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A double time-of-flight (E−ΔE) mass spectrometer
Nuclear Instruments and Methods 193 (1982) 513-520 North-Holland Publishing Company
513
A DOUBLE TIME-OF-FLIGHT ( E - A E ) MASS SPECTROMETER * Youhanna FARES ** and M.L. MUGA Chemistry Department, University o f Florida, Gainesville, Florida 32601, U.S.A.
Received 7 May 1981 and in revised form 7 September 1981
A correlated (E-AE) double time-of-flight mass spectrometer was designed and used in the dE/dx measurements of 23su fission fragments in nickel as a function of mass and energy. The system has a time resolution of 1 ns, a mass resolution of ,~u~/L= 2.24 amu, AMH = 1.54 amu, and energy resolution of A E L = 1.56 MeV, AEH = 1.08 MeV for the median light and median heavy fission fragments, respectively. A simple Range-Energy relation of the form R = KE °~ was fitted to the data points. K was found to be mass dependent and decreases with the increase of the ion mass. For a = 0.75, best range-energy results were obtained for K = 0.176 mg em2/MeVO.75. K was determined from the values of the ranges and kinetic energies of the median light and median heavy fragments and then extrapolated to other masses.
1. Introduction The study of the passage of charged particles through matter continues to be of importance in atomic and nuclear physics. A knowledge o f fission fragments ranges and their energy loss in solid and gaseous media is not only important for mass identification o f heavy ions but also essential to the application o f fission fragment-induced chemical reactions, i.e. o f fissio-chemistry. Such knowledge is directly applicable to energy conversion in the fission electric cell as well as to radiation induced ionization for space-charge neutralization in thermionics [1]. Experiments in the above mentioned fields require accurate reference data on particle penetration, and conversely, they feed back new information on the penetration process itself. An example of the feed-back is the discovery of difference in the penetration o f hyperons o f opposite charge [2]. For the development o f a universal range-energy relation for heavy charged particles, the starting point would be the accurate measurement of stopping cross sections as a function o f energy and mass. The choice o f time-offlight technique coupled with solid-state barrier
* Work supported in part by the US Atomic Energy Commission. ** Present address: Biosystems Research Division, Dept. of Industrial Engineering, Texas A and M University, College Station, Texas 77843, USA. 0029-554X/82/0000-0000/$02.75
© 1982 North-Holland
detector seems to provide the best method for accuracy in the measurements of velocity, energy, and mass of fission fragments. The main goal o f this work was to: (1) devise and construct a double time-offlight (TOF) ( E - A E ) correlated system; (2) determine accurately the stopping powers o f the spectrum of fission fragments obtained from thermal neutron induced fission of 23SU; (3) find an empirical relation of the stopping power o f a heavy charged particle as a function o f its important parameters, i.e., its energy (velocity), mass and possibly its atomic number, as well as the mass o f the stopping material. In this paper we describe the experiment and the construction and the characteristics of the spectrometer. The results of the other two goals will be reported elsewhere.
2. Principle of experiment, electronic system and signal procedures The principle o f this experiment consists o f the measurements of the times-of-flight, TOF, over accurately known distances (hence, velocities), of a fission fragment before and after its passage through a nickel foil o f a given thickness, and the measurement of the fragment's residual kinetic energy by means of a surface barrier solid-state detector. The final TOF, together with the residual kinetic energy, gives the mass o f the fission fragment, via E = ~ M V 2 . The mass
514
Y. Fares, M.L. Muga / A double TOF (E- AE) mass spectrometer
Fig. 1. Experimental layout and sequence of events from left to right. Notation: F = fuel box of UFTR; S = 23SU source; C = collimating tube; B = beam tube; T1, T2 = flight tube No. 1 and No. 2, respectively; E.L.1, E.L.2, E.L.3 = electron Lenses No. 1, No. 2, and No. 3, respectively; S.S.D. = solid state detector; C.P.A. = charge sensitive preamplifier; F.C.F. = fast cathode follow preamplifier; F.A. = fast amplifier; F.D. = fast discriminator; F.C. = fast coincidence; S.C. = slow coincidence;'L.A. = linear amplifier; L.D.A. = linear delay amplifier; B.A. = bias amplifier; S.C.A. = single channel analyzer; T.H.C. = time-to-pulse height converter; S.P.R. = start-pulse pile-up reject; A.D.C. = analogue to digital converter. of the fission fragment, together with its initial velocity, gives the initial energy of the fragment. Hence, we obtain AE, the energy lost when the fragment passes through the nickel foil. Fig. 1. represents the layout of the experiment and the sequence of events. Fission was induced in the 23Su source "S" (in the central fuel core region of the University of Florida Training Reactor). The fission fragment traveled up a 300 cm flight tube to meet electron lens No. 1 (EL No. l) (located just outside the reactor shielding), which gave an output pulse directed partly to the start line of a time to pulse height converter, THC No. 1 and partly to a start pileup reject unit, SPR. The fragment continued its flight until it encountered electron lens No. 2 (EL No. 2). The output pulse of that lens was routed to the stop side of THC No. 1 and the start side of THC No. 2. At a very short distance from the cathode of EL No. 2, nickel foils mounted on two rotating discs, "D", intercepted the fragment and reduced its energy. When the fragment traversed the third electron lens (EL No. 3) a pulse was generated that was directed to the stop side of THC No. 2. The fragments were then stopped by the solid state detector,
SSD, that was placed 5.08 cm away from the cathode of EL No. 3. The solid state detector system generated two pulses - a slow pulse, E, proportional to the residual energy of the stopped fragment, and a fast timing signal, FP. In addition to that, the ouput of each electron lens was used as such to produce pulse height distributions of the fission fragments, PH(1), PH(2) and PH(3). Hence, we had eight output signals to be treated in coincidence: ]'(1) and T(2), the outputs of THC No. 1 and THC No. 2, which gave the initial and final TOF of the fragments, respectively; E, the residual energy of the fragments; PH(1), PH(2), and PH(3) which served to indicate the proper functioning of the three electron lenses and have a measure of their resolution; the start pile-up reject served to reject any pulse override. The fast timing signal, FP, from the solid state detector, when taken in coincidence with the output of electron lens No. 3, indicated that the slow output of the solid state detector was genuine and due to the same fission fragment that had just traversed electron lens No. 3. A block diagram of the complete electronic circuitry is shown in fig. 1. The eight output pulses from the system were amplified and made to trigger indi.
515
Y. Fares, M.L. Muga / A double TOF (E-AE) mass spectrometer
vidual single channel analyzers, SCAs. The output of the SCA acted as an input to an eight-fold coincidence unit which served to open the gate of the sixparameter analyzer. The analyzer in turn allowed the output of the six linear amplifiers carrying the required information to pass through for subsequent analysis. The six signal inputs to the ADSs were the three pulse height distributions from the time detectors, PH(1), PH(2) and PH(3), the first and second TOFs, T(1) and T(2), respectively, and the residual energy E from the solid state detectors. Each pulse was delayed the proper time by means of linear delay amplifiers (with reference to the last signal in the sequence, i.e., E) in order to gain acceptance to the six-parameter analyzer unit. The analyzer which was used in this experiment is a six-parameter analyzer having 256 X 256 X 256 X 256 X 256 X 256 channel resolution. It is capable of analyzing and storing, simultaneously, six parameters in a maximum time of 245 /~s. When the six-stage digital conversion of each of 256 events is completed, automatic readout takes place or, if desired, data can be read out by manual selection.
3. Time detector
The basic design of the time detector used in this experiment was developed by Stein and Leachman [3]. Fig. 2 shows the structure of the time detector which consisted essentially of three parts: (a) cathode - the cathode was made of VYNS f'gm 10 ~g/cm 2 thick mounted on an aluminum washer, 5.08 cm outside diameter and 2.54 cm inside diameter onto which was evaporated a gold layer of 20 /.tg/cm2. The VYNS is a polyvinyl chloride acetate copolymer (85% chloride, 15% acetate) and the technique of making thin films from VYNS was described by Pate and Yaffe [4] in their review paper. The aluminum washer carrying the f'dm was secured in position to a copper holder. The copper holder was connected to the negative high voltage of -11 kV. (b) Electrostatic l e n s - the electrostatic lens consisted of three rings of 2.54 cm diameter arranged in an isopotential Einzel system as described by Casslett [5] and shown in fig. 2J The first ring to which the cathode is attached is made negative at -11 kV potential, the other rings were grounded and separated from the first one by means of ceramic posts. The system had the advantage of a very short and constant focal length.
PHOTOMULTIPLIRE TUBE~ COLLAR~
L
I
G
H
T
PIPE
ANOI~~
SCINTILLATOR
HIGHVOLTAGELEAD ~CATHODESUPPORTED ONCOPPERRING FRAGMENTTRAJECTORY k
Fig. 2. Time detector (electron lens).
(c) A n o d e - the anode was a 2.54 cm diameter disc of NE-102 plastic scintillator of 0.00127 em thickness. The scintillator film was mounted on a highly polished plexiglass light pipe which was placed in contact with the glass face of a 56 AVP Amprex photomultiplier tube. Good optical contact between the scintillator and the light pipe and between the latter and phototube was achieved by means of high quality optical grease. The anode assembly was placed very close to the third ring and in the center because of the focusing properties of the Einzel lens system. The scintillator posts and the phototube were were kept out of the fragment beam from the reactor to avoid the undesirable result of high background noise. The photomultiplier performed with a signal rise time of less than 2 ns and its voltage divider network was designed for high gain. The optimum high voltage going to the voltage divider for a maximum output pulse with a low level of noise generally varies from one phototube to another. In this experiment best results were obtained at high voltage of 2.25 kV for all three systems. When the highly charged fission fragments traverse the cathode of an electron lens, electrons are promptly emitted from both surfaces of the foil. The number of electrons ejected varies as the square of the charge of the fragment and according to Stein and Leachman [3] about 70 electrons are ejected from
516
Y. Fares, M.L. Muga / A double TOF (E- AE) mass spectrometer
gold, silver, copper, aluminum and nickel to within +-10%. These electrons are then accelerated and focussed on the scintillator anode which enable the photomultiplier to give a time signal. Two properties of this type of time detector determine the quality of its performance: (1)efficiency and (2) resolution. The efficiency of the time detector is defined as the ratio of the number of signals from the time detector to the number of fragments detected by the solid state detector, which was found to be very nearly 100% for all three time detectors when the negative potential of the cathode was ~>10 kV.
4. System resolution 4.1. Energy resolution
In order to determine the energy resolution, an energy spectrum for a given mass should be collected and the fwhm of the distribution is estimated. Since this is not feasible with the present experimental arrangement, the dispersion in the energy is indirectly evaluated from the dispersion in time-of-flight. The experimentally evaluated dispersion in time (see later) is of the order of 1 ns and if we consider the average time-of-flight to be 125 ns, then ( A T / T ) = 0.008. If we make use of the relation (AE/E)i = 2(A T/T)i
(1)
i.e. zSEi = 2Ei(,ST/T)i
(2)
where i refers to the median light (L) or median heavy (H) fragment. On the basis of EL = 97.8 MeV and En = 67.9 MeV for the final median light and median heavy fragments respectively [6], we obtain AE L = 2 × 97.8 × 0.008 = 1.56 MeV,
(3)
AE H = 2 × 67.9 × 0.008 = 1..08 MeV.
(4)
most important factors that might have contributed to the dispersions in the TOF measurements: (1) ATs is the average increase in fragment flight time caused by the energy lost in the source. (2) ATn is the average spread in flight time as a result of neutron recoil. (3) ATf is the average increase in time-of-flight due to energy lost in the VYNS foils. (4) ATa is any change in the apparent time-offlight introduced by the pulse height analyzer including effects of finite channel width. (5) ATj is the uncertainty in the flight time caused by the variations in the time at which the THC is triggered when the start pulse arrives and in the time at which the THC shuts off due to the arrival of the stop pulse, i.e., due to the timing "jitter" in the THC. It is known that the angular distribution of the neutron emission from a fission fragment moving in its center of mass system is isotropic and has a little effect on the average velocity of fission fragments [7,8]. Also we are not actually measuring the absolute velocities of the fragment, but just its velocity between two points after the fragment has evaporated its excess neutrons and acquired its average post neutrons velocity. In view of these two points, it seems reasonable to neglect the effects of energy loss in the source and the effect of neutron emission. It was estimated that the median light fragment loses 1.3 MeV and median heavy loses 0.91 MeV in a VYNS foil, i.e., (AT/T)L = 0.65% and (AT/T)H = 0.66%. The initial velocity V1 is affected only by the energy lost in the first foil, while V2, the final velocity, is affected by the energy lost in both the first and second foils. This means that V~ has to be corrected by an average amount of 0.65%, while V2 should be corrected by 1.31% due to energy loss in the VYNS foils alone. If we may combine A T a and ATj and call both ATe, i.e. dispersion in time-of-flight due to electronic instrumentation, then ATe = AT a + ATj.
4.2. Dispersions in time measurements
The fwhm of TOF distribution for a very short flight distance is a measure of the dispersion in the TOF measurements. Under ideal conditions and for a fixed energy, the time-of-flight spectrum of the fragment should be a line, but because of the various sources of energy loss and instrumental dispersions, a broadened distribution results. The following are the
(5)
(ATj/T) was avaluated for the THCs used to be 0.1% of the full scale, i.e. for the 300 ns range of the THCs used in this experiment, therefore ATj = 0.3 ns.
(6)
In a separate experiment using 2S2cf, lhc TOF distribution for a short flight distance of 5.08 cm was found to be 1 ns (fwhm). If we consider that tlfis
Y. Fares, M.L. Muga / A double TOF (1:'-AE) mass spectrometer
value corresponds to ATe, ATa is of the order of 0.7 ns. In fact, this 0.7 ns dispersion should also include other instrumental dispersions. We conclude, therefore, that after correcting the energies and velocities for the energy lost in the VYNS foils, the dominant source of dispersion in the TOF is the time "jitter" in the THC and the channel width of the multichannel analyzer which is of the order of 1 ns. All other uncertainties in the flight itme measurement are negligible compared with this.
517
and dT/T = AT/T
(15)
then in general, for any fission fragment i, we have (AM/M)i = 2( A T/T)i.
(16)
For the median light fission fragment the uncertainty in the evaluated mass is AML = 2 X 96 X 0.008 = 1.54 amu
(17)
and for the median heavy fragment 4.3. Mass resolution
AMH = 2 × 140 × 0.008 = 2.24 amu.
The masses of the individual fission fragments were calculated from following equation:
(18)
Therefore, the expected uncertainty in the evaluated mass is of the order of +-2 amu.
(ax + b )
M - (k/2) I/~ - (a'x + b')
(7)
Eq. (7) was derived from the relation given by Schmitt et at. [9] for the calibration of surface barrier detectors i.e. E =(ax + b ) +(a'x + b ' ) M .
(8)
By equating eq. (8) to eq. (9) /f = (k/2) MV~2 = (k/2) M(d2/T2) 2
(9)
eq. (7) is obtained, a, a', b and b' are constants for a particular detector that can be determined by the general method described by Schmitt et al. [10],M is the mass of the fission fragment in amu, x is the energy channel number, E is the corresponding energy in MeV, V2 is the velocity of the fission fragment in cm/ns over the second flight path of length d2 cm. k is a unit conversion constant of value 1.037 MeV ns 2/amu cm 2. From eq. (9) we obtain the relation M = k'ET~,
(10)
where k' = 2/kd].
(11)
For a given energy, dM = 2 k ' E T u d T
(12)
and aM/M = 2dT/T2
(13)
and if we let dM/M = AM/M
(14)
5. Results and discussion A list of average fragment energies, masses, velocities and some of the distribution widths obtained from this experiment is given in table 1. Comparisons of these quatities with those of Schmitt et at. [11], and Khan and Forgue [1], show good agreement; comparisons of these quantities with the results of Milton and Fraser [6], show some discrepancies which are understood in terms of the effects of fragment scattering from the walls of the flight tubes in the double velocity experiment of Milton and Fraser [6]. These authors described the possible effects of scattering [12], but the surprisingly large magnitude of the probability for fragment scattering at small angles to the surface was only recently found explicitly in the measurements of Englekemeier [13]. Such scattering produces tailing towards lower velocities and energies, somewhat broadens the measured mass distributions and alters the total kinetic energies, i.e. decreases them over most of the mass range. The mass distribution for 235U thermally induced fission is shown in fig. 3. The observed peak to valley ratio for the present distribution is about 500, as compared with 450 of Schmitt et al. [11 ], and about 650 for the radiochemical distribution. Fig. 4 shows the velocity distribution of the 23 s U fission fragments in cm/ns with peak to valley ratio of nearly 1000. The kinetic energy distribution of 23Su fission fragments spectrum is shown in fig. 5 with peak to valley ratio of 19 which indicated the good energy resoluttion of the solid state detector. Figs. (6a, b and 7a, b) show the residual energy distribution and mass distribution of the 23Su fission fragments after passing
Y. Fares, M.L. Muga / A double TOF (E- AE) mass spectrometer
518 2000
5000
1800 4500
~600 4000
1400 3500
1200 to
,,~,tO00 i,~ 2 5 0 0
"6 800 Z 600
1500
400
1000
200
500
o
0
Fig. 3. Mass distribution of 2 a s u fission fragments (uninterrupted) in amu for a typical run.
.64 .
9 6 1.12 I Z~8 1.44 IBO
Fig. 4. Velocity distribution of 2 as U fission fragments in cm/ ns for a typical run. (The fine structure observed in the peaks will be discussed elsewhere.)
Table 1 Mean values and rms widths of the distributions of all the runs. Quantity
(E K) oEK (EL) ag L (E H) t~H OIL) aML O/H) aMH (VL) (VH)
This work
170.37 5.19 100.26 3.55 70.11 3.78 95.01 5.24 138.49 5.24 1.426 0.988
± 1.04 * ± 0.98 * ± 0.34 * ± 0.13 * ± 0.03 * ± 0.03 ± 0.026
Khan and Forgue
Pre-neutron emission Schmitt et al. [11]
Milton and Frazer
100.13
171.9 x 1.4 10.9 101.56 ± 0.7
69.63
70.34 ± 0.5
(168.3 -+ 1.7) MeV 11.4 (99.8 ± 1.0) MeV 6.0 (68.4 ± 0.7) MeV 7.5 (96.08 ± 0.10) amu 5.829 (139.92 _+0.1) ainu 5.829 (1,409 ± 0,062) cm/ns (0.966 ± 0.07) cm/ns
169.76
95.34 138.14
96.57 5.36 139,43 5.36
Y. Fares, M.L. Muga / A double TOF (E-AE) mass spectrometer
519
4000
2000
3500
1800
3000 1600
g 1400 ~ 2000 ~
1200 t500
[~ I 0 0 0 LO00
"6 E 800
#
jS
500
rt
400
200
0 0
32
I 96
64
Fig. 5. Energy distribution of for a typical run. 1600
9~
t 12~
Fig. 7. (a) Energy distribution of 23su fission fragments in MeV after passing through a 1.01 mg/cm2 Ni foil. (b) Mass distribution of 23sU fission fragments in amu after passing through a 1.01 mg/cm2 Ni foil.
/{28
160
fission fragments in MeV
23SU
b
a
64
o
through nickel foils o f thickness 0.618 and 1.01 rag/ cm 2 respectively. When e was kept constant at 0.67 as suggested by Alexander et al. [13], and the fit made by adjusting K, it was found that K varies by as much as 6.5%. The variation o f K was much less (about 2%) when
1400
,~o 1200
0,26 o
= =0 6 7 ,ooo
0.25
o
o o
~ 800 Z
"~ 0.24 Od
600
I
400 018
a :075 O
0
0____0
0
0
0
0
'
' [~o' ilo' i~,o ' t~o * Mass Number of Fission Fragmer~
0----0
0,17 0
o
~
14
~
o
~
6'4
~
,16
~o 0.16
Fig. 6 (a) Energy distribution of 23su fission fragments in MeV after passing through a 0.618 mg/cm2 Ni foil. (b) Mass distribution of 23 s U fission fragments in amu after passing through a 0.618 mg/cm2 Ni foil.
9oL
~o
'
Fig. 8. Dependence of constant "K" on mass number of the fission fragment.
52~)
Y. Fares, M.L. Muga / A double TOF (E-AE) mass spectrometer
c~ was kept constant at 0.75. In general, it is seen that K is mass dependent and for a given constant a, K decreases with the increase in the ion's mass. Fig. 8 shows the variation of K with mass number A. From this, and on the basis of the more reliable data for the median light and median heavy masses, i.e. A = 95 and 140 ainu, the value of K = 0 . 1 7 6 mg cm-2/ MeV °'75 gives the best results.
(3) Range-energy correlations were presented graphically for the same mass range of the fission fragments as above. The points of these graphs were fitted to a simple range-energy relation with two adjustable parameters of the form R = KI:"~. K proved to be mass dependent and decreases slowly with the increase of the ion mass when a is kept constant at 0.75.
6. Conclusions
We express our gratitude to the UFTR Staff and the UF Computer Center. The f'mancial support of the AEC and the Chemistry Department of the University of Florida is greatly appreciated.
Several conclusions were arrived at in the previous section. These conclusions will be summarized below and their importance pointed out. (1) The correlated energy-time-of-flight technique for measuring velocities, masses and energies of fission fragments - or any other ions for this matter -proved to be an excellent method that yielded very accurate results with very good resolutions as shown in table 1. The results of this experiment for the various distribution of the 23su fission fragments are in close agreement with the most reliable values reported in the literature. In addition, the importance of time-of-flight in this experiment lies in the fact that is was possible to simultaneously evaluate the energy loss of the fragment and its mass, as well as energy. This means that the system can be used as both a mass spectrometer and as an ( E - AE) telescope as well. (2) "Stopping powers" of the 23Su fission fragments in nickel as a function of energy or velocity for the range of masses 80 through 110 and 130 through 145 amu were correlated in the form of plots. These graphs show the general trend observed by other authors and may possibly by fitted to some general polynomial or exponential funciton. (Will be reported elsewhere).
References [1] S. Kahn and V. Forgue, Phys. Rev. 163 (1967) 163. [2] Walter H. Barkas, John N. Dyer and Harry H. Heckman, Phys. Rev. Lett. 11 (1963) 26. [3] W.E. Stein and R.B. Leachman, Rev. Sci. Instr. 27 (1956) 1049. [4] B.D. Pate and L. Yaffe, Can. J. Chem. 33 (1955) 15. [5] V.E. Casslett, Introduction to electron optics, 2nd ed. (Clarendon, Oxford). [6] J.C.D. Milton and J.S. Fraser, Can. J. Phys. 40 (1962) 1626. [7] H.R. Bowman, S.G. Thompson, J.C.D. Milton and W.J. Swiatecki, Phys. Rev. 126 (1962) 2129. [8] J. TerreU, Phys. Rev. 127 (1962) 880. [9] H.W. Schmitt, W.M. Gibson, J.H. Neiler, F.J. Walter and T.D. Thomas, in: Proc. Conf. Phys. and chem. of fission, Salzburg, Austria (IAEA, 1965). [10] H.W. Schmitt, W.E. Kiker and C.W. Williams, Phys. Rev. 137. [11] H.W. Schmitt, J.H. Neiler and F.J. Walter, Phys. Rev. 141 (1966) 1146. [12] J.S. Fraser, in Proc. IAEA Conf. on Phys. and chem. of fission, Salzburg, Austria (IAEA, 1965). [13] D. Engelkemeier and G.N. Walton, Report AERE-R 4716 (1964) unpublished. [14] J.M. Alexander and M.F. Gazdik, Phys. Rev. 120 (1960) 874.
513
A DOUBLE TIME-OF-FLIGHT ( E - A E ) MASS SPECTROMETER * Youhanna FARES ** and M.L. MUGA Chemistry Department, University o f Florida, Gainesville, Florida 32601, U.S.A.
Received 7 May 1981 and in revised form 7 September 1981
A correlated (E-AE) double time-of-flight mass spectrometer was designed and used in the dE/dx measurements of 23su fission fragments in nickel as a function of mass and energy. The system has a time resolution of 1 ns, a mass resolution of ,~u~/L= 2.24 amu, AMH = 1.54 amu, and energy resolution of A E L = 1.56 MeV, AEH = 1.08 MeV for the median light and median heavy fission fragments, respectively. A simple Range-Energy relation of the form R = KE °~ was fitted to the data points. K was found to be mass dependent and decreases with the increase of the ion mass. For a = 0.75, best range-energy results were obtained for K = 0.176 mg em2/MeVO.75. K was determined from the values of the ranges and kinetic energies of the median light and median heavy fragments and then extrapolated to other masses.
1. Introduction The study of the passage of charged particles through matter continues to be of importance in atomic and nuclear physics. A knowledge o f fission fragments ranges and their energy loss in solid and gaseous media is not only important for mass identification o f heavy ions but also essential to the application o f fission fragment-induced chemical reactions, i.e. o f fissio-chemistry. Such knowledge is directly applicable to energy conversion in the fission electric cell as well as to radiation induced ionization for space-charge neutralization in thermionics [1]. Experiments in the above mentioned fields require accurate reference data on particle penetration, and conversely, they feed back new information on the penetration process itself. An example of the feed-back is the discovery of difference in the penetration o f hyperons o f opposite charge [2]. For the development o f a universal range-energy relation for heavy charged particles, the starting point would be the accurate measurement of stopping cross sections as a function o f energy and mass. The choice o f time-offlight technique coupled with solid-state barrier
* Work supported in part by the US Atomic Energy Commission. ** Present address: Biosystems Research Division, Dept. of Industrial Engineering, Texas A and M University, College Station, Texas 77843, USA. 0029-554X/82/0000-0000/$02.75
© 1982 North-Holland
detector seems to provide the best method for accuracy in the measurements of velocity, energy, and mass of fission fragments. The main goal o f this work was to: (1) devise and construct a double time-offlight (TOF) ( E - A E ) correlated system; (2) determine accurately the stopping powers o f the spectrum of fission fragments obtained from thermal neutron induced fission of 23SU; (3) find an empirical relation of the stopping power o f a heavy charged particle as a function o f its important parameters, i.e., its energy (velocity), mass and possibly its atomic number, as well as the mass o f the stopping material. In this paper we describe the experiment and the construction and the characteristics of the spectrometer. The results of the other two goals will be reported elsewhere.
2. Principle of experiment, electronic system and signal procedures The principle o f this experiment consists o f the measurements of the times-of-flight, TOF, over accurately known distances (hence, velocities), of a fission fragment before and after its passage through a nickel foil o f a given thickness, and the measurement of the fragment's residual kinetic energy by means of a surface barrier solid-state detector. The final TOF, together with the residual kinetic energy, gives the mass o f the fission fragment, via E = ~ M V 2 . The mass
514
Y. Fares, M.L. Muga / A double TOF (E- AE) mass spectrometer
Fig. 1. Experimental layout and sequence of events from left to right. Notation: F = fuel box of UFTR; S = 23SU source; C = collimating tube; B = beam tube; T1, T2 = flight tube No. 1 and No. 2, respectively; E.L.1, E.L.2, E.L.3 = electron Lenses No. 1, No. 2, and No. 3, respectively; S.S.D. = solid state detector; C.P.A. = charge sensitive preamplifier; F.C.F. = fast cathode follow preamplifier; F.A. = fast amplifier; F.D. = fast discriminator; F.C. = fast coincidence; S.C. = slow coincidence;'L.A. = linear amplifier; L.D.A. = linear delay amplifier; B.A. = bias amplifier; S.C.A. = single channel analyzer; T.H.C. = time-to-pulse height converter; S.P.R. = start-pulse pile-up reject; A.D.C. = analogue to digital converter. of the fission fragment, together with its initial velocity, gives the initial energy of the fragment. Hence, we obtain AE, the energy lost when the fragment passes through the nickel foil. Fig. 1. represents the layout of the experiment and the sequence of events. Fission was induced in the 23Su source "S" (in the central fuel core region of the University of Florida Training Reactor). The fission fragment traveled up a 300 cm flight tube to meet electron lens No. 1 (EL No. l) (located just outside the reactor shielding), which gave an output pulse directed partly to the start line of a time to pulse height converter, THC No. 1 and partly to a start pileup reject unit, SPR. The fragment continued its flight until it encountered electron lens No. 2 (EL No. 2). The output pulse of that lens was routed to the stop side of THC No. 1 and the start side of THC No. 2. At a very short distance from the cathode of EL No. 2, nickel foils mounted on two rotating discs, "D", intercepted the fragment and reduced its energy. When the fragment traversed the third electron lens (EL No. 3) a pulse was generated that was directed to the stop side of THC No. 2. The fragments were then stopped by the solid state detector,
SSD, that was placed 5.08 cm away from the cathode of EL No. 3. The solid state detector system generated two pulses - a slow pulse, E, proportional to the residual energy of the stopped fragment, and a fast timing signal, FP. In addition to that, the ouput of each electron lens was used as such to produce pulse height distributions of the fission fragments, PH(1), PH(2) and PH(3). Hence, we had eight output signals to be treated in coincidence: ]'(1) and T(2), the outputs of THC No. 1 and THC No. 2, which gave the initial and final TOF of the fragments, respectively; E, the residual energy of the fragments; PH(1), PH(2), and PH(3) which served to indicate the proper functioning of the three electron lenses and have a measure of their resolution; the start pile-up reject served to reject any pulse override. The fast timing signal, FP, from the solid state detector, when taken in coincidence with the output of electron lens No. 3, indicated that the slow output of the solid state detector was genuine and due to the same fission fragment that had just traversed electron lens No. 3. A block diagram of the complete electronic circuitry is shown in fig. 1. The eight output pulses from the system were amplified and made to trigger indi.
515
Y. Fares, M.L. Muga / A double TOF (E-AE) mass spectrometer
vidual single channel analyzers, SCAs. The output of the SCA acted as an input to an eight-fold coincidence unit which served to open the gate of the sixparameter analyzer. The analyzer in turn allowed the output of the six linear amplifiers carrying the required information to pass through for subsequent analysis. The six signal inputs to the ADSs were the three pulse height distributions from the time detectors, PH(1), PH(2) and PH(3), the first and second TOFs, T(1) and T(2), respectively, and the residual energy E from the solid state detectors. Each pulse was delayed the proper time by means of linear delay amplifiers (with reference to the last signal in the sequence, i.e., E) in order to gain acceptance to the six-parameter analyzer unit. The analyzer which was used in this experiment is a six-parameter analyzer having 256 X 256 X 256 X 256 X 256 X 256 channel resolution. It is capable of analyzing and storing, simultaneously, six parameters in a maximum time of 245 /~s. When the six-stage digital conversion of each of 256 events is completed, automatic readout takes place or, if desired, data can be read out by manual selection.
3. Time detector
The basic design of the time detector used in this experiment was developed by Stein and Leachman [3]. Fig. 2 shows the structure of the time detector which consisted essentially of three parts: (a) cathode - the cathode was made of VYNS f'gm 10 ~g/cm 2 thick mounted on an aluminum washer, 5.08 cm outside diameter and 2.54 cm inside diameter onto which was evaporated a gold layer of 20 /.tg/cm2. The VYNS is a polyvinyl chloride acetate copolymer (85% chloride, 15% acetate) and the technique of making thin films from VYNS was described by Pate and Yaffe [4] in their review paper. The aluminum washer carrying the f'dm was secured in position to a copper holder. The copper holder was connected to the negative high voltage of -11 kV. (b) Electrostatic l e n s - the electrostatic lens consisted of three rings of 2.54 cm diameter arranged in an isopotential Einzel system as described by Casslett [5] and shown in fig. 2J The first ring to which the cathode is attached is made negative at -11 kV potential, the other rings were grounded and separated from the first one by means of ceramic posts. The system had the advantage of a very short and constant focal length.
PHOTOMULTIPLIRE TUBE~ COLLAR~
L
I
G
H
T
PIPE
ANOI~~
SCINTILLATOR
HIGHVOLTAGELEAD ~CATHODESUPPORTED ONCOPPERRING FRAGMENTTRAJECTORY k
Fig. 2. Time detector (electron lens).
(c) A n o d e - the anode was a 2.54 cm diameter disc of NE-102 plastic scintillator of 0.00127 em thickness. The scintillator film was mounted on a highly polished plexiglass light pipe which was placed in contact with the glass face of a 56 AVP Amprex photomultiplier tube. Good optical contact between the scintillator and the light pipe and between the latter and phototube was achieved by means of high quality optical grease. The anode assembly was placed very close to the third ring and in the center because of the focusing properties of the Einzel lens system. The scintillator posts and the phototube were were kept out of the fragment beam from the reactor to avoid the undesirable result of high background noise. The photomultiplier performed with a signal rise time of less than 2 ns and its voltage divider network was designed for high gain. The optimum high voltage going to the voltage divider for a maximum output pulse with a low level of noise generally varies from one phototube to another. In this experiment best results were obtained at high voltage of 2.25 kV for all three systems. When the highly charged fission fragments traverse the cathode of an electron lens, electrons are promptly emitted from both surfaces of the foil. The number of electrons ejected varies as the square of the charge of the fragment and according to Stein and Leachman [3] about 70 electrons are ejected from
516
Y. Fares, M.L. Muga / A double TOF (E- AE) mass spectrometer
gold, silver, copper, aluminum and nickel to within +-10%. These electrons are then accelerated and focussed on the scintillator anode which enable the photomultiplier to give a time signal. Two properties of this type of time detector determine the quality of its performance: (1)efficiency and (2) resolution. The efficiency of the time detector is defined as the ratio of the number of signals from the time detector to the number of fragments detected by the solid state detector, which was found to be very nearly 100% for all three time detectors when the negative potential of the cathode was ~>10 kV.
4. System resolution 4.1. Energy resolution
In order to determine the energy resolution, an energy spectrum for a given mass should be collected and the fwhm of the distribution is estimated. Since this is not feasible with the present experimental arrangement, the dispersion in the energy is indirectly evaluated from the dispersion in time-of-flight. The experimentally evaluated dispersion in time (see later) is of the order of 1 ns and if we consider the average time-of-flight to be 125 ns, then ( A T / T ) = 0.008. If we make use of the relation (AE/E)i = 2(A T/T)i
(1)
i.e. zSEi = 2Ei(,ST/T)i
(2)
where i refers to the median light (L) or median heavy (H) fragment. On the basis of EL = 97.8 MeV and En = 67.9 MeV for the final median light and median heavy fragments respectively [6], we obtain AE L = 2 × 97.8 × 0.008 = 1.56 MeV,
(3)
AE H = 2 × 67.9 × 0.008 = 1..08 MeV.
(4)
most important factors that might have contributed to the dispersions in the TOF measurements: (1) ATs is the average increase in fragment flight time caused by the energy lost in the source. (2) ATn is the average spread in flight time as a result of neutron recoil. (3) ATf is the average increase in time-of-flight due to energy lost in the VYNS foils. (4) ATa is any change in the apparent time-offlight introduced by the pulse height analyzer including effects of finite channel width. (5) ATj is the uncertainty in the flight time caused by the variations in the time at which the THC is triggered when the start pulse arrives and in the time at which the THC shuts off due to the arrival of the stop pulse, i.e., due to the timing "jitter" in the THC. It is known that the angular distribution of the neutron emission from a fission fragment moving in its center of mass system is isotropic and has a little effect on the average velocity of fission fragments [7,8]. Also we are not actually measuring the absolute velocities of the fragment, but just its velocity between two points after the fragment has evaporated its excess neutrons and acquired its average post neutrons velocity. In view of these two points, it seems reasonable to neglect the effects of energy loss in the source and the effect of neutron emission. It was estimated that the median light fragment loses 1.3 MeV and median heavy loses 0.91 MeV in a VYNS foil, i.e., (AT/T)L = 0.65% and (AT/T)H = 0.66%. The initial velocity V1 is affected only by the energy lost in the first foil, while V2, the final velocity, is affected by the energy lost in both the first and second foils. This means that V~ has to be corrected by an average amount of 0.65%, while V2 should be corrected by 1.31% due to energy loss in the VYNS foils alone. If we may combine A T a and ATj and call both ATe, i.e. dispersion in time-of-flight due to electronic instrumentation, then ATe = AT a + ATj.
4.2. Dispersions in time measurements
The fwhm of TOF distribution for a very short flight distance is a measure of the dispersion in the TOF measurements. Under ideal conditions and for a fixed energy, the time-of-flight spectrum of the fragment should be a line, but because of the various sources of energy loss and instrumental dispersions, a broadened distribution results. The following are the
(5)
(ATj/T) was avaluated for the THCs used to be 0.1% of the full scale, i.e. for the 300 ns range of the THCs used in this experiment, therefore ATj = 0.3 ns.
(6)
In a separate experiment using 2S2cf, lhc TOF distribution for a short flight distance of 5.08 cm was found to be 1 ns (fwhm). If we consider that tlfis
Y. Fares, M.L. Muga / A double TOF (1:'-AE) mass spectrometer
value corresponds to ATe, ATa is of the order of 0.7 ns. In fact, this 0.7 ns dispersion should also include other instrumental dispersions. We conclude, therefore, that after correcting the energies and velocities for the energy lost in the VYNS foils, the dominant source of dispersion in the TOF is the time "jitter" in the THC and the channel width of the multichannel analyzer which is of the order of 1 ns. All other uncertainties in the flight itme measurement are negligible compared with this.
517
and dT/T = AT/T
(15)
then in general, for any fission fragment i, we have (AM/M)i = 2( A T/T)i.
(16)
For the median light fission fragment the uncertainty in the evaluated mass is AML = 2 X 96 X 0.008 = 1.54 amu
(17)
and for the median heavy fragment 4.3. Mass resolution
AMH = 2 × 140 × 0.008 = 2.24 amu.
The masses of the individual fission fragments were calculated from following equation:
(18)
Therefore, the expected uncertainty in the evaluated mass is of the order of +-2 amu.
(ax + b )
M - (k/2) I/~ - (a'x + b')
(7)
Eq. (7) was derived from the relation given by Schmitt et at. [9] for the calibration of surface barrier detectors i.e. E =(ax + b ) +(a'x + b ' ) M .
(8)
By equating eq. (8) to eq. (9) /f = (k/2) MV~2 = (k/2) M(d2/T2) 2
(9)
eq. (7) is obtained, a, a', b and b' are constants for a particular detector that can be determined by the general method described by Schmitt et al. [10],M is the mass of the fission fragment in amu, x is the energy channel number, E is the corresponding energy in MeV, V2 is the velocity of the fission fragment in cm/ns over the second flight path of length d2 cm. k is a unit conversion constant of value 1.037 MeV ns 2/amu cm 2. From eq. (9) we obtain the relation M = k'ET~,
(10)
where k' = 2/kd].
(11)
For a given energy, dM = 2 k ' E T u d T
(12)
and aM/M = 2dT/T2
(13)
and if we let dM/M = AM/M
(14)
5. Results and discussion A list of average fragment energies, masses, velocities and some of the distribution widths obtained from this experiment is given in table 1. Comparisons of these quatities with those of Schmitt et at. [11], and Khan and Forgue [1], show good agreement; comparisons of these quantities with the results of Milton and Fraser [6], show some discrepancies which are understood in terms of the effects of fragment scattering from the walls of the flight tubes in the double velocity experiment of Milton and Fraser [6]. These authors described the possible effects of scattering [12], but the surprisingly large magnitude of the probability for fragment scattering at small angles to the surface was only recently found explicitly in the measurements of Englekemeier [13]. Such scattering produces tailing towards lower velocities and energies, somewhat broadens the measured mass distributions and alters the total kinetic energies, i.e. decreases them over most of the mass range. The mass distribution for 235U thermally induced fission is shown in fig. 3. The observed peak to valley ratio for the present distribution is about 500, as compared with 450 of Schmitt et al. [11 ], and about 650 for the radiochemical distribution. Fig. 4 shows the velocity distribution of the 23 s U fission fragments in cm/ns with peak to valley ratio of nearly 1000. The kinetic energy distribution of 23Su fission fragments spectrum is shown in fig. 5 with peak to valley ratio of 19 which indicated the good energy resoluttion of the solid state detector. Figs. (6a, b and 7a, b) show the residual energy distribution and mass distribution of the 23Su fission fragments after passing
Y. Fares, M.L. Muga / A double TOF (E- AE) mass spectrometer
518 2000
5000
1800 4500
~600 4000
1400 3500
1200 to
,,~,tO00 i,~ 2 5 0 0
"6 800 Z 600
1500
400
1000
200
500
o
0
Fig. 3. Mass distribution of 2 a s u fission fragments (uninterrupted) in amu for a typical run.
.64 .
9 6 1.12 I Z~8 1.44 IBO
Fig. 4. Velocity distribution of 2 as U fission fragments in cm/ ns for a typical run. (The fine structure observed in the peaks will be discussed elsewhere.)
Table 1 Mean values and rms widths of the distributions of all the runs. Quantity
(E K) oEK (EL) ag L (E H) t~H OIL) aML O/H) aMH (VL) (VH)
This work
170.37 5.19 100.26 3.55 70.11 3.78 95.01 5.24 138.49 5.24 1.426 0.988
± 1.04 * ± 0.98 * ± 0.34 * ± 0.13 * ± 0.03 * ± 0.03 ± 0.026
Khan and Forgue
Pre-neutron emission Schmitt et al. [11]
Milton and Frazer
100.13
171.9 x 1.4 10.9 101.56 ± 0.7
69.63
70.34 ± 0.5
(168.3 -+ 1.7) MeV 11.4 (99.8 ± 1.0) MeV 6.0 (68.4 ± 0.7) MeV 7.5 (96.08 ± 0.10) amu 5.829 (139.92 _+0.1) ainu 5.829 (1,409 ± 0,062) cm/ns (0.966 ± 0.07) cm/ns
169.76
95.34 138.14
96.57 5.36 139,43 5.36
Y. Fares, M.L. Muga / A double TOF (E-AE) mass spectrometer
519
4000
2000
3500
1800
3000 1600
g 1400 ~ 2000 ~
1200 t500
[~ I 0 0 0 LO00
"6 E 800
#
jS
500
rt
400
200
0 0
32
I 96
64
Fig. 5. Energy distribution of for a typical run. 1600
9~
t 12~
Fig. 7. (a) Energy distribution of 23su fission fragments in MeV after passing through a 1.01 mg/cm2 Ni foil. (b) Mass distribution of 23sU fission fragments in amu after passing through a 1.01 mg/cm2 Ni foil.
/{28
160
fission fragments in MeV
23SU
b
a
64
o
through nickel foils o f thickness 0.618 and 1.01 rag/ cm 2 respectively. When e was kept constant at 0.67 as suggested by Alexander et al. [13], and the fit made by adjusting K, it was found that K varies by as much as 6.5%. The variation o f K was much less (about 2%) when
1400
,~o 1200
0,26 o
= =0 6 7 ,ooo
0.25
o
o o
~ 800 Z
"~ 0.24 Od
600
I
400 018
a :075 O
0
0____0
0
0
0
0
'
' [~o' ilo' i~,o ' t~o * Mass Number of Fission Fragmer~
0----0
0,17 0
o
~
14
~
o
~
6'4
~
,16
~o 0.16
Fig. 6 (a) Energy distribution of 23su fission fragments in MeV after passing through a 0.618 mg/cm2 Ni foil. (b) Mass distribution of 23 s U fission fragments in amu after passing through a 0.618 mg/cm2 Ni foil.
9oL
~o
'
Fig. 8. Dependence of constant "K" on mass number of the fission fragment.
52~)
Y. Fares, M.L. Muga / A double TOF (E-AE) mass spectrometer
c~ was kept constant at 0.75. In general, it is seen that K is mass dependent and for a given constant a, K decreases with the increase in the ion's mass. Fig. 8 shows the variation of K with mass number A. From this, and on the basis of the more reliable data for the median light and median heavy masses, i.e. A = 95 and 140 ainu, the value of K = 0 . 1 7 6 mg cm-2/ MeV °'75 gives the best results.
(3) Range-energy correlations were presented graphically for the same mass range of the fission fragments as above. The points of these graphs were fitted to a simple range-energy relation with two adjustable parameters of the form R = KI:"~. K proved to be mass dependent and decreases slowly with the increase of the ion mass when a is kept constant at 0.75.
6. Conclusions
We express our gratitude to the UFTR Staff and the UF Computer Center. The f'mancial support of the AEC and the Chemistry Department of the University of Florida is greatly appreciated.
Several conclusions were arrived at in the previous section. These conclusions will be summarized below and their importance pointed out. (1) The correlated energy-time-of-flight technique for measuring velocities, masses and energies of fission fragments - or any other ions for this matter -proved to be an excellent method that yielded very accurate results with very good resolutions as shown in table 1. The results of this experiment for the various distribution of the 23su fission fragments are in close agreement with the most reliable values reported in the literature. In addition, the importance of time-of-flight in this experiment lies in the fact that is was possible to simultaneously evaluate the energy loss of the fragment and its mass, as well as energy. This means that the system can be used as both a mass spectrometer and as an ( E - AE) telescope as well. (2) "Stopping powers" of the 23Su fission fragments in nickel as a function of energy or velocity for the range of masses 80 through 110 and 130 through 145 amu were correlated in the form of plots. These graphs show the general trend observed by other authors and may possibly by fitted to some general polynomial or exponential funciton. (Will be reported elsewhere).
References [1] S. Kahn and V. Forgue, Phys. Rev. 163 (1967) 163. [2] Walter H. Barkas, John N. Dyer and Harry H. Heckman, Phys. Rev. Lett. 11 (1963) 26. [3] W.E. Stein and R.B. Leachman, Rev. Sci. Instr. 27 (1956) 1049. [4] B.D. Pate and L. Yaffe, Can. J. Chem. 33 (1955) 15. [5] V.E. Casslett, Introduction to electron optics, 2nd ed. (Clarendon, Oxford). [6] J.C.D. Milton and J.S. Fraser, Can. J. Phys. 40 (1962) 1626. [7] H.R. Bowman, S.G. Thompson, J.C.D. Milton and W.J. Swiatecki, Phys. Rev. 126 (1962) 2129. [8] J. TerreU, Phys. Rev. 127 (1962) 880. [9] H.W. Schmitt, W.M. Gibson, J.H. Neiler, F.J. Walter and T.D. Thomas, in: Proc. Conf. Phys. and chem. of fission, Salzburg, Austria (IAEA, 1965). [10] H.W. Schmitt, W.E. Kiker and C.W. Williams, Phys. Rev. 137. [11] H.W. Schmitt, J.H. Neiler and F.J. Walter, Phys. Rev. 141 (1966) 1146. [12] J.S. Fraser, in Proc. IAEA Conf. on Phys. and chem. of fission, Salzburg, Austria (IAEA, 1965). [13] D. Engelkemeier and G.N. Walton, Report AERE-R 4716 (1964) unpublished. [14] J.M. Alexander and M.F. Gazdik, Phys. Rev. 120 (1960) 874.