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Energy resolution and nuclear charge resolving power of an ionisation chamber for fission products
NUCLEAR
INSTRUMENTS
AND
ENERGY RESOLUTION IONISATION
CHAMBER
METHODS
I33
(I976) I63-I69;
AND NUCLEAR FOR FISSION
©
NORTH-HOLLAND
CHARGE RESOLVING
POWER
PUBLISHING
CO.
OF AN
PRODUCTS
K. S I S T E M I C H *
lnstitut fiir Kernphysik, Kernforschungsanlage Jii6ch, D-517 Jiilich, Germany P. A R M B R U S T E R
Gesellschaft fiir Schwerionenforsehung, D-61 Darmstadt, Germany and J.P. BOCQUETt,
CH. CHAUVIN?
and Y. G L A I Z E
lnstitut Laue-Langevin, F-38 Grenoble, France The L O H E N G R I N Collaboration, Institut Laue-Langevin, F-38 Grenoble, France Received 7 N o v e m b e r 1975 A t r a n s m i s s i o n ionisation c h a m b e r with high energy resolution has been developed a n d its nuclear charge resolving power for fission products has been investigated. T h e c h a m b e r was tested with the m a s s and energy separated fission products o f the parabola spectrograph L O H E N G R I N o f the Institut L a u e - L a n g e v i n , Grenoble. T h e obtained energy resolution a m o u n t e d to 1.7% for 90Kr fission products which deposit inside the c h a m b e r a b o u t 75% o f their initial energy o f 83.6 MeV. T h e energy straggling in the A r + C H 4 a n d A r + CO2 fillings o f the c h a m b e r turned out to be m u c h larger t h a n predicted and to limit the resolution o f the chamber. T h e nuclear charge resolving power was determined to be Z / A Z ~ 30.
1. Introduction
2. The method
The determination of the nuclear charge Z of nuclear reaction products is necessary in many investigations. With the onset of the studies at heavy ion accelerators the determination of Z for heavy products has gained increasing importance. One of the commonly used methods for the identification of Z is based on the measurement of the energy loss AE in an absorbing medium, as AE depends on Z. An ionisation chamber was developed in order to investigate the Z-resolving power which can be obtained for fission products. The chamber was adapted to the fission product mass separator L O H E N G R I N 1-3) at the high flux reactor of the Institut Laue-Langevin, Grenoble. The separator is well suited for the test of such detectors. It provides beams of fission products with one single mass and a well defined kinetic energy, but with different nuclear charges. Many different masses and an extended region of kinetic energies are available. The chamber was designed for a high energy resolution. The energy straggling turned out to be the most serious limitation for the energy resolution. It was investigated in some detail.
The differential energy loss dE/dx of heavy ions passing through matter is determined by the velocity v and the ionic charge q of the ions and by the nuclear charge ZT of the target:
* D u r i n g the preparation o f the c h a m b e r at the lnstitut L a u e Langevin, on leave o f absence. ? Also from D R F C E N Grenoble.
dE dx
-
f ( v , q , ZT).
(1)
As the velocity is given by the mass A and the kinetic energy E of the ions and as the ionic charge is a function of their nuclear charge Z, dE/dx can be written as dE dx
-
f ( x / [ 2 E / A ] , Z, ZT).
(2)
The integration ofeq. (2) leads to the energy loss in the target of thickness x. With dE/dx = - - v z K F ( Z T ) , 0 < K < 2, which holds approximately in the mass and energy region of the fission products according to the experimental results of ref. 4, the integration results in AE = Vo F ( Z T ) Z K x - F z ( Z T ) z 2 K x 2 , 2A
(3)
where v0 is the initial velocity of the ions. This equation can be used for the calculation of the energy loss at any value of x, if the factor F(ZT)Z K is determined by a measurement of AE for one value of x. It should be
164
K. SISTEMICH et al.
emphasized that AE depends both on A and Z even for fission products with identical initial velocity, whereas dE/dx depends only on Z in this case. The resolving power Z/AZ of the ionisation chamber is defined by the condition that its energy resolution (SAE)m should be equal to the difference of the energy losses of two neighbouring isobars with equal kinetic energy:
(4)
(SAE),c = (6AE)Az=,.
This equality can be transformed into the relationship Z --
AZ
l =
Fz
,
(SAE)~c/AE
(5)
where F z = dlnAE/dlnZ is the charge dispersion and (6AE)~c/AE is the energy resolution of the chamber. These parameters have to be determined experimentally.
3. Experimental arrangement
resolving power a large energy loss AE inside the active volume and a large charge dispersion Fz were needed (sect. 4). Both AE and Fz are highest for high energy and, hence, the beginning of the path of the fission products in the gas was the most effective part of the range. All plates were made of copper. Their thickness amounted to 1 ram. A small value of this thickness guaranteed a small capacity between the collector and the guard ring, the overall capacity of the collector against the surroundings being 60 pF. The Frisch grid was made of steel wires of 0.3 mm diameter and 1.5 mm distance. The distances to the collector and to the cathode amounted to 7 and 10 ram. With these dimensions and the given geometry of the plates and the beam a reduction of the electrostatic induction of charge on the collector by the produced positive gas ions to less than 1% was obtained7). The wires were clamped into a stainless steel frame. The frame was open at the entrance, again in order to minimize the distance between window and active volume. The area covered by the wires amounted to 70 mm times 160 ram. At the grid a negative potential
3.1. [ONISATIONCHAMBER The ionisation chamber was of the parallel-plate type with a Frisch grid 5) and a guard ring6). It is shown schematically in fig. 1. The plate system was adapted to the long and narrow focus of L O H E N G R I N of about 700 times 10 mm 2. A section of 30 times 4 mm 2 of this focus could be accepted by the chamber. The fission products entered the chamber through a thin window. Their paths were parallel to the plates, and they were stopped in an ORTEC surface barrier detector for fission products. The electrons which they produced in the gas below the collector plate were used for the AE-information. The influence of the positive ions was suppressed by the grid. The collector plate was 60 mm wide and 140 mm long (fig. l b). It was grounded through an O R T E C preamplifier for heavy ion detectors. The cathode was at a negative potential between 15 and 200 V. Its size covered that of the collector and of the guard ring which surrounded the collector. The purpose of the guard ring was a good homogeneity of the electrostatic field below the collector• Inhomogeneities at the borders of the active volume of the chamber (the region below the collector) would have influenced the resolution (sect. 3.2). The width of the guard ring was 20 mm at the sides and the end of the system. At the entrance the guard ring was only 10 mm wide in order to minimize the distance between the window and the active volume to 13 mm. The energy loss in this region influenced the quality of the chamber as for a high Z
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ENERGY
RESOLUTION
was applied which could be varied between 0 and the potential of the cathode. The grid and the plates were isolated against each other and positioned by glass supports. In order to avoid field disturbances by parasitic charge, very little of the glass surface was seen from inside the active volume of the chamber. As gas fillings of the chamber, A r + 5 % CO2 and A r + 10% C H 4 w e r e used up to now. The pressure in the chamber was varied in order to investigate the dependence of the energy resolution and of the Zresolving power on the energy loss. Pressures up to 60 torr were applied which lead to energy losses of up to 75% of the initial energy of the fission products. The gas was flowing through the chamber with a rate of about ½ cm3/s at atmospheric pressure. The gas density in the chamber and, hence, the thickness x of the absorbing gas in the active volume were kept constant by an automatic pressure controller of Granville-Phillips. This controller maintained the I
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1,0
0,8
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0,l.
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pressure of the gas equal to that of a volume of reference, which was filled in the beginning of each experiment with the gas at the working pressure and was sealed. The volume of reference of 1 1 was inside the ionisation chamber volume of 18 1. Therefore, the temperature of the gas in both volumes was also equal. The comparison of the pressures of the two volumes was performed precisely by a capacitance manometer. Gases of high purity were used. All parts of the vacuum arrangement were carefully cleaned, outgasing materials as plastics were avoided. The double window with intermediate pumping was made of foils of Rovinal, which turned out to provide the most stable foils. They were supported by Cu-grids of 80 % transmission. With an overall thickness of 45 pg/cm 2 the window withstood pressures of up to 70 torr with negligible leaks. 3.2. EXPECTEDENERGYRESOLUTION Several effects contributed to the limitation of the energy resolution of the ionisation chamber. The contributions are plotted in fig. 2 as a function of the relative energy loss AE/E o. The calculations were performed for fission products of mass A = 90, which were used for the measurements. The initial energy E 0 of these products (behind the window) amounted to 83.6 MeV, the energy loss in the window was about 3 MeV. All contributions are the full widths at half maximum. Curve 1: The statistical fluctuation of the energy loss was calculated according to ref. 8. Curve 2: The uncertainty of the initial energy Eo amounted to 0.7 MeV. The spread of the beam of L O H E N G R I N was 0.35 MeV, the window caused a fluctuation of 0.6 MeV on account of inhomogeneities of its thickness.
0,6
0.2
165
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~
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t
10
20
30
4.0
50
60
70
80
90
100
&E/Eo[%] Fig. 2. T h e d i f f e r e n t c o n t r i b u t i o n s to the e n e r g y r e s o l u t i o n o f t h e chamber: curve 1 = energy straggling, curve 2 = fluctuations of E 0 , c u r v e 3 = e l e c t r o n i c a l noise, c u r v e 4 = t h i c k n e s s f l u c t u a t i o n s , c u r v e 5 = n u c l e a r collisions, c u r v e 6 = i n f l u e n c e o f t h e p o s i t i v e ions, c u r v e 7 = e x p e c t e d e n e r g y r e s o l u t i o n ( s u p e r p o s i t i o n o f c u r v e s 1-6).
Curve 3: The electronical noice was measured. Curve 4: The fluctuation 5x of the thickness of the absorbing gas was composed of remaining fluctuations of the gas density and of differences in the path lengths of single fission products. The remaining fluctuations of the gas density 6x/x were less than 0.3% as the pressure was kept constant within ___0.1 torr and the temperature difference between the chamber and the volume of reference was smaller than 1 °C. The different path lengths are due to inhomogeneities of the electrostatic field and to the divergence of the beam. In spite of the
166
K. S I S T E M I C H et al.
guard ring the field lines were curved slightly at the entrance boundary of the active volume. On account of the extension of the beam in the field direction the curvature caused slightly different path lengths of the fission products in the active volume. The contribution was estimated to be 5 x / x = 0 . 5 % . The influence of the different path lengths due to divergences of the beam including the deflections from scattering events in the window and in the gas was limited by the coincidence requirement between the chamber and the surface barrier detector to 0.15 %. Curve 5: The nuclear collisions with the gas atoms lead to large energy losses in single collisions of the fission products of up to a few percent of their energy. A considerable amount of energy was transferred to the knocked-on atoms. They produced additional charge which was observed in the chamber. As the energy, which was transferred to the gas atoms, varied for identical fission products, the energy resolution was affected. Curve 5 is an upper limit. It was calculated considering the mean energy loss of the fission products 4) in nuclear collisions and the range of the accelerated gas atoms in the active volume of the chamber. Curve 6: In spite of the Frisch grid, a contribution of 0.4% fwhm to the limitation of the energy resolution of the chamber was estimated to be due to the electrostatic induction from the positively charged gas atoms. The sum of the curves 1-6 (quadratically added) is shown as curve 7 in fig. 2. It represents the expected energy resolution of the chamber.
The potentials for the cathode and the Frisch grid were optimized experimentally to - 2 5 V and - 2 0 V, respectively. For these values maximum charge collection was obtained. (The plateau of the chamber ranged from - 2 0 to - 1 5 0 V, where the gas multiplication began.) The measured pulse height agreed reasonably well with the one, which was estimated from the produced charge. This proved that the charge collection was good, and that effects like recombination and electron capture by impurities in the gas did not affect the measurements noticeably. With the above mentioned potential at the collector the ratio of the field strength to the gas pressure was about 0.3 (V/cm) torr. At this ratio the electrons are expected to have a maximum drift velocity in the field'l). A standard electronical arrangement was used. It consisted of charge sensitive preamplifiers, main amplifiers, a slow coincidence curcuit and a 1024channel analyzer. The energy loss AE in the active volume of the chamber was calculated from eq. (3) after the factor F ( Z T ) Z K had been determined from the energy loss of the product in the whole gas region with the use of the surface barrier detector. The measured energy resolution is shown in fig. 3. for A r + C H , . The results for A r + C O 2 were identical within the uncertainties. The expected resolution (curve 7 of fig. 2) is also plotted. The disagreement between the experimental and the calculated resolution is by far beyond the uncertainties. It is due to a nonadequate consideration of the energy straggling in sect. 3.2. 4.2. ENERGY STRAGGLING Curve 1 of fig. 2 takes into account the statistical uncertainty of the number of electron-ion pairs produclO
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4. Results 4.1. MEASUREDENERGY RESOLUTION The energy resolution of the ionisation chamber was measured as a function of the gas pressure for fission products of the mass A = 90. At this mass LOHENG R I N provided an almost isotopically pure beam. The fractional independent fission yields for this mass chain are about 9'1°) r/ar=0.5% , q K r = 4 . 4 % and r/Rb-- 0.9 % . Consequently the Z-dependence of the energy loss did not affect the determination of the energy resolution.
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AE/Eo[%)
Fig. 3. The measured energy resolution, open circles = extrapolated values. The full line is used for the calculation o f the straggling curve in fig. 5b and of the nuclear charge resolving power, fig. 7. Hatched line = expected resolution (curve 7 of
fig. 2).
ENERGY
167
RESOLUTION
ed in the chamber. It does not consider the influence of the fluctuations of the ionic charge of the fission products along their path through the active volume. On account of charge changing collisions, the ionic charge of the fission products varies very often. According to eq. (1) the specific energy loss varies correspondingly. The overall loss A E can be calculated with an average charge q for the whole path as is done in eq. (3). For identical fission products with equal initial energy, q fluctuates, however, slightly. This fluctuation is well known to be important for the mass resolution of gasfilled isotope separators12-14), where separation in a magnetic field depends on ?/. Its contribution to the energy straggling of heavy ions has been pointed out recently by experiments on O and S ions 15) and on N, Ne and Ar ions16). It is reproduced by Monte Carlo calculations on the energy loss of ions in this mass regiontf'17). In order to verify the assumption that the discrepancy between the measured energy resolution and the expected one is due to an underestimation of the energy straggling, this contribution was determined separately. The energy straggling (fiAE)s (fwhm) shows up as an increase of the width (6E)sB of the pulse height distribution of the surface barrier detector for the fission products having passed through the gas compared to the width (fiE)sBo for monoenergetic ions of the same energy:
products at L O H E N G R I N cannot be lowered below 75 MeV without a degrader, (fiE)sBo was determined with the iodine beam of the Cologne tandem accelerator for low energies, fig. 4. One calibration run was performed before and another one after the experiments at L O H E N G R I N . (fiE)sB and (fiE)s ' w were determined again for fission products with A = 90 and E o = 83.6 MeV. The obtained results for the straggling are plotted in fig. 5a in the absolute scale and in fig. 5b relative to A E . The data result from four runs at different distances between the entrance window and the surface barrier detector. The large values at A E / E o = 14, 24, 48 and 62 were all obtained at the largest distance (25 cm). A systematic error may be the reason of their deviation from the general trend. It is apparent from fig. 5b that the straggling exceeds considerably the purely statistical part, curve 1 of fig. 2. The solid line in fig. 5b shows the energy straggling as it is deduced from the energy resolution of the chamber. It is the (quadratic) difference between the measured I ~
2.0 -
(6E)s2,w] ÷
-
(6)
(fiE)s,w is the contribution of the separator and the window to the width, it is measured without gas in the chamber. As the initial energy of the fission
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Fig. 4. Intrinsic width of the surface barrier detector. Open and closed circles = results obtained at the tandem accelerator before and after the experiments at LOHENGRIN, triangles = values from LOHENGRIN. The line is used in eq. (6).
100
]
(b) F i g . 5. T h e e n e r g y direct determination
straggling. The points are the results of the f r o m e q . ( 6 ) , t h e c u r v e is d e d u c e d from the
energy resolution of the ionisation chamber.
168
K. S I S T E M I C H et al.
resolution and the contributions of curves 2-6 of fig. 2. The agreement between this curve and the directly measured energy straggling is good. The straggling has also been determined for CH4, Kr, Xe and pure Ar. The results do not differ from those of fig. 5 by more than the experimental uncertainties. The Monte Carlo program of ref. 14 was applied for the calculation of the energy straggling of light fission products in Ar. The results were in general even slightly larger than the measured energy straggling. This is supposed to be due to the particular choice of the input data for the energy loss cross sections and the charge changing cross sections, which are not measured for these combinations of incoming particles and target material in the given energy range. 4.3. NUCLEAR CHARGE RESOLVING POWER
The energy losses of fission products with neighbouring nuclear charges could not be resolved with the ionisation chamber. Therefore the nuclear charge resolving power was calculated from the energy resolution of the chamber according to eq. (5). The charge dispersion Fz was obtained in the following way. From eq. (3), F z is deduced to be Fz = FA -- I - - ( 1 - A E / E o ) ~ 1 + (1 - AE/Eo) ~'
(7)
where F a is the mass dispersion of the chamber. Relationship (7) is valid under the condition that ZocA, which is fulfilled for fission products within 1%. The dispersion Fa = d l n A E / d l n A = [(6AE)A/AE]/[AA/ A] is deduced from the measured difference (SAE)A of the energy loss of fission products with the masses A = 90 and 99 to be 0.85+0.02 at AE/E o = 0.75. This corresponds to a value of 0.75+0.02 for the quantity '1,0
--
I
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f
I
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K in eq. (3). Fig. 6 shows the resultant Fz as a function of the energy loss in the active volume of the chamber. The nuclear charge resolving power is given in fig. 7. The maximum obtainable resolution is Z / A Z = 30.0+ 1.5, the uncertainty resulting from the experimental uncertainties of (6AE)~c and of Fz. It is reached for an energy loss in the chamber of about 75 % of the initial energy. The Z/AZ-curve is extrapolated above AE/E o = 75 %. A measurement of the energy resolution at these energy losses was not possible as the final energy of the fission products was too low for the necessary coincidence between the surface barrier detector and the chamber. The extrapolation of the energy resolution (fig. 5b) is based on the calculated curves 2-6 of fig. 2 and on an extrapolation of the straggling. For the same fission products a maximum resolving power of Z / A Z ~ 4 5 has been obtained for A E = 60 MeV with detectors measuring the energy loss in solid state absorbersl°'ls). The corresponding energy resolution amounted to about 1.2% compared to 1.7% for the ionisation chamber. The difference is supposed to be mainly due to a smaller contribution from the energy straggling in solids. The straggling in carbon foils was measured to be 1.5 times smaller than for Ar19). This points to either a smaller width of the ionic charge distribution of heavy ions in solids or to a larger cross section for charge changing collisions in solids than in gases2°). Comparative measurements of the energy straggling in solids and in gases thus may give some information of the actual state of an ion passing through a solid. 5. Conclusions
The investigations have shown that a transmission N 40
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Fig. 7. The nuclear charge resolving power.
ENERGY RESOLUTION
ionisation chamber for fission products could be brought to an energy resolution of 1.7 % and to a nuclear charge resolving power of Z/AZ~ 30. The most important limitation of the energy resolution is the energy straggling which is larger than in solids and which exceeds considerably the predicted values of ref. 8. Hence, the results are valid for any gas-filled detector for heavy ions with a mass around A = 100 and an initial energy E/A ~ 1 MeV per nucleon. About the same resolution has been obtained recently el) with a similar ionisation chamber for reaction products having Z < 3 0 and an energy of about 100MeV. For reaction products in the Kr-region with higher energies, however, a resolution of Z/AZ > 40 was reported zz) for this chamber. The increased resolution is probably due to a larger value of K [see eq. (3)] leading to a larger Fz, as the reaction products are more strongly ionized, and to a smaller influence of the energy straggling, as the straggling tends towards a saturation (fig. 5a). Further experiments and calculations on the energy straggling are needed before transferring the present results to other mass and energy regions. The authors acknowledge with pleasure the support from Drs M. Asghar, G. Bailleul, J. P. Gautheron, E. Moll, H. Schrader, G. Siegert and from Dipl. Phys. J. Greif, Mr H. Hammers, Mr A. Beynet and Mr J. L. Coquin of the Institut Laue-Langevin in the development of the ionisation chamber and in the experiments at L O H E N G R I N . Many members of the technical staffs of the Institut Laue-Langevin and of the lnstitut ffir Kernphysik of the Kernforschungsanlage Jfilich, especially Mr W. Klein and Mr G. Schuba, contributed decisively to parts of the chamber. The authors want also to thank Dipl. Phys. S. Hagmann and Dipl. Phys. U. Scharfer for supporting the calibration runs at Cologne, as well as Prof. Dr H. Wollnik for fruitful discussions. Refcrcnces
1) E. Moll, H. Ewald, H. Wollnik, P. Armbruster, G. Fiebig and H. Lawin, Proc. Int. Conf. on Electromagnetic isotope
2)
3)
4) 5) 6) 7) s) 9) 10)
al) 12) 13) 14)
15) 16)
169
separators and the techniques of their applications, Marburg, 1970 (eds. H. Wagner and W. Walcher; report B M B W - F B K 70-28, 1970) p. 241. E. Moll, G. Siegert, M. Asghar, G. Bailleul, J. P. Bocquet, J . P . Gautheron, J. Greif, H. Hammers, H. Schrader, P. Armbruster, G. Fiebig, H. Lawin and K. Sistemich, Proc. 8th Int. EMIS Conf. on Low energy ion accelerators and mass separators, Sk6vde, 1973 (eds. G. Andersson and G. Holm6n) p. 249. E. Moll, H. Schrader, G. Siegert, M. Asghar, J. P. Bocquet, G. Bailleul, J . P . Gautheron, J. Greif, G . I . Crawford, C. Chauvin, H. Ewald, H. Wollnik, P. Armbruster, G. Fiebig, H. Lawin and K. Sistemich, Nucl. Instr. and Meth. 123 (1975) 615. T. E. Pierce and M. Blann, Phys. Rev. 173 (1968) 390. O. Frisch, British Atomic Energy report BR-49 (1944). H. Neuert, Kernphysikalische Meflverfi~hren (Verlag G. Braun, Karlsruhe, 1966). O. Bunemann, T . E . Cranshaw and J . A . Harvey, Can. J. Res. A27 (1949) 191. C. Tschalfir, Nucl. Instr. and Meth. 61 (1968) 141. S. Amiel and H. Feldstein, in Physics and chemistry of fission 1973, vol 2 (IAEA, Vienna, 1974) p. 65. G. Siegert, H. Wollnik, J. Greif, G. Fiedler, M. Asghar, G. Bailleul, J . P . Bocquet, J . P . Gautheron, H. Schrader, H. Ewald and P. Armbruster, Phys. Lett. 53B (1974) 45. H. W. Fulbright, in Encyclopedia of physics, vol. 45 (Springer Verlag, Berlin, 1958). p. Armbruster, Nukleonik 3 (1961) 188. H. Lawin, Thesis (Universit/it zu K61n, 1976). G. Fiebig, Proc. 8th Int. EMIS Conf. on Low energy ion accelerators and mass separators, SkOvde, 1973 (eds. G. Andersson and G. Holm6n) p. 232. H. Schmidt-B6cking and C. Gtith, Annual report 1973 of the Max-Planck Institut ftir Kernphysik, Heidelberg, p. 104. B. Efken, D. Hahn, D. Hilscher and G. Wiistefeld; Nucl. Instr. and Meth. 129 (1975) 219.
17) O. Vollmer, Nucl. Instr. and Meth. 121 (1974) 373. is) H.-G. Clerc, K.-H. Schmidt, H. Wohlfarth, W. Lang, H. Schrader, K. E. Pferdekfimper, R. Jungmann, M. Asghar, J. P. Bocquet and G. Siegert, Nucl. Instr. and Meth. 124 (1975) 607. 19) K.-H. Schmidt, Thesis (Technische Hochschule Darmstadt, 1975). z0) p. Armbruster, K. Sistemich, J. P. Bocquet, Ch. Chauvin and Y. Glaize, Proc. 6th Int. Conf. on Atomic collisions in solids, Amsterdam, 1975, Nucl. Instr. and Meth. 132 (1976) 129. 21) M. M. Fowler and R. C. Jared, Nucl. Instr. and Meth. 124 (1975) 341. 22) L. Moretto, LBL Berkeley, private communication.
INSTRUMENTS
AND
ENERGY RESOLUTION IONISATION
CHAMBER
METHODS
I33
(I976) I63-I69;
AND NUCLEAR FOR FISSION
©
NORTH-HOLLAND
CHARGE RESOLVING
POWER
PUBLISHING
CO.
OF AN
PRODUCTS
K. S I S T E M I C H *
lnstitut fiir Kernphysik, Kernforschungsanlage Jii6ch, D-517 Jiilich, Germany P. A R M B R U S T E R
Gesellschaft fiir Schwerionenforsehung, D-61 Darmstadt, Germany and J.P. BOCQUETt,
CH. CHAUVIN?
and Y. G L A I Z E
lnstitut Laue-Langevin, F-38 Grenoble, France The L O H E N G R I N Collaboration, Institut Laue-Langevin, F-38 Grenoble, France Received 7 N o v e m b e r 1975 A t r a n s m i s s i o n ionisation c h a m b e r with high energy resolution has been developed a n d its nuclear charge resolving power for fission products has been investigated. T h e c h a m b e r was tested with the m a s s and energy separated fission products o f the parabola spectrograph L O H E N G R I N o f the Institut L a u e - L a n g e v i n , Grenoble. T h e obtained energy resolution a m o u n t e d to 1.7% for 90Kr fission products which deposit inside the c h a m b e r a b o u t 75% o f their initial energy o f 83.6 MeV. T h e energy straggling in the A r + C H 4 a n d A r + CO2 fillings o f the c h a m b e r turned out to be m u c h larger t h a n predicted and to limit the resolution o f the chamber. T h e nuclear charge resolving power was determined to be Z / A Z ~ 30.
1. Introduction
2. The method
The determination of the nuclear charge Z of nuclear reaction products is necessary in many investigations. With the onset of the studies at heavy ion accelerators the determination of Z for heavy products has gained increasing importance. One of the commonly used methods for the identification of Z is based on the measurement of the energy loss AE in an absorbing medium, as AE depends on Z. An ionisation chamber was developed in order to investigate the Z-resolving power which can be obtained for fission products. The chamber was adapted to the fission product mass separator L O H E N G R I N 1-3) at the high flux reactor of the Institut Laue-Langevin, Grenoble. The separator is well suited for the test of such detectors. It provides beams of fission products with one single mass and a well defined kinetic energy, but with different nuclear charges. Many different masses and an extended region of kinetic energies are available. The chamber was designed for a high energy resolution. The energy straggling turned out to be the most serious limitation for the energy resolution. It was investigated in some detail.
The differential energy loss dE/dx of heavy ions passing through matter is determined by the velocity v and the ionic charge q of the ions and by the nuclear charge ZT of the target:
* D u r i n g the preparation o f the c h a m b e r at the lnstitut L a u e Langevin, on leave o f absence. ? Also from D R F C E N Grenoble.
dE dx
-
f ( v , q , ZT).
(1)
As the velocity is given by the mass A and the kinetic energy E of the ions and as the ionic charge is a function of their nuclear charge Z, dE/dx can be written as dE dx
-
f ( x / [ 2 E / A ] , Z, ZT).
(2)
The integration ofeq. (2) leads to the energy loss in the target of thickness x. With dE/dx = - - v z K F ( Z T ) , 0 < K < 2, which holds approximately in the mass and energy region of the fission products according to the experimental results of ref. 4, the integration results in AE = Vo F ( Z T ) Z K x - F z ( Z T ) z 2 K x 2 , 2A
(3)
where v0 is the initial velocity of the ions. This equation can be used for the calculation of the energy loss at any value of x, if the factor F(ZT)Z K is determined by a measurement of AE for one value of x. It should be
164
K. SISTEMICH et al.
emphasized that AE depends both on A and Z even for fission products with identical initial velocity, whereas dE/dx depends only on Z in this case. The resolving power Z/AZ of the ionisation chamber is defined by the condition that its energy resolution (SAE)m should be equal to the difference of the energy losses of two neighbouring isobars with equal kinetic energy:
(4)
(SAE),c = (6AE)Az=,.
This equality can be transformed into the relationship Z --
AZ
l =
Fz
,
(SAE)~c/AE
(5)
where F z = dlnAE/dlnZ is the charge dispersion and (6AE)~c/AE is the energy resolution of the chamber. These parameters have to be determined experimentally.
3. Experimental arrangement
resolving power a large energy loss AE inside the active volume and a large charge dispersion Fz were needed (sect. 4). Both AE and Fz are highest for high energy and, hence, the beginning of the path of the fission products in the gas was the most effective part of the range. All plates were made of copper. Their thickness amounted to 1 ram. A small value of this thickness guaranteed a small capacity between the collector and the guard ring, the overall capacity of the collector against the surroundings being 60 pF. The Frisch grid was made of steel wires of 0.3 mm diameter and 1.5 mm distance. The distances to the collector and to the cathode amounted to 7 and 10 ram. With these dimensions and the given geometry of the plates and the beam a reduction of the electrostatic induction of charge on the collector by the produced positive gas ions to less than 1% was obtained7). The wires were clamped into a stainless steel frame. The frame was open at the entrance, again in order to minimize the distance between window and active volume. The area covered by the wires amounted to 70 mm times 160 ram. At the grid a negative potential
3.1. [ONISATIONCHAMBER The ionisation chamber was of the parallel-plate type with a Frisch grid 5) and a guard ring6). It is shown schematically in fig. 1. The plate system was adapted to the long and narrow focus of L O H E N G R I N of about 700 times 10 mm 2. A section of 30 times 4 mm 2 of this focus could be accepted by the chamber. The fission products entered the chamber through a thin window. Their paths were parallel to the plates, and they were stopped in an ORTEC surface barrier detector for fission products. The electrons which they produced in the gas below the collector plate were used for the AE-information. The influence of the positive ions was suppressed by the grid. The collector plate was 60 mm wide and 140 mm long (fig. l b). It was grounded through an O R T E C preamplifier for heavy ion detectors. The cathode was at a negative potential between 15 and 200 V. Its size covered that of the collector and of the guard ring which surrounded the collector. The purpose of the guard ring was a good homogeneity of the electrostatic field below the collector• Inhomogeneities at the borders of the active volume of the chamber (the region below the collector) would have influenced the resolution (sect. 3.2). The width of the guard ring was 20 mm at the sides and the end of the system. At the entrance the guard ring was only 10 mm wide in order to minimize the distance between the window and the active volume to 13 mm. The energy loss in this region influenced the quality of the chamber as for a high Z
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ENERGY
RESOLUTION
was applied which could be varied between 0 and the potential of the cathode. The grid and the plates were isolated against each other and positioned by glass supports. In order to avoid field disturbances by parasitic charge, very little of the glass surface was seen from inside the active volume of the chamber. As gas fillings of the chamber, A r + 5 % CO2 and A r + 10% C H 4 w e r e used up to now. The pressure in the chamber was varied in order to investigate the dependence of the energy resolution and of the Zresolving power on the energy loss. Pressures up to 60 torr were applied which lead to energy losses of up to 75% of the initial energy of the fission products. The gas was flowing through the chamber with a rate of about ½ cm3/s at atmospheric pressure. The gas density in the chamber and, hence, the thickness x of the absorbing gas in the active volume were kept constant by an automatic pressure controller of Granville-Phillips. This controller maintained the I
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pressure of the gas equal to that of a volume of reference, which was filled in the beginning of each experiment with the gas at the working pressure and was sealed. The volume of reference of 1 1 was inside the ionisation chamber volume of 18 1. Therefore, the temperature of the gas in both volumes was also equal. The comparison of the pressures of the two volumes was performed precisely by a capacitance manometer. Gases of high purity were used. All parts of the vacuum arrangement were carefully cleaned, outgasing materials as plastics were avoided. The double window with intermediate pumping was made of foils of Rovinal, which turned out to provide the most stable foils. They were supported by Cu-grids of 80 % transmission. With an overall thickness of 45 pg/cm 2 the window withstood pressures of up to 70 torr with negligible leaks. 3.2. EXPECTEDENERGYRESOLUTION Several effects contributed to the limitation of the energy resolution of the ionisation chamber. The contributions are plotted in fig. 2 as a function of the relative energy loss AE/E o. The calculations were performed for fission products of mass A = 90, which were used for the measurements. The initial energy E 0 of these products (behind the window) amounted to 83.6 MeV, the energy loss in the window was about 3 MeV. All contributions are the full widths at half maximum. Curve 1: The statistical fluctuation of the energy loss was calculated according to ref. 8. Curve 2: The uncertainty of the initial energy Eo amounted to 0.7 MeV. The spread of the beam of L O H E N G R I N was 0.35 MeV, the window caused a fluctuation of 0.6 MeV on account of inhomogeneities of its thickness.
0,6
0.2
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10
20
30
4.0
50
60
70
80
90
100
&E/Eo[%] Fig. 2. T h e d i f f e r e n t c o n t r i b u t i o n s to the e n e r g y r e s o l u t i o n o f t h e chamber: curve 1 = energy straggling, curve 2 = fluctuations of E 0 , c u r v e 3 = e l e c t r o n i c a l noise, c u r v e 4 = t h i c k n e s s f l u c t u a t i o n s , c u r v e 5 = n u c l e a r collisions, c u r v e 6 = i n f l u e n c e o f t h e p o s i t i v e ions, c u r v e 7 = e x p e c t e d e n e r g y r e s o l u t i o n ( s u p e r p o s i t i o n o f c u r v e s 1-6).
Curve 3: The electronical noice was measured. Curve 4: The fluctuation 5x of the thickness of the absorbing gas was composed of remaining fluctuations of the gas density and of differences in the path lengths of single fission products. The remaining fluctuations of the gas density 6x/x were less than 0.3% as the pressure was kept constant within ___0.1 torr and the temperature difference between the chamber and the volume of reference was smaller than 1 °C. The different path lengths are due to inhomogeneities of the electrostatic field and to the divergence of the beam. In spite of the
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K. S I S T E M I C H et al.
guard ring the field lines were curved slightly at the entrance boundary of the active volume. On account of the extension of the beam in the field direction the curvature caused slightly different path lengths of the fission products in the active volume. The contribution was estimated to be 5 x / x = 0 . 5 % . The influence of the different path lengths due to divergences of the beam including the deflections from scattering events in the window and in the gas was limited by the coincidence requirement between the chamber and the surface barrier detector to 0.15 %. Curve 5: The nuclear collisions with the gas atoms lead to large energy losses in single collisions of the fission products of up to a few percent of their energy. A considerable amount of energy was transferred to the knocked-on atoms. They produced additional charge which was observed in the chamber. As the energy, which was transferred to the gas atoms, varied for identical fission products, the energy resolution was affected. Curve 5 is an upper limit. It was calculated considering the mean energy loss of the fission products 4) in nuclear collisions and the range of the accelerated gas atoms in the active volume of the chamber. Curve 6: In spite of the Frisch grid, a contribution of 0.4% fwhm to the limitation of the energy resolution of the chamber was estimated to be due to the electrostatic induction from the positively charged gas atoms. The sum of the curves 1-6 (quadratically added) is shown as curve 7 in fig. 2. It represents the expected energy resolution of the chamber.
The potentials for the cathode and the Frisch grid were optimized experimentally to - 2 5 V and - 2 0 V, respectively. For these values maximum charge collection was obtained. (The plateau of the chamber ranged from - 2 0 to - 1 5 0 V, where the gas multiplication began.) The measured pulse height agreed reasonably well with the one, which was estimated from the produced charge. This proved that the charge collection was good, and that effects like recombination and electron capture by impurities in the gas did not affect the measurements noticeably. With the above mentioned potential at the collector the ratio of the field strength to the gas pressure was about 0.3 (V/cm) torr. At this ratio the electrons are expected to have a maximum drift velocity in the field'l). A standard electronical arrangement was used. It consisted of charge sensitive preamplifiers, main amplifiers, a slow coincidence curcuit and a 1024channel analyzer. The energy loss AE in the active volume of the chamber was calculated from eq. (3) after the factor F ( Z T ) Z K had been determined from the energy loss of the product in the whole gas region with the use of the surface barrier detector. The measured energy resolution is shown in fig. 3. for A r + C H , . The results for A r + C O 2 were identical within the uncertainties. The expected resolution (curve 7 of fig. 2) is also plotted. The disagreement between the experimental and the calculated resolution is by far beyond the uncertainties. It is due to a nonadequate consideration of the energy straggling in sect. 3.2. 4.2. ENERGY STRAGGLING Curve 1 of fig. 2 takes into account the statistical uncertainty of the number of electron-ion pairs produclO
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4. Results 4.1. MEASUREDENERGY RESOLUTION The energy resolution of the ionisation chamber was measured as a function of the gas pressure for fission products of the mass A = 90. At this mass LOHENG R I N provided an almost isotopically pure beam. The fractional independent fission yields for this mass chain are about 9'1°) r/ar=0.5% , q K r = 4 . 4 % and r/Rb-- 0.9 % . Consequently the Z-dependence of the energy loss did not affect the determination of the energy resolution.
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fig. 2).
ENERGY
167
RESOLUTION
ed in the chamber. It does not consider the influence of the fluctuations of the ionic charge of the fission products along their path through the active volume. On account of charge changing collisions, the ionic charge of the fission products varies very often. According to eq. (1) the specific energy loss varies correspondingly. The overall loss A E can be calculated with an average charge q for the whole path as is done in eq. (3). For identical fission products with equal initial energy, q fluctuates, however, slightly. This fluctuation is well known to be important for the mass resolution of gasfilled isotope separators12-14), where separation in a magnetic field depends on ?/. Its contribution to the energy straggling of heavy ions has been pointed out recently by experiments on O and S ions 15) and on N, Ne and Ar ions16). It is reproduced by Monte Carlo calculations on the energy loss of ions in this mass regiontf'17). In order to verify the assumption that the discrepancy between the measured energy resolution and the expected one is due to an underestimation of the energy straggling, this contribution was determined separately. The energy straggling (fiAE)s (fwhm) shows up as an increase of the width (6E)sB of the pulse height distribution of the surface barrier detector for the fission products having passed through the gas compared to the width (fiE)sBo for monoenergetic ions of the same energy:
products at L O H E N G R I N cannot be lowered below 75 MeV without a degrader, (fiE)sBo was determined with the iodine beam of the Cologne tandem accelerator for low energies, fig. 4. One calibration run was performed before and another one after the experiments at L O H E N G R I N . (fiE)sB and (fiE)s ' w were determined again for fission products with A = 90 and E o = 83.6 MeV. The obtained results for the straggling are plotted in fig. 5a in the absolute scale and in fig. 5b relative to A E . The data result from four runs at different distances between the entrance window and the surface barrier detector. The large values at A E / E o = 14, 24, 48 and 62 were all obtained at the largest distance (25 cm). A systematic error may be the reason of their deviation from the general trend. It is apparent from fig. 5b that the straggling exceeds considerably the purely statistical part, curve 1 of fig. 2. The solid line in fig. 5b shows the energy straggling as it is deduced from the energy resolution of the chamber. It is the (quadratic) difference between the measured I ~
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100
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(b) F i g . 5. T h e e n e r g y direct determination
straggling. The points are the results of the f r o m e q . ( 6 ) , t h e c u r v e is d e d u c e d from the
energy resolution of the ionisation chamber.
168
K. S I S T E M I C H et al.
resolution and the contributions of curves 2-6 of fig. 2. The agreement between this curve and the directly measured energy straggling is good. The straggling has also been determined for CH4, Kr, Xe and pure Ar. The results do not differ from those of fig. 5 by more than the experimental uncertainties. The Monte Carlo program of ref. 14 was applied for the calculation of the energy straggling of light fission products in Ar. The results were in general even slightly larger than the measured energy straggling. This is supposed to be due to the particular choice of the input data for the energy loss cross sections and the charge changing cross sections, which are not measured for these combinations of incoming particles and target material in the given energy range. 4.3. NUCLEAR CHARGE RESOLVING POWER
The energy losses of fission products with neighbouring nuclear charges could not be resolved with the ionisation chamber. Therefore the nuclear charge resolving power was calculated from the energy resolution of the chamber according to eq. (5). The charge dispersion Fz was obtained in the following way. From eq. (3), F z is deduced to be Fz = FA -- I - - ( 1 - A E / E o ) ~ 1 + (1 - AE/Eo) ~'
(7)
where F a is the mass dispersion of the chamber. Relationship (7) is valid under the condition that ZocA, which is fulfilled for fission products within 1%. The dispersion Fa = d l n A E / d l n A = [(6AE)A/AE]/[AA/ A] is deduced from the measured difference (SAE)A of the energy loss of fission products with the masses A = 90 and 99 to be 0.85+0.02 at AE/E o = 0.75. This corresponds to a value of 0.75+0.02 for the quantity '1,0
--
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K in eq. (3). Fig. 6 shows the resultant Fz as a function of the energy loss in the active volume of the chamber. The nuclear charge resolving power is given in fig. 7. The maximum obtainable resolution is Z / A Z = 30.0+ 1.5, the uncertainty resulting from the experimental uncertainties of (6AE)~c and of Fz. It is reached for an energy loss in the chamber of about 75 % of the initial energy. The Z/AZ-curve is extrapolated above AE/E o = 75 %. A measurement of the energy resolution at these energy losses was not possible as the final energy of the fission products was too low for the necessary coincidence between the surface barrier detector and the chamber. The extrapolation of the energy resolution (fig. 5b) is based on the calculated curves 2-6 of fig. 2 and on an extrapolation of the straggling. For the same fission products a maximum resolving power of Z / A Z ~ 4 5 has been obtained for A E = 60 MeV with detectors measuring the energy loss in solid state absorbersl°'ls). The corresponding energy resolution amounted to about 1.2% compared to 1.7% for the ionisation chamber. The difference is supposed to be mainly due to a smaller contribution from the energy straggling in solids. The straggling in carbon foils was measured to be 1.5 times smaller than for Ar19). This points to either a smaller width of the ionic charge distribution of heavy ions in solids or to a larger cross section for charge changing collisions in solids than in gases2°). Comparative measurements of the energy straggling in solids and in gases thus may give some information of the actual state of an ion passing through a solid. 5. Conclusions
The investigations have shown that a transmission N 40
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Fig. 7. The nuclear charge resolving power.
ENERGY RESOLUTION
ionisation chamber for fission products could be brought to an energy resolution of 1.7 % and to a nuclear charge resolving power of Z/AZ~ 30. The most important limitation of the energy resolution is the energy straggling which is larger than in solids and which exceeds considerably the predicted values of ref. 8. Hence, the results are valid for any gas-filled detector for heavy ions with a mass around A = 100 and an initial energy E/A ~ 1 MeV per nucleon. About the same resolution has been obtained recently el) with a similar ionisation chamber for reaction products having Z < 3 0 and an energy of about 100MeV. For reaction products in the Kr-region with higher energies, however, a resolution of Z/AZ > 40 was reported zz) for this chamber. The increased resolution is probably due to a larger value of K [see eq. (3)] leading to a larger Fz, as the reaction products are more strongly ionized, and to a smaller influence of the energy straggling, as the straggling tends towards a saturation (fig. 5a). Further experiments and calculations on the energy straggling are needed before transferring the present results to other mass and energy regions. The authors acknowledge with pleasure the support from Drs M. Asghar, G. Bailleul, J. P. Gautheron, E. Moll, H. Schrader, G. Siegert and from Dipl. Phys. J. Greif, Mr H. Hammers, Mr A. Beynet and Mr J. L. Coquin of the Institut Laue-Langevin in the development of the ionisation chamber and in the experiments at L O H E N G R I N . Many members of the technical staffs of the Institut Laue-Langevin and of the lnstitut ffir Kernphysik of the Kernforschungsanlage Jfilich, especially Mr W. Klein and Mr G. Schuba, contributed decisively to parts of the chamber. The authors want also to thank Dipl. Phys. S. Hagmann and Dipl. Phys. U. Scharfer for supporting the calibration runs at Cologne, as well as Prof. Dr H. Wollnik for fruitful discussions. Refcrcnces
1) E. Moll, H. Ewald, H. Wollnik, P. Armbruster, G. Fiebig and H. Lawin, Proc. Int. Conf. on Electromagnetic isotope
2)
3)
4) 5) 6) 7) s) 9) 10)
al) 12) 13) 14)
15) 16)
169
separators and the techniques of their applications, Marburg, 1970 (eds. H. Wagner and W. Walcher; report B M B W - F B K 70-28, 1970) p. 241. E. Moll, G. Siegert, M. Asghar, G. Bailleul, J. P. Bocquet, J . P . Gautheron, J. Greif, H. Hammers, H. Schrader, P. Armbruster, G. Fiebig, H. Lawin and K. Sistemich, Proc. 8th Int. EMIS Conf. on Low energy ion accelerators and mass separators, Sk6vde, 1973 (eds. G. Andersson and G. Holm6n) p. 249. E. Moll, H. Schrader, G. Siegert, M. Asghar, J. P. Bocquet, G. Bailleul, J . P . Gautheron, J. Greif, G . I . Crawford, C. Chauvin, H. Ewald, H. Wollnik, P. Armbruster, G. Fiebig, H. Lawin and K. Sistemich, Nucl. Instr. and Meth. 123 (1975) 615. T. E. Pierce and M. Blann, Phys. Rev. 173 (1968) 390. O. Frisch, British Atomic Energy report BR-49 (1944). H. Neuert, Kernphysikalische Meflverfi~hren (Verlag G. Braun, Karlsruhe, 1966). O. Bunemann, T . E . Cranshaw and J . A . Harvey, Can. J. Res. A27 (1949) 191. C. Tschalfir, Nucl. Instr. and Meth. 61 (1968) 141. S. Amiel and H. Feldstein, in Physics and chemistry of fission 1973, vol 2 (IAEA, Vienna, 1974) p. 65. G. Siegert, H. Wollnik, J. Greif, G. Fiedler, M. Asghar, G. Bailleul, J . P . Bocquet, J . P . Gautheron, H. Schrader, H. Ewald and P. Armbruster, Phys. Lett. 53B (1974) 45. H. W. Fulbright, in Encyclopedia of physics, vol. 45 (Springer Verlag, Berlin, 1958). p. Armbruster, Nukleonik 3 (1961) 188. H. Lawin, Thesis (Universit/it zu K61n, 1976). G. Fiebig, Proc. 8th Int. EMIS Conf. on Low energy ion accelerators and mass separators, SkOvde, 1973 (eds. G. Andersson and G. Holm6n) p. 232. H. Schmidt-B6cking and C. Gtith, Annual report 1973 of the Max-Planck Institut ftir Kernphysik, Heidelberg, p. 104. B. Efken, D. Hahn, D. Hilscher and G. Wiistefeld; Nucl. Instr. and Meth. 129 (1975) 219.
17) O. Vollmer, Nucl. Instr. and Meth. 121 (1974) 373. is) H.-G. Clerc, K.-H. Schmidt, H. Wohlfarth, W. Lang, H. Schrader, K. E. Pferdekfimper, R. Jungmann, M. Asghar, J. P. Bocquet and G. Siegert, Nucl. Instr. and Meth. 124 (1975) 607. 19) K.-H. Schmidt, Thesis (Technische Hochschule Darmstadt, 1975). z0) p. Armbruster, K. Sistemich, J. P. Bocquet, Ch. Chauvin and Y. Glaize, Proc. 6th Int. Conf. on Atomic collisions in solids, Amsterdam, 1975, Nucl. Instr. and Meth. 132 (1976) 129. 21) M. M. Fowler and R. C. Jared, Nucl. Instr. and Meth. 124 (1975) 341. 22) L. Moretto, LBL Berkeley, private communication.