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Response of thin film plastic scintillator to fission fragments
NUCLEAR INSTRUMENTS AND METHODS I24 (1975) IOI-I06; © NORTH-HOLLAND PUBLISHING CO.
R E S P O N S E OF T H I N F I L M P L A S T I C S C I N T I L L A T O R T O F I S S I O N F R A G M E N T S R. K. BATRA and A. C. SHOTTER
Physics Department, University of Edinburgh, Edinburgh, Scotland
Received 21 August 1974 Thin plasUc scintillator films of d~fferentthicknesses are prepared and their characteristic response to fission fragments from ~s2Cf is studied. It is observed that the peak to valley ratio of fission fragments depends non-linearly upon the thickness of thin films. This ratio for the heavy fragments IS larger than that of the light fragments by a factor of 1.5-1.9 for the whole range
of thickness investigated. The laminated structure of the films shows shghtly higher gain (~ 5 %) but the resolution remains practically the same. The energy loss of fission fragments in plastic scintdlator (NE 102A) is measured. A simple system for a "start-detector", employing such a thin film and a photomultipher, is also described for the time-of-flight experiments.
I . Introduction
teristic versus energy loss of F.F. The thin films investigated cover the thickness region up to the range of these fragments. Single and laminated films of equivalent thickness have also been studied. The energy loss of fission fragments in the scintillator films (NE 102A) is measured. Finally, we describe also a system, comprising of such a thin film and a single photomultiplier, which is capable of acting as a startdetector in any time-of-flight experiment.
The silicon transmission detectors in conjunction with thick semi-conductor detectors have been used for the identification of light charged particles 1'2) for the last few years. Due to practical difficulties, such silicon detectors cannot easily be made thin enough to transmit heavy ions of medium energy range. Thin film plastic scintillators might be an alternative to overcome this problem. Also, thin plastic scintillators are useful as timing detectors 3'4) in the time-of-flight experiments with heavy ions such as the measurements of massyields of fission fragments. During the past few years, Muga and co-workers 5-8) have investigated the energy and time response characteristics of thin film plastic (NE 102) for various ions. Their thin film detector consisted of a thin scintillator film attached to a hemi-cylindrical lucite support placed perpendicular to the parallel faces of two opposing photomultipliers. They demonstrated that a thin film scintillator detector resolves the light and the heavy fission fragments. Using such three detectors in series, Muga 5) observed that the heavy fragments to valley ratio improves. However, such an observation does not indicate clearly whether the improvement in resolution is due to the larger energy loss of the fission fragments in the three scintillator films or the larger collection of light by the six photomultipliers or both. Further, they pointed out that the lamination of the films and the perpendicular mounting of the film to the photomultlpller might be helping in photon production or collection. Therefore, it will be useful to establish systematically the true characteristic of thin films to transmitting fission fragments. We studied the fission fragments response of these detectors when mounted directly on the photomultiplier (section 3) as a function of detector thickness, i.e. response charac-
2. Making the film
The solution of plastic scintillator is prepared in the same way as prescribed by Muga et al. 6) and a slightly different procedure to that of Muga's 6) for making thin films is used. A porcelain pan (22 c m x 16 cm x x 5 cm) is filled with distilled water up to ~ of its depth. On the stationary surface of water, an "O-ring" (i.d. 8 cm) is floated gently. The " O - r i n g " helps to stretch the water surface. A few drops of the scintillator solution are poured m succession onto the stretched surface of water. It is found that the solution spreads more uniformly over the stretched surface as compared to the unstretched surface (tried without "O-ring") and a film of single thickness results, depending upon the strength of the solution. The uniformity of the film can be further improved by placing a second " O - r i n g " (i.d. 4.5 cm) onto the film when it is nearly dry. When the film is completely dry, a rectangular aluminium frame is dipped inclined in the water such that the film under the second "O-ring" just rests over the frame. The film is released from the "O-ring" (second) by piercing the boundary with a pointed aluminium needle having dipped in ethyl acetate. The film may now be conveniently hfted by the Al-frame. It is worth noting that the films made by this process 101
102
R. K. B A T R A A N D A. C. S H O T T E R
are very clear. Also, the use of second "O-ring" is not essential when making thicker films. The uniformity of these films was determined by studying the absorption of alpha particles through different regions of the films. It was found that a typical film had a uniformity better than ___3.5%. Having determined the uniformity of a film, it was transferred to a perspex rectangular frame (22 m m × 18 m m x x 1.5 mm). A thin layer of the optical coupler ensured the attachment. The sides of the perspex frame were painted with a reflector paint. The thin film plastic scintillator attached to the perspex frame will be referred to as "T.F.P.S." detector in this paper.
3. Details of experiment Looking at the arrangements of the thin film detector used by Muga et al.5'6), it seems that a quite good number of the emitted photons are not being received by the photomultipliers. Therefore, we would prefer to mount the "T.F.P.S." onto the face of a photomultiplier so as to optimize the resolution characteristtcs of thin films for fission fragments. The "T.F.P.S." is mounted at the centre of a 56AVP multiplier using the optical coupler (Dow Coming 20057). The simple arrangement of the apparatus is shown in fig. 1. It is important that the thin film is neither touching the photomultipher face nor the coupler. The whole system is placed in a vacuum chamber maintained at 10 -5 torr. A collimated beam of fission fragments from 252Cf impinges on a thin film scintillator. The energy signals are derived from the 12th dynode of the photomultiplier. Many thin scintillator films, both single and laminated, covering the range of fission fragments, were prepared as described in section 2. Their thicknesses were determined by studying the absorption of alpha particles (6.11 MeV from 252Cf) m the films, using a surface barrier semi-conductor counter and making use of range energy tables of
Northcliffe and Schilling9). Also, with a view to know the actual energy deposited by fission fragments in these films, the unslowed and slowed spectra of 252Cf were recorded by a solid state counter in a separate set-up. Having known the thickness of each film, each "T.F.P.S." was mounted to the photomultiplier individually and the corresponding spectra of fission fragments were recorded by a multi-channel analyser.
4. Data and error analysis The data of each spectrum of 252Cf recorded by a "T.F.P.S." is smoothed n times using the subroutine " D S G 13 ". One such smoothed spectrum recorded by a "T.F.P.S." of thickness ~250/~g/cm 2 is shown in fig. 2. The fission fragments peaks are distinctly resolved. The extrapolated dotted line clearly indicates that the background due to alpha particles and ~,-rays from the source has very little interference with the spectrum. The spectra are corrected for the background. The n-times smoothing over any channel means that the resulting smoothed value has been averaged over ( 2 n + 1) adjacent channels. Thus, the statistical error on the counts rate y at any channel x of the spectrum when smoothed n times, wdl be given approximately as as(y ) ~
a 2 (2n+l)
,
(l)
I...l=X--n
where a, denotes the statistical error on the unsmoothed value y,. It was found that the values of as(Pa), as(Pv) and as(PL) on the smoothed values of PH, Pv and PL are less than ___3%.
I00'
o BACKGROUND
COR RECTED .
PERSPEX FRAME
.
.
.
o
C OL/IMATOR
>.
'
!o".5"6 2~P ~'",,
z Z
(3 FILM
OPIICAL COUPLER
\ 5b
1OO
CHAN NO
150
Fig. 2. Fission spectrum from 2~2Cf recorded by thin film Fig. 1. Mounting of a thin film of plastic scintillator on the photomultipher face.
sclntdlator
(250ffg/cm2). • Uncorrected, background.
® corrected
for
RESPONSE OF THIN FILM P L A S T I C S C I N T I L L A T O R
5. Results and discussions Fig. 3 shows the response characteristics of light and heavy fission fragments as they pass through various thin films. It is clear that the pulse height of light fragments increases as their energy loss in the scintillator increases until a plateau region is obtained. This observation is in favour of the earlier predictionsa) of the relative pulse height of fission fragments versus scintillator thickness obtained on the basis of specific ionization of fission fragments, where the authors 3) did not differentiate between the light and heavy fragments. Fig. 3 shows also that the pulse height of heavy fission fragments varies linearly with the energy loss experienced by them in traversing the scintillator films. Recently, Muga et al.7'a) observed that the pulse heights of light and heavy accelerated ions, suffering energy loss up to 15 MeV in the scintdlator films, fol/ows the trends of the theoretical energy lossesg); and predicted the agreement between the theory and the experiment for the larger energy loss region also. Our observations of thin films' response as a function of energy loss [varied by varying the thickness of the films and using range-energy tables9)] to light fragments (average mass = 106.5, average energy 104.5 MeV) suffering energy loss up to 102 MeV; have the same pattern for larger energy loss region as demonstrated by Muga et al.'s experiment for smaller energy loss region. The pulse height versus energy loss plot of heavy fragments (average mass 141.7, average energy = 80.1 MeV) experiencing energy loss up to 70 MeV also verifies the anticipated behaviour for heavy ions. As a measure of resolution, the peak to valley ratios i.e. Pn/Pv and PL/Pv are obtained from each recorded
Z
o LIGHT
FRAG
• HEAVy
FRAG
103
spectrum. The plots of these ratios* as a function of thin film thickness for light and heavy fission fragments are shown in fig. 4. This observation clearly indicates that the energy resolution of fission fragments registered by "T.F.P.S." varies non-linearly with the amount of energy deposited by them. Also, the peak to valley ratio of heavy fragments is greater (by a factor of 1.5-1.9) than that of the light fragments for any "T.F.P.S.". This study of resolution also points out that for experiments requiring high resolution of fission fragments, a "T.F.P.S." of thickness lying between 0.5 t o 1.35 m g / c m 2 is most suitable. There are no measurements known in the literature about the energy loss of fission fragments in the scintillator NE 102A. One has to rely upon the interpolated values obtained either from the range-energy tables 9) or from the measured values in the neighbouring materials1°). In order to know the precise values of energy loss of fission fragments in the scintillator films (NE 102A), we smoothed the unslowed and slowed spectra of 252Cf recorded by a semi-conductor counter and estimated the energy loss of F.F. in different films using Schmitt's equation 12) among the energy, the pulse height and the average mass of the fragments. For convenience, the residual energies of light and heavy fission fragments passing through the films are displayed in fig. 5 as a function of the measured energy loss of 6.11 MeV alpha particles. The graph is important in the sense that by measuring only the energy loss * The errors bars are the statistical errors on the ratio values. 1o
/•
~ LIGHT FRAG AG
300 5-
~ 200"r
o_.-~
Z
10o
a."r
c
2"~ 5b J5 . ;do ho ENERGY LOSS (MeV) OF FISSION FRAGMENTS Fig. 3. Pulse heights of hght and heavy fission fragments versus energy loss of F.F.
100o
SCINTILLATORTHICKNESS
,1'500
2o00
IN ~gm/cm2
Fig. 4. Peak-to-valley ratzos of light and heavy fission fragments as a function of scintillator thickness.
104
R.K.
B A T R A A N D A. C. S H O T T E R
of alpha particles in any film of NE 102A, one can readily know the energy deposited by the mean light and mean heavy fragments. The measurements have been done only up to the region 12) where the calibration procedure of semi-conductor detector for heavy ions remains linear.
6. Comparison with other experiments Muga et al. 5) observed that the values of heavy fragment to valley and light fragment to valley for a four laminations film of thickness 0.4 mg/cm 2 were 4.0 and 3.0 respectively (see ref. 6, fig. 6). For comparison with our results, the values of PH/Pv and PL/Pv for a single film 0.4 mg/cm 2 thickness are about 5.05+_0.06 and 3.57+_0.06 as obtained from fig. 4. Thus, one finds that the response of a thin film mounted as described in section 3 is better than when the mounting procedure of Muga et al. 5-s) is used. To examine the improvement in the response of F.F. passing through thin scintdlators, Muga 5) arranged three scintillator films in series and mixed the signals from the six photomultipliers employed in the experiment (see ref. 5, figs. 8 and 9). He concluded that for about 40% of the total energy loss of F.F. in three T.F.D.s, the values of PH/Pv and PL/Pv are 7.5 and 4.25 respectively. Making use of fig. 5, one finds that the fission fragments lose about 40% of their energy in about 0.55 mg/cm 2 of the scintillator material. The resolution curves (fig. 4) observed by us indicate the values of PH/Pv and PL/Pv for about 0.55 mg/cm 2 as
,25]
7.0_ 0.07 and 4.2_ 0.06 respectively. This implies that a "T.F.P.S." mounted directly on photomultipher tube shows practically the same resolution as the stacked detector system 6) for the same total thickness of the film. The above comparison between our results and Muga's experiment with stacks of detectors 6) is not direct because the geometrical arrangement in the two experiments differ. However, it is shown in section 8 that the perpendicular mounting arrangement for transmission detector and direct mounting do in fact yield identical resolutions. Thus, we see that the larger scintillator thickness, i.e. larger energy loss of F.F. ~s responsible for the improved values of peaks to valley in the case of stacked detectors5). Hence, for the improved differentiation between two adjacent heavy ions, one need not have to use stacked T.F.D.s. but instead, a "T.F.P.S." of appropriate thickness will solve the same purpose.
7. Influence of laminated structure It is interesting to investigate how the lamination of films affects the response of "T.F.P.S." to fission fragments. To do that, we made the scintillator solutions of different strengths and prepared a couple of single and laminated films and determined their thicknesses. For instance, a single film (thickness m 1.2 mg/ cm 2) and another film of three laminations (total thickness m 1.16 mg/cm 2) were tested independently. It is found that the spectrum of 252Cf recorded from the laminated film is slightly shifted to higher channels (m5%) as compared with that recorded by the single film, while the peak to valley ratios in both the cases are practically the same. Probably, the tlansmission of the scintillations produced in the multilayers of the
IOO'~
COLLIMATOR
>.. 50" ~ ~
PERSPEX IIOAIA E
" HEAVYFRAG
z
560
lobo
~5oo
ALPHA ENERGYLOSS(KeV)
Fig. 5. Residual energy of F.F. vs energy loss o f 6.11 MeV alpha particles in the scintillator (NE 102A).
6
O~.ALC
'
25ram
Fig. 6. A n arrangement showing the use of thin film scintdlator as a transmission detector for heavy ions studies.
RESPONSE OF THIN FILM P L A S T I C S C I N T I L L A T O R
laminated film might be helping in spreading the light more uniformly over the used area of the photocathode. This may be the cause for the slightly large pulse height for laminated structure as compared to the single film response. 8. " T . F . P . S . " as transmission detector The obvious dis advantage of mounting the "T. F. P.S." directly on the photomultiplier face is that this arrangement cannot be used for the transmission purposes. We describe below a simple and economical system, employing only one photomultiplier tube, to use "T.F.P.S." as a transmission detector. The arrangement of the transmission detector is shown in fig. 6. The rectangular perspex frame having a T.F.S. mounted over it, is inserted perpendicularly into a groove at the centre of a cylindrical well-polished base B, made of perspex. The base B acting as small light guide is mounted on the face of the photomultiplier with the help of optical coupler. The curved surfaces of the base are painted white with the reflector paint (NE 560). An inverted hollow cylinder made of perspex is placed in a circular groove over the base. This cyhnder has an entrance and an exit opening at appropriate height. The cylinder is polished thoroughly (both inside and outside) and is covered with an aluminium cap having entrance and exit openings matching to the cylinder. The dimensions of the cylinder are chosen such that the loss of light due to multiple reflections at the walls should be as small as possible. Also, the openings in the cylinder allow
I00THREE LAMINATIONS FILM TOTAL THICKNESS = 1 O8mg~cm 2
105
about 10% of the total scintillations to be lost through them. As an exploratory measure of the system, a thicker plastic scintillator (thickness ~ 25 mg/cm z) as supplied by the manufacturer is mounted onto the perspex frame and is housed in the cylinder. The whole system shown in fig. 6 is placed in a vacuum chamber. The spectra of alpha particles of 6.11 MeV from 252Cf and 5.48 MeV from 24~Am were recorded both with and without closing the exit window with an aluminium foil. From the analysis of these spectra, it is found that the energy resolution (fwhm) of 5.48 MeV alpha particle is 7.4% when the exit window was open and 7.0% for the case when the exit window was shut. This result is in good agreement with the observations of Muga et al. 6) and Bertolini et al.X°). Thus, the arrangement of fig. 6 is established to be practically equivalent to the direct mounting result 1°) and the perpendicular mounting results6). Then, the thicker scintillator is replaced by a "T.F.P.S." prepared as described in this paper. The spectrum of fission fragments from 252Cf w a s recorded for different thin film plastic scintillators, keeping the exit window open. One representative spectrum recorded by a "T.F.P.S." comprising of three laminations amounting to about 1.08 mg/cm 2 is shown in fig. 7. The fission fragment peaks are well resolved and the values of PH/Pv and PL/Pv a r e found to be in fair agreement with the corresponding values indicated in fig. 4. Thus, the perpendicular mounting of the type of apparatus used in fig. 6 is practically equivalent to direct mounting case. So, principally, this system can be used as a start detector in the time-of-flight experiments and another detector for the stop signal may easily be placed after the exit window of the transmission detector housing.
N .-J
:E
9. Conclusions
8
z z z
100
200
300
(CHAN NO.)
Fig. 7. Spectrum of ~52Cf responded by a three laminations scintillator film (total thickness ~ 1080/~g/cm 2) when mounted in the arrangement of fig. 6.
We draw the following conclusions from the above experiments: The measured pulse height of the light fission fragments from 252Cf passing through different films of plastic scintillator (NE 102A) increases with the increase in energy loss until a kind of plateau is obtained for higher energy deposition; while the pulse height of the heavy fragments rises slowly over the energy loss region studied (see fig. 3). The peak-to-valley ratios of the light and the heavy fragments are established systematically as a function of the scintillator film thickness. Typically, a single scintillator film of 0.4 mg/cm 2 thickness when mounted
106
R. K. BATRA AND A. C. SHOTTER
directly on the face of a photomultiplier, gives P H / P v -~ 5.05_+0.06 and P•/Pv = 3.57_+0.06, as compared with the corresponding values 4.0 and 3.0 respectively for an equally thick four laminations film mounted perpendicularlyS,6) between the faces of two photomultipliers. Thus, the direct mounting of the thin films described in this paper shows better resolution response than the perpendicular mounting arrangement of Muga et al.6). A scintillator film of appropriate thickness to achieve the optimum differentiation between the light and the heavy fragments can be selected by the inspection of fig. 4. For the same amount of energy loss of fission fragments in the scintillator, our values of PH/Pvand PL/Pv are in close agreement with Muga's measurements using stacks of detectors 5) (see section 6). Thus, we conclude that the improvement in resolution of fission fragments using stacks of thin film detectors 5) is not due to the different geometrical collection of light but is simply related to the total energy loss in the films. Another important conclusion is that the single and laminated scintillator films of equivalent thickness show practically the similar response to 252Cf. A simple transmission detector system using a thin film and a single photomultiplier has been described
for its application in a time-of-flight spectrometer for heavy ions. The authors are very grateful to Prof. N. Feather and Prof. W. Cochran for their encouragement and support. References 1) F. S. Goulding, D. A. Landis, J. Cerny and R. H. Pehl, Nucl. Instr. and Meth. 31 (1964) 1. 2) A. M. Pskanzer, S. W. Cosper, E. K. Hyde and J. Cerny, Phys. Rex,. Letters 17 (1966) 1271. a) L. Bridwell and M. E. Wyman, Rev. SCL Instr. 31 (1966) 1145. 4) j. C. D. Milton and J. S. Fraser, Chalk River Nuclear Laboratories, Sm-60/45 (1964). 5) M. L. Muga, Nucl. Instr. and Meth. 95 (1971) 349. 6) M. L. Muga, D. J. Burnsed and W. E Steeger, Nucl. Instr. and Meth. 104 (1972) 605. 7) M. L. Muga and G. Griffith, Nucl. Instr. and Meth. 109 (1973) 289. s) M. L. Muga and G. L. Griffith, Nucl. Instr. and Meth. 111 (1973) 581. 9) L. C. Northchffe and R. F. Schllhng, Nucl. Data A7 (1971) 233. 10) R. Muller and F. Gonnenwem, Nucl. Instr. and Meth. 91 (1971) 357. 11) G. Gertolim, A. M. Delturco and G. Restelli, Nucl. Instr. and Meth. 7 (1960) 350. 12) H. W. Schmltt, W. E. Klker and C. W. Wdliams, Phys. Rev. 137B (1967) B837.
R E S P O N S E OF T H I N F I L M P L A S T I C S C I N T I L L A T O R T O F I S S I O N F R A G M E N T S R. K. BATRA and A. C. SHOTTER
Physics Department, University of Edinburgh, Edinburgh, Scotland
Received 21 August 1974 Thin plasUc scintillator films of d~fferentthicknesses are prepared and their characteristic response to fission fragments from ~s2Cf is studied. It is observed that the peak to valley ratio of fission fragments depends non-linearly upon the thickness of thin films. This ratio for the heavy fragments IS larger than that of the light fragments by a factor of 1.5-1.9 for the whole range
of thickness investigated. The laminated structure of the films shows shghtly higher gain (~ 5 %) but the resolution remains practically the same. The energy loss of fission fragments in plastic scintdlator (NE 102A) is measured. A simple system for a "start-detector", employing such a thin film and a photomultipher, is also described for the time-of-flight experiments.
I . Introduction
teristic versus energy loss of F.F. The thin films investigated cover the thickness region up to the range of these fragments. Single and laminated films of equivalent thickness have also been studied. The energy loss of fission fragments in the scintillator films (NE 102A) is measured. Finally, we describe also a system, comprising of such a thin film and a single photomultiplier, which is capable of acting as a startdetector in any time-of-flight experiment.
The silicon transmission detectors in conjunction with thick semi-conductor detectors have been used for the identification of light charged particles 1'2) for the last few years. Due to practical difficulties, such silicon detectors cannot easily be made thin enough to transmit heavy ions of medium energy range. Thin film plastic scintillators might be an alternative to overcome this problem. Also, thin plastic scintillators are useful as timing detectors 3'4) in the time-of-flight experiments with heavy ions such as the measurements of massyields of fission fragments. During the past few years, Muga and co-workers 5-8) have investigated the energy and time response characteristics of thin film plastic (NE 102) for various ions. Their thin film detector consisted of a thin scintillator film attached to a hemi-cylindrical lucite support placed perpendicular to the parallel faces of two opposing photomultipliers. They demonstrated that a thin film scintillator detector resolves the light and the heavy fission fragments. Using such three detectors in series, Muga 5) observed that the heavy fragments to valley ratio improves. However, such an observation does not indicate clearly whether the improvement in resolution is due to the larger energy loss of the fission fragments in the three scintillator films or the larger collection of light by the six photomultipliers or both. Further, they pointed out that the lamination of the films and the perpendicular mounting of the film to the photomultlpller might be helping in photon production or collection. Therefore, it will be useful to establish systematically the true characteristic of thin films to transmitting fission fragments. We studied the fission fragments response of these detectors when mounted directly on the photomultiplier (section 3) as a function of detector thickness, i.e. response charac-
2. Making the film
The solution of plastic scintillator is prepared in the same way as prescribed by Muga et al. 6) and a slightly different procedure to that of Muga's 6) for making thin films is used. A porcelain pan (22 c m x 16 cm x x 5 cm) is filled with distilled water up to ~ of its depth. On the stationary surface of water, an "O-ring" (i.d. 8 cm) is floated gently. The " O - r i n g " helps to stretch the water surface. A few drops of the scintillator solution are poured m succession onto the stretched surface of water. It is found that the solution spreads more uniformly over the stretched surface as compared to the unstretched surface (tried without "O-ring") and a film of single thickness results, depending upon the strength of the solution. The uniformity of the film can be further improved by placing a second " O - r i n g " (i.d. 4.5 cm) onto the film when it is nearly dry. When the film is completely dry, a rectangular aluminium frame is dipped inclined in the water such that the film under the second "O-ring" just rests over the frame. The film is released from the "O-ring" (second) by piercing the boundary with a pointed aluminium needle having dipped in ethyl acetate. The film may now be conveniently hfted by the Al-frame. It is worth noting that the films made by this process 101
102
R. K. B A T R A A N D A. C. S H O T T E R
are very clear. Also, the use of second "O-ring" is not essential when making thicker films. The uniformity of these films was determined by studying the absorption of alpha particles through different regions of the films. It was found that a typical film had a uniformity better than ___3.5%. Having determined the uniformity of a film, it was transferred to a perspex rectangular frame (22 m m × 18 m m x x 1.5 mm). A thin layer of the optical coupler ensured the attachment. The sides of the perspex frame were painted with a reflector paint. The thin film plastic scintillator attached to the perspex frame will be referred to as "T.F.P.S." detector in this paper.
3. Details of experiment Looking at the arrangements of the thin film detector used by Muga et al.5'6), it seems that a quite good number of the emitted photons are not being received by the photomultipliers. Therefore, we would prefer to mount the "T.F.P.S." onto the face of a photomultiplier so as to optimize the resolution characteristtcs of thin films for fission fragments. The "T.F.P.S." is mounted at the centre of a 56AVP multiplier using the optical coupler (Dow Coming 20057). The simple arrangement of the apparatus is shown in fig. 1. It is important that the thin film is neither touching the photomultipher face nor the coupler. The whole system is placed in a vacuum chamber maintained at 10 -5 torr. A collimated beam of fission fragments from 252Cf impinges on a thin film scintillator. The energy signals are derived from the 12th dynode of the photomultiplier. Many thin scintillator films, both single and laminated, covering the range of fission fragments, were prepared as described in section 2. Their thicknesses were determined by studying the absorption of alpha particles (6.11 MeV from 252Cf) m the films, using a surface barrier semi-conductor counter and making use of range energy tables of
Northcliffe and Schilling9). Also, with a view to know the actual energy deposited by fission fragments in these films, the unslowed and slowed spectra of 252Cf were recorded by a solid state counter in a separate set-up. Having known the thickness of each film, each "T.F.P.S." was mounted to the photomultiplier individually and the corresponding spectra of fission fragments were recorded by a multi-channel analyser.
4. Data and error analysis The data of each spectrum of 252Cf recorded by a "T.F.P.S." is smoothed n times using the subroutine " D S G 13 ". One such smoothed spectrum recorded by a "T.F.P.S." of thickness ~250/~g/cm 2 is shown in fig. 2. The fission fragments peaks are distinctly resolved. The extrapolated dotted line clearly indicates that the background due to alpha particles and ~,-rays from the source has very little interference with the spectrum. The spectra are corrected for the background. The n-times smoothing over any channel means that the resulting smoothed value has been averaged over ( 2 n + 1) adjacent channels. Thus, the statistical error on the counts rate y at any channel x of the spectrum when smoothed n times, wdl be given approximately as as(y ) ~
a 2 (2n+l)
,
(l)
I...l=X--n
where a, denotes the statistical error on the unsmoothed value y,. It was found that the values of as(Pa), as(Pv) and as(PL) on the smoothed values of PH, Pv and PL are less than ___3%.
I00'
o BACKGROUND
COR RECTED .
PERSPEX FRAME
.
.
.
o
C OL/IMATOR
>.
'
!o".5"6 2~P ~'",,
z Z
(3 FILM
OPIICAL COUPLER
\ 5b
1OO
CHAN NO
150
Fig. 2. Fission spectrum from 2~2Cf recorded by thin film Fig. 1. Mounting of a thin film of plastic scintillator on the photomultipher face.
sclntdlator
(250ffg/cm2). • Uncorrected, background.
® corrected
for
RESPONSE OF THIN FILM P L A S T I C S C I N T I L L A T O R
5. Results and discussions Fig. 3 shows the response characteristics of light and heavy fission fragments as they pass through various thin films. It is clear that the pulse height of light fragments increases as their energy loss in the scintillator increases until a plateau region is obtained. This observation is in favour of the earlier predictionsa) of the relative pulse height of fission fragments versus scintillator thickness obtained on the basis of specific ionization of fission fragments, where the authors 3) did not differentiate between the light and heavy fragments. Fig. 3 shows also that the pulse height of heavy fission fragments varies linearly with the energy loss experienced by them in traversing the scintillator films. Recently, Muga et al.7'a) observed that the pulse heights of light and heavy accelerated ions, suffering energy loss up to 15 MeV in the scintdlator films, fol/ows the trends of the theoretical energy lossesg); and predicted the agreement between the theory and the experiment for the larger energy loss region also. Our observations of thin films' response as a function of energy loss [varied by varying the thickness of the films and using range-energy tables9)] to light fragments (average mass = 106.5, average energy 104.5 MeV) suffering energy loss up to 102 MeV; have the same pattern for larger energy loss region as demonstrated by Muga et al.'s experiment for smaller energy loss region. The pulse height versus energy loss plot of heavy fragments (average mass 141.7, average energy = 80.1 MeV) experiencing energy loss up to 70 MeV also verifies the anticipated behaviour for heavy ions. As a measure of resolution, the peak to valley ratios i.e. Pn/Pv and PL/Pv are obtained from each recorded
Z
o LIGHT
FRAG
• HEAVy
FRAG
103
spectrum. The plots of these ratios* as a function of thin film thickness for light and heavy fission fragments are shown in fig. 4. This observation clearly indicates that the energy resolution of fission fragments registered by "T.F.P.S." varies non-linearly with the amount of energy deposited by them. Also, the peak to valley ratio of heavy fragments is greater (by a factor of 1.5-1.9) than that of the light fragments for any "T.F.P.S.". This study of resolution also points out that for experiments requiring high resolution of fission fragments, a "T.F.P.S." of thickness lying between 0.5 t o 1.35 m g / c m 2 is most suitable. There are no measurements known in the literature about the energy loss of fission fragments in the scintillator NE 102A. One has to rely upon the interpolated values obtained either from the range-energy tables 9) or from the measured values in the neighbouring materials1°). In order to know the precise values of energy loss of fission fragments in the scintillator films (NE 102A), we smoothed the unslowed and slowed spectra of 252Cf recorded by a semi-conductor counter and estimated the energy loss of F.F. in different films using Schmitt's equation 12) among the energy, the pulse height and the average mass of the fragments. For convenience, the residual energies of light and heavy fission fragments passing through the films are displayed in fig. 5 as a function of the measured energy loss of 6.11 MeV alpha particles. The graph is important in the sense that by measuring only the energy loss * The errors bars are the statistical errors on the ratio values. 1o
/•
~ LIGHT FRAG AG
300 5-
~ 200"r
o_.-~
Z
10o
a."r
c
2"~ 5b J5 . ;do ho ENERGY LOSS (MeV) OF FISSION FRAGMENTS Fig. 3. Pulse heights of hght and heavy fission fragments versus energy loss of F.F.
100o
SCINTILLATORTHICKNESS
,1'500
2o00
IN ~gm/cm2
Fig. 4. Peak-to-valley ratzos of light and heavy fission fragments as a function of scintillator thickness.
104
R.K.
B A T R A A N D A. C. S H O T T E R
of alpha particles in any film of NE 102A, one can readily know the energy deposited by the mean light and mean heavy fragments. The measurements have been done only up to the region 12) where the calibration procedure of semi-conductor detector for heavy ions remains linear.
6. Comparison with other experiments Muga et al. 5) observed that the values of heavy fragment to valley and light fragment to valley for a four laminations film of thickness 0.4 mg/cm 2 were 4.0 and 3.0 respectively (see ref. 6, fig. 6). For comparison with our results, the values of PH/Pv and PL/Pv for a single film 0.4 mg/cm 2 thickness are about 5.05+_0.06 and 3.57+_0.06 as obtained from fig. 4. Thus, one finds that the response of a thin film mounted as described in section 3 is better than when the mounting procedure of Muga et al. 5-s) is used. To examine the improvement in the response of F.F. passing through thin scintdlators, Muga 5) arranged three scintillator films in series and mixed the signals from the six photomultipliers employed in the experiment (see ref. 5, figs. 8 and 9). He concluded that for about 40% of the total energy loss of F.F. in three T.F.D.s, the values of PH/Pv and PL/Pv are 7.5 and 4.25 respectively. Making use of fig. 5, one finds that the fission fragments lose about 40% of their energy in about 0.55 mg/cm 2 of the scintillator material. The resolution curves (fig. 4) observed by us indicate the values of PH/Pv and PL/Pv for about 0.55 mg/cm 2 as
,25]
7.0_ 0.07 and 4.2_ 0.06 respectively. This implies that a "T.F.P.S." mounted directly on photomultipher tube shows practically the same resolution as the stacked detector system 6) for the same total thickness of the film. The above comparison between our results and Muga's experiment with stacks of detectors 6) is not direct because the geometrical arrangement in the two experiments differ. However, it is shown in section 8 that the perpendicular mounting arrangement for transmission detector and direct mounting do in fact yield identical resolutions. Thus, we see that the larger scintillator thickness, i.e. larger energy loss of F.F. ~s responsible for the improved values of peaks to valley in the case of stacked detectors5). Hence, for the improved differentiation between two adjacent heavy ions, one need not have to use stacked T.F.D.s. but instead, a "T.F.P.S." of appropriate thickness will solve the same purpose.
7. Influence of laminated structure It is interesting to investigate how the lamination of films affects the response of "T.F.P.S." to fission fragments. To do that, we made the scintillator solutions of different strengths and prepared a couple of single and laminated films and determined their thicknesses. For instance, a single film (thickness m 1.2 mg/ cm 2) and another film of three laminations (total thickness m 1.16 mg/cm 2) were tested independently. It is found that the spectrum of 252Cf recorded from the laminated film is slightly shifted to higher channels (m5%) as compared with that recorded by the single film, while the peak to valley ratios in both the cases are practically the same. Probably, the tlansmission of the scintillations produced in the multilayers of the
IOO'~
COLLIMATOR
>.. 50" ~ ~
PERSPEX IIOAIA E
" HEAVYFRAG
z
560
lobo
~5oo
ALPHA ENERGYLOSS(KeV)
Fig. 5. Residual energy of F.F. vs energy loss o f 6.11 MeV alpha particles in the scintillator (NE 102A).
6
O~.ALC
'
25ram
Fig. 6. A n arrangement showing the use of thin film scintdlator as a transmission detector for heavy ions studies.
RESPONSE OF THIN FILM P L A S T I C S C I N T I L L A T O R
laminated film might be helping in spreading the light more uniformly over the used area of the photocathode. This may be the cause for the slightly large pulse height for laminated structure as compared to the single film response. 8. " T . F . P . S . " as transmission detector The obvious dis advantage of mounting the "T. F. P.S." directly on the photomultiplier face is that this arrangement cannot be used for the transmission purposes. We describe below a simple and economical system, employing only one photomultiplier tube, to use "T.F.P.S." as a transmission detector. The arrangement of the transmission detector is shown in fig. 6. The rectangular perspex frame having a T.F.S. mounted over it, is inserted perpendicularly into a groove at the centre of a cylindrical well-polished base B, made of perspex. The base B acting as small light guide is mounted on the face of the photomultiplier with the help of optical coupler. The curved surfaces of the base are painted white with the reflector paint (NE 560). An inverted hollow cylinder made of perspex is placed in a circular groove over the base. This cyhnder has an entrance and an exit opening at appropriate height. The cylinder is polished thoroughly (both inside and outside) and is covered with an aluminium cap having entrance and exit openings matching to the cylinder. The dimensions of the cylinder are chosen such that the loss of light due to multiple reflections at the walls should be as small as possible. Also, the openings in the cylinder allow
I00THREE LAMINATIONS FILM TOTAL THICKNESS = 1 O8mg~cm 2
105
about 10% of the total scintillations to be lost through them. As an exploratory measure of the system, a thicker plastic scintillator (thickness ~ 25 mg/cm z) as supplied by the manufacturer is mounted onto the perspex frame and is housed in the cylinder. The whole system shown in fig. 6 is placed in a vacuum chamber. The spectra of alpha particles of 6.11 MeV from 252Cf and 5.48 MeV from 24~Am were recorded both with and without closing the exit window with an aluminium foil. From the analysis of these spectra, it is found that the energy resolution (fwhm) of 5.48 MeV alpha particle is 7.4% when the exit window was open and 7.0% for the case when the exit window was shut. This result is in good agreement with the observations of Muga et al. 6) and Bertolini et al.X°). Thus, the arrangement of fig. 6 is established to be practically equivalent to the direct mounting result 1°) and the perpendicular mounting results6). Then, the thicker scintillator is replaced by a "T.F.P.S." prepared as described in this paper. The spectrum of fission fragments from 252Cf w a s recorded for different thin film plastic scintillators, keeping the exit window open. One representative spectrum recorded by a "T.F.P.S." comprising of three laminations amounting to about 1.08 mg/cm 2 is shown in fig. 7. The fission fragment peaks are well resolved and the values of PH/Pv and PL/Pv a r e found to be in fair agreement with the corresponding values indicated in fig. 4. Thus, the perpendicular mounting of the type of apparatus used in fig. 6 is practically equivalent to direct mounting case. So, principally, this system can be used as a start detector in the time-of-flight experiments and another detector for the stop signal may easily be placed after the exit window of the transmission detector housing.
N .-J
:E
9. Conclusions
8
z z z
100
200
300
(CHAN NO.)
Fig. 7. Spectrum of ~52Cf responded by a three laminations scintillator film (total thickness ~ 1080/~g/cm 2) when mounted in the arrangement of fig. 6.
We draw the following conclusions from the above experiments: The measured pulse height of the light fission fragments from 252Cf passing through different films of plastic scintillator (NE 102A) increases with the increase in energy loss until a kind of plateau is obtained for higher energy deposition; while the pulse height of the heavy fragments rises slowly over the energy loss region studied (see fig. 3). The peak-to-valley ratios of the light and the heavy fragments are established systematically as a function of the scintillator film thickness. Typically, a single scintillator film of 0.4 mg/cm 2 thickness when mounted
106
R. K. BATRA AND A. C. SHOTTER
directly on the face of a photomultiplier, gives P H / P v -~ 5.05_+0.06 and P•/Pv = 3.57_+0.06, as compared with the corresponding values 4.0 and 3.0 respectively for an equally thick four laminations film mounted perpendicularlyS,6) between the faces of two photomultipliers. Thus, the direct mounting of the thin films described in this paper shows better resolution response than the perpendicular mounting arrangement of Muga et al.6). A scintillator film of appropriate thickness to achieve the optimum differentiation between the light and the heavy fragments can be selected by the inspection of fig. 4. For the same amount of energy loss of fission fragments in the scintillator, our values of PH/Pvand PL/Pv are in close agreement with Muga's measurements using stacks of detectors 5) (see section 6). Thus, we conclude that the improvement in resolution of fission fragments using stacks of thin film detectors 5) is not due to the different geometrical collection of light but is simply related to the total energy loss in the films. Another important conclusion is that the single and laminated scintillator films of equivalent thickness show practically the similar response to 252Cf. A simple transmission detector system using a thin film and a single photomultiplier has been described
for its application in a time-of-flight spectrometer for heavy ions. The authors are very grateful to Prof. N. Feather and Prof. W. Cochran for their encouragement and support. References 1) F. S. Goulding, D. A. Landis, J. Cerny and R. H. Pehl, Nucl. Instr. and Meth. 31 (1964) 1. 2) A. M. Pskanzer, S. W. Cosper, E. K. Hyde and J. Cerny, Phys. Rex,. Letters 17 (1966) 1271. a) L. Bridwell and M. E. Wyman, Rev. SCL Instr. 31 (1966) 1145. 4) j. C. D. Milton and J. S. Fraser, Chalk River Nuclear Laboratories, Sm-60/45 (1964). 5) M. L. Muga, Nucl. Instr. and Meth. 95 (1971) 349. 6) M. L. Muga, D. J. Burnsed and W. E Steeger, Nucl. Instr. and Meth. 104 (1972) 605. 7) M. L. Muga and G. Griffith, Nucl. Instr. and Meth. 109 (1973) 289. s) M. L. Muga and G. L. Griffith, Nucl. Instr. and Meth. 111 (1973) 581. 9) L. C. Northchffe and R. F. Schllhng, Nucl. Data A7 (1971) 233. 10) R. Muller and F. Gonnenwem, Nucl. Instr. and Meth. 91 (1971) 357. 11) G. Gertolim, A. M. Delturco and G. Restelli, Nucl. Instr. and Meth. 7 (1960) 350. 12) H. W. Schmltt, W. E. Klker and C. W. Wdliams, Phys. Rev. 137B (1967) B837.