dE-anode and that induced on the wires of the 0-grid adjacent to the drift path of the electrons. The inductor L and the capacitor C are connected to each terminal of the wires of the &grid, and the LC delay line is constructed as shown in the figure. To avoid deterioration of the waveform of the 0-signal by the reflection cf the signal at the opposite terminal of the LC delay line, a charge sensitive preamplifier was connected also to the opposite terminal and impedance matching by Radeka's "cooled damping" method IS) was done by-adjusting the value of Co in the relation shown in the lower-right part of the figure. For the present measurements of positions, L and Cr were 200 #H and 4 pF, respectively, and C as well as Co was varied in each measurement as will be mentioned in the next section. Fig. 6 shows a block diagram of the electronics system
...... ~S
A E-ANODE
\, \\'~x
~
ix
FRI SCH-GRID
~
ENTRANCEWINDOW
CATh ODE
Fig. 7. Cross sections of the ionization chamber presently used for the identification of atomic number Z of the fission fragments in the coincidence measurements with the gamma rays.
IONIZATION
129
C H A M B E R TELESCOPES
the anode into AE and E and the shape of the entrance window are made to draw concentric circles with the source for their center. The distance from the source to the window is 20 m m and the radial effective lengths of AE-anode and E-anode are 80 m m and 90 m m , respectively. The 0-grid was not used in this chamber, and the position of only the x-coordinate was read out. Methane gas of 99% purity flowed through the chamber at a pressure of 90 torr which was controlled within an accuracy of 1 tort. The relations under the above conditions between the average energies which were used in the present 3E-E telescope are shown in fig. 1 by the arrows for the light and heavy fragments, respectively, in 252Cf spontaneous fission. The average energy losses in the AEcounter are 2/3 and 3/4 of the incident energies of light and heavy fragments, respectively. The pulse height values of the AE-signal were corrected after the measurements by the respective values of x measured simultaneously. Nuclear charge Z of the fragments was assigned on the two-dimensional AE. E~-plane, where AE is the corrected value and E~ is AE plus E. The Frisch-grid was constructed by stretching the wires parallel at intervals of l mm. To the anode and the cathode were applied + 2 5 0 V and - 1 2 5 0 V, respectively, and the ratios of the electric field to the pressure were 2.32 and 1 . 1 6 V / c m . t o r r , respectively. The circumference of the space between the cathode and the Frisch-grid was surrounded by 23 field wires, to each of which was applied the proper dc bias to make the lines of electric force in any position in the space (especially near the window) normal to the electrodes. The whole detector system was put in a small thermostatic chamber and its temperature was controlled at 26°C within an accuracy of 0.3 °C.
SOLIRCE COLLIMATOR
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250 3(]0 CHANNEL NUMBER
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.,.,350
Fig. g. x-position spectrum for fission fragments measured with the "0°-detector" (fig. 3). The schematic view of the arrangement of the source, the source collimator and the detector collimator is also shown in the upper part 'of the figure. The measured width is equal to the geometrical one within the experimental uncertainty. The intrinsic resolution (fwhm) of vposition of the "o°-detector" is smaller than 0.5 ram.
AE-anode signals were amplified by a timing filter amplifier (r~,~ = 1/is, zdm.= 120 ns), and a constant fraction discriminator (ARC timing, f = 0.55, delay 13 ns) was used to derive the start signals to a time-to-amplitude converter. The time constants of the T F A for the cathode signals were 0.9/~s (z~n~) and 50 ns (rd~rf), and a fast leading-edge discriminator was used to derive the stop signals to the TAC. The time range in the measurement of the spectrum in fig. 8 was about 1/~s which corresponded to about 500 channels. As shown in fig. 9, the shape of a peak in the position spectrum is elliptical from the geometrical Z
81
3. Measurements and results 3.1. MEAStmEMENTOV THV X-COORDINATE Fig. 8 shows the x-position spectrum for the fis.,;ion fragments from a 252Cf source measured with lhe 0°-detector (fig. 3 or (A) in fig. 2). The measurement was done with the source and detector collimators in an arrangement as shown in the upper part of fig. 8. The source collimator o f 2 m m diameter was used because the source area was large and the diameter was 8 ram. The detector collimafor had five holes of 2.0 m m diameter at intervals of 4.5 ram, and was put just behind the entrance window of the ionization chamber, as shown. The
.5 r 14~ o'i
Cf $OURCE~ L _
DETECTOR COLLIMATOR
1 'T' b/2
I~c~c~i
I
(FWHM)GEO = ,/~ a =(,~t2)D
Fig. 9. Illustration of the shape of a peak in the position spectrum obtained from the geometrical consideration when particles pass through the circular hole of the collimator of diameter D. Its full width at half-maximum [fwhm]geo is as shown.
130
v. YOSHIDA et al.
consideration of the circular hole of the collimator, if we neglect the width caused by the intrinsic character of the detector ([fwhm]m~,~) and the width caused by the dimension of the source. Its full width at half-maximum [fwhm]g~o is equal to that of the circular hole of the collimator immediately after particles have passed through the hole. Then, [fwhm]g~o - x/3 D ,-~ 1.73 mm ('." D=2.0 mm). 2 As shown in fig. 8, the present result obtained by the measurement I1fwhm]me,~) is 1.72mm and agrees very well with the geometrical one ([fwhm]~eo). If we consider the experimental uncertainty to be 4%, then [ f w h m (x)]i.~t~ ~ ~ / ( [ f w h m ] 2. . . . - [ f w h m ] g 2e o )
< 1.72x/(1.042 - 12) = 0.49 mm. Therefore, the intrinsic x-position resolution of the 0°-detector for the fission fragments ([fwhm(x)L~,~) is estimated to be smaller than 0.5 ram. 3.2. MEASUREMENT OF THE 0-COORDINATE Fig. 10 shows the 0-position spectrum for the fission fragments from a 2s2Cf source measured with the 90°-detector (fig. 4 or (B) or (C) in fig. 2). i
,~
/
7,,
i
0o
,:: I1' H~" 50
_j10C z <1 I u d3
z50
1 [fwhm(0 or Y)]geo = 1.50 mm x zx/3 = 1.30 mm (or 1.30mm x 8.7°/7 m m = 1.6°).
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/l~41q¢' [, -'~ /DETECTOR~ COLLIMATOR
SOURCE COLLIMATOR
As shown in fig. 10, the present result obtained by the measurement ([fwhm(0 or y)l. . . . ) is 1.20 mm or 1.50 ° and agrees well with the geometrical one within the experimental uncertainty. If we consider the experimental uncertainty to be 6%, then
} +8.70 I FWHM , =1.2 ornm= 1.5 °
/ -43.2 ° -34.6 °
-26.0 ° T' i
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,
T
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[fwhm(y)]~nstr < 1.20 x ,/(1.062- 12) = 0.42 mm, or
[fwhm(0)]i,~tr < 1.5 ° x \/(1.062- 12) = 0.53 °.
i
t"
The measurement was done by using the source collimator and the detector collimator which had circular holes of 1.5 mm diameter as shown in fig. 10. The detector collimator just behind the entrance window of the ionization chamber has eleven holes at intervals of about 8.7 ° or 7 m m as shown. The values of C and Co shown in fig. 5 were 0.4pF and 150pF, respectively. The time constants of the timing filter amplifiers for the AE-anode and 0-grid signals were equal and also equal to those for the AE-anode signals in the measurement of the x-coordinate mentioned in section 3.1. A constant fraction discriminator (ARC timing, f = 0.55, delay 50 ns) was used for the 0-grid signals to derive the stop signals to a time-to-amplitude converter. The time range in the measurement of the spectrum in fig. 10 was about 800 ns which corresponded to 500 channels. The inequality of each peak area in fig. 10 was caused by the inequality of the solid angle and of the source density subtended by each hole of the detector collimator, and the detection efficiency is considered to be independent of the incident angle 0 of the fragments. In the same way as in the case of x-position in section 3.1, the geometrical width is calculated from the relation shown in fig. 9, and
.. 200 300 CHANNEL NUMBER
:
? "1 400
Fig. 10. 0-(or y-) position spectrum for fission fragments measured with the "'90°-detector '' (fig. 4). The schematic view of the arrangement of the source, the source collimator and the detector collimator is also shown in the upper-left part of the figure. The inequality of each peak area is caused by the inequality of the solid angle and of the source density subtended by each hole of the detector collimator. The measured width is equal to the geometrical one within the experimental uncertainty. The intrinsic resolution of 0-(or y-) position o f the "90°-detector '' is smaller than 0.5 ° (or 0.4 mm).
Therefore, the intrinsic 0- or y-position resolution of the 90°-detector for the fission fragments is estimated to be equal to or smaller than about 0.5 ° or 0.4 m m , respectively. The intrinsic 0- or y-position resolution of the 0°-detector [(A)] obtained in the simultaneous measurement with the x-position spectrum shown in fig. 8 w a s about 0.7 ° or 0.8 m m and larger than that of the 90°-detector [(B) or (C)]. In this case, the values of C and Co shown in fig. 5 were 22 pF and 27 pF, respectively.
IONIZATION
131
C H A M B E R TELESCOPES
ments, they were corrected by the respective values of x after the measurements. Correction by the values of 0 was not necessary because the shape of the entrance window and the separation of the anode into AE and E were made to draw concentric circles with the source for their center as mentioned in section 2. Each value of LIE was empirically corrected by the respective values of x using the pulse height spectra of AE obtained experimentally as a function of x. The values of the correction agreed rather well with those obtained from the geometrical consideration. The long-term components of the variations of the values of AE and E due to the variations of the pressure and temperature of the flowing gas of the counter were also corrected in common to each magnetic tape by making equal to each other the peak positions of the spectra of AE or E collected and summed for each tape. The residual error (fwhm) of each value of A E or E due to those variations is estimated to be about 0.4% after the above corrections. Fig. 11 shows a contour map of the A E - E t spectrum and the projected spectra for the fission fragments which were in coincidence with the gamma rays, where Et is A E plus E. This figure was obtained after the above corrections from the exper-
3.3. MEASUREMENT OF THE ATOMIC NUMBER Z OF THE FISSION FRAGMENTS
As stated in section 2, the resolving power of the ionization chamber to identify the atomic number Z of fission fragments was investigated by coincidence measurements of the fragments with the already assigned gamma rayst9), which were emitted from the specific fragments of the atomic and mass numbers of Z and A. The fission fragments were measured with the ionization chamber shown in fig. 7 which was almost twice the size of (A) and had a thin entrance window of 50 # g / c m 2. The gamma rays were measured with a Ge(Li) detector having a sensitive volume of about 60 cm 3, which was put adjacent to and on the right of the ionization chamber shown in the lower-right part of fig. 7. The five-dimensional coincidence measurements by list mode of E, AE, x, T and Ey were made by using an electronics system similar to that shown in fig. 6, where E, AE and x are the corresponding values for the fission fragments, T is the time difference between the fragments and the gamma rays from them, and E~ is the energy of the gamma rays. The resuits were recorded on 51 magnetic tapes 2400 feet long. As the values of d E were measured different according to the incident positions of the frag-
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132
Y. YOSHIDA et al.
imental results collected and summed for only one magnetic tape. The upper-right part of the contour map corresponds to the light fragment region and the lower-left part to the heavy one. The energy values are written in the figure as an approximate standard, which was obtained by the calculation from the output pulse heights when pulses from the pulser were fed to the ionization chamber. Each energy value of the peaks in the projected spectrum of E, is considerably smaller than the respective incident energies of light and heavy fragments. This is considered to be mainly due to the pulse height defect of the detector 2°,2]) and the energy loss of fragments in the entrance window. The right figure shown in fig. 11 shows the expanded view of the main part of the light fragment region which was divided into 8 by 15. The respective intensities of the already assigned m) twelve gamma rays from the fragments of Xe, Ba and Ce were obtained for each of the 120 gamma spectra in total which was in coincidence with the fragments in each division obtained above. The "nuclear charge parameter" N was assigned as shown lor the identification of the atomic number Z of the fragments on the basis of each set obtained by linking together the divisions in which the coincidence intensities of the individual gamma rays mentioned above were the largest among the neighboring divisions. The nineteen coincidence gamma spectra, each of which was distinguished by N, were obtained by summing up all the gamma spectra that had been in coincidence with the fragments in each division on the corresponding N~ and recorded on the 51 magnetic tapes. For simplicity, fig. 12 shows only the six gamma spectra in coincidence with the light fission fragments, each of which was obtained by adding up three of the eighteen coincidence spectra distinguished by N in the order of N. These spectra were collected and summed from the 51 magnetic tapes in total. The energy range of the gamma rays measured was about 70-890 keV, and fig. 12 shows only a part of the range (262-400 keV). When the light fragments are measured with the ionization chamber, :the heavy ones fly in the opposite direction and are stopped in the source backing of platinum plate. Then, most gamma rays in the above energy range from the heavy fragments could be measured without Doppler shift or broadening because the flight time is smaller than 1 ps. However, the gamma rays from
the light ones were measured at the smaller energy with a little broadening due to the Doppler effect because they were emitted in a direction opposite to the flight direction of the fragments. Therefore, each spectrum sorted by N contains gamma rays with the smaller energy by about 5% which are emitted from the light fragments of the atomic numer Z L corresponding to the number N, and also the unshifted gamma rays from the heavy ones of the atomic number Z , equal to 98Z L, where 98 is the atomic number of Cf. For example, in the spectrum sorted by the N numbers of 7 to 9 in fig. 12 are contained the gamma rays from s~Ba isotopes and those from 42Mo isotopes. For the latter are written in the figure the energy values Doppler shifted and lowered by about 5%. The Z and A numbers and the transitions of the fragments emitting the respective gamma rays are ....
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Fig. 12. A part of the coincidence gamma spectra sorted by nuclear charge parameter N. Each spectrum contains the gamma rays with the energy smaller by about 5% emitted from the moving light fragments of-the atomic n u m b e r Z L corresponding to N, and also the gamma rays not Doppler-shifted from the stopped heavy ones of the atomic n u m b e r Z H equal to 98 Z k. The g a m m a transitions assigned by ref. 19 and by the present work are shown in the spectra.
I O N I Z A T I O N CHAMBER TELESCOPES
also written in the figure, which were assigned by ref. 19 and the present work. Each intensity of gamma rays was obtained for each of the 19 coincidence spectra distinguished by N. From the intensities each gamma transition intensity was calculated by using the internal conversion coefficients of the transitions. Some exampies of the transition intensities and their values converted to the yields per fission (%) are shown in fig. 13 against the nuclear charge parameter N. ]'he assignment of each transition shown in the figure is according to ref. 19. It is clear from the figure that each Z number of the fragments emitting the respective gamma rays can be determined irrespective of the mass number from the N values of the corresponding peak positions, if the fragments are known to be light or heavy• The rmclear charge resolution of the present ionization chamber for the light fragments [fwhm(Z)] is obtained to be about 2.0 from the figure, but it is about 1.6 if only two or three divisions on each N are used to obtain the gamma transition intensities.
133
Similar procedures were followed also in the heavy fission fragment region, but the Z numbers of the fragments could not be determined on the AE.Ecplane. Therefore, the heavy fragment region was divided into only four divisions, and each coincidence gamma spectrum with the fragments in the respective divisions was obtained and compared with those obtained from the light fragment region. By confirming for each gamma ray the energy difference due to Doppler shift and the consistency of intensities between the peak in the coincidence spectrum with the heavy fragments and the peak with the light ones, it was determined whether each gamma ray was emitted from the light fragments or the heavy ones. From the above results and the previously mentioned results with the light fragments, the respective Z numbers of the light and heavy fragments which emit each gamma ray were determined. The gamma yields per fission and the Z numbers of the fragments emitting each gamma ray could be determined for about one hundred prompt gamma transitions following the spontaneous fission of 252Cf 22).
•5
4. Conclusion It has been shown for fission fragments that ionization chambers are very suitable for the measurent of incident position (two space coordinates) v and the identification of atomic number. When we Z o considzr their excellent properties in variety of .0 ~ geometry and dimension, in freedom from radia,7 tion damage and in easiness of construction, and ~also their rather good properties in energy and lad a. time resolution, ionization chambers are useful in '-'.~ many applications for the spectrometry of fission ~.5 Wfragments or other heavy ions.
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Fig. 13. Gamma transition intensities against N, which were obtained from the present coincidence measurement of the fragments with the gammas from 252Cf fission. The assignment of each transition is according to re['. 19. It is clear from the figure that the atomic number Z of the fragments can be determined by the N value of the corresponding peak. The nuclear charge resolution of the present ionization chamber telescope ([fwhm(z)]) is obtained from the figure to be about 2.0 for the light fragments.
We are very much indebted to Mr. Y. Matsumote for the fabrication of the electronics•
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6) p. Armbruster et al., Nucl. Instr. and Meth. 132 (1976) 129. 7) H. G. Clerc et al., Nucl. Instr. and Meth. 124 (1975) 607. 8) G. Siegert et al., Phys. Lett. 53B (1974) 45. 9) j. Barrette, P. Braun-Munizinger and C. K. Gelbke, Nucl. Instr. and Meth. 126 (1975) 181. 10) p. Gl~issel, R. C. Jared and L. G. Morretto, Nucl. Instr. and Meth. 142 (1977) 569. I1) D. Shapira, R. M. Devries, H. W. Fulbright, J. Toke and M. R. Clover, Nucl. Instr. and Meth. 129 (1975) 123. 12) A. Gamp, W. Bohne, P. Braun-Munizinger and C. K. Gelbke, Nucl. Instr. and Meth. 120 (1974) 281. 13) C. Tschal~ir, Nucl. Instr. and Meth. 61 (1968) 141. 14) O. Vollmer, Nucl. Instr. and Meth. 121 (1974) 373.
15) K. Tsuji, Y. Yoshida, F. Toyofuku and A. Katase, to be submitted. 16) L. C. Northcliffe and R. F. Shilling, Nucl. Data Tables A7 (1970) 233. 17) M. Fujioka and K. Hisatake, INS TL-89 (1966). 18) V. Radeka, IEEE Trans. Nucl. Sci. NS-21, no. 1 (1974) 51. 19) E. Cheiftez, J. B. Wilhelmy, R. C. Jared and S. G. Thomson, Phys. Rev. C4 (1971) 1913. 20) E. C. Finch, Nucl. Instr. and Meth. 113 (1973) 43. 21) H. W. Schmidt and R. B. Leachman, Phys. Rev. 102 (1956) 183. 22) F. Toyofuku, Y. Yoshida, K. Tsuji and A. Katase, to be submitted to Nucl. Phys.