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Response of various thin-film scintillators to low-energy particles
N U C L E A R I N S T R U M E N T S AND METHODS
I2 5
([975) 413-417; © N O R T H - H O L L A N D P U B L I S H I N G CO.
R E S P O N S E OF VARIOUS T H I N - F I L M S C I N T I L L A T O R S T O L O W - E N E R G Y PARTICLES JOHN W. K O H L
Applied Physics Laboratory, The Johns Hopkins University, Silver Spring, Maryland 20910, U.S.A. Received l0 February 1975 The response of several different types of plastic thin-film scintillator detectors (TFD) to low-energy protons and alpha particles is presented. Film thicknesses vary from ~ 5 0 p g / c m 2 to ~ 4 0 0 # g / c m 2. Particle energies vary from ~0.05 MeV/ nucleon to ~ 2 . 5 MeV/nucleon and are both penetrating and non-penetrating.
The results extend previously published work and are consistent with it 1, 2). Response of the TFDs shows the effect of ionization quenching at low energies and the non-linearity of light output with deposited energy (dE). The double-valued nature of specific luminescence (dL/dx) as a function of specific energy loss (dE/dx) suggested by Muga and Griffith 1) is confirmed.
1. Introduction In recent years, the determination of the charge and isotopic composition of the low-energy (~< 10 MeV/ nucleon) interplanetary particle population has become of interest. Previous instruments for this investigation used the dE/dx-E technique for isotopic identification. However, the dE/dx measurement becomes unreliable for heavy particles at low energies, because the charge of the incident particle varies as it traverses matter. Hence the d E signatures are very difficult to identify. Also, present instrumentation has severe low-energy restrictions due to the limitations on thickness and area of present transmission detectors. It turns out that velocity and total energy (which is more easily measured at low energies) will work well, so that this method is probably dictated by the energyloss mechanism as a means of identifying low-energy charged particles. Such techniques have been described by Muga et al. a) and by Gelbke et alff) using thin scintillating films as the transmission detector. The advantages of this type of transmission detector are obvious: (1) they can be made very thin (,,~25 #g/cm 2 allowing measurement of particles down to -,~50 keV/ nucleon), and (2) they can be made in fairly large areas. The response of an NE102 thin-film detector (TFD) to light and heavy ions has been presented by Muga and Griffithl). The response of an N E l l l T F D to 160 ions has been presented by Braun-Munzinger and Gelbke2). In this work, the response of T F D s made from the plastic scintillators N E l l 0 , N E l l l , Pilot M, and Pilot Y* to protons and alpha particles will be pre-
sented. T F D thicknessses varied from ~,50 p g / c m 2 t o ,-,400 pg/cm 2. Incident particle energies varied from ,-~50 keV to upper limits of ,-, 1 MeV and ,,~3 MeV for protons and alphas, respectively. Basically this work represents this laboratory's initial investigation into the characteristics of TFDs and the feasibility of utilizing them in either a dE/dx-E or time-of-flight (TOF) low-energy heavy-particle spectrometer.
* Available from Nuclear Enterprises Inc., San Carlos, California, U.S.A.
Fig. 1. Exploded view of TFD assembly showing films and lucite light guide.
2. Apparatus and procedures The TFDs used in this study were made using the recipe by MugaS). Some difficulty was encountered with Pilot M plastic material in that no film <400/zg/ cm 2 could be obtained without tearing during removal from the water. For this reason, results will be presented
413
J
1.800 LUCITE
VYNS FILM
00
/
SCINTILLATOR FILM LUCITE DIMENSIONS IN INCHES
414
J . w . KOHL
only for a thick film of Pilot M compared to the other materials (50-130/Jg/cm2). Pilot Y, which seemed to have a greater material strength, was added, until a sufficiently strong film was obtained. The resulting mixture was approximately 50% each and will be referred to in the following discussion as Pilot MY. The thicknesses of the TFDs were determined from the energy loss of 5.477 MeV alpha particles from 241Am using the table of Northcliffe et al. 6) for polyethylene. The lucite film-holder and light guide is similar to that used by MugaS), except for some minor dimensional and geometrical modifications. A sketch o f the film holder is shown in fig. 1. A VYNS film acted as a bond and light coupler to one lucite segment, and silicon grease was used on the other half. The outside surfaces of the lucite were coated with a white reflective paint. Silicon grease was also used as the coupling agent between the lucite and the single RCA 8850 photomultiplier used to view the film. Because the l, 2, 3, etc. photoelectron peaks could be identified in the noise spectrum of the photomultiplier tube, some of the units related to light output could be calibrated in terms of photoelectrons and will be presented as such in the following discussion. The basic electronic arrangement which was used to measure T F D response and Ein-AETFD in the solidstate detector (SSD) is shown in fig. 2. As can be seen, it is a simple arrangement intended for initial study of T F D characteristics. Noise was eliminated, for penetrating particles, by relying on a signal from the SSD to provide a gating pulse. For non-penetrating particles, the gate was disabled, the resulting data being somewhat more ambiguous. Not shown in fig. 2 is an SSD, which is removable from the beam path, to provide information on the initial energy of the particles. Preliminary checks for proper operation and cali-
brations were made with 241Am and 22aTh radioactive sources which provide alpha particles ranging in energy from 5.4 to 8.8 MeV. The data to be presented here were obtained on one of the National Aeronautics and Space Administration/Goddard Space Flight Center Van de Graaff accelerators, which provides protons and alpha particles from 50 keV to 3 MeV. 3. Observations
The response (in units of channel number) of the I NE 111 132 #gm/cm 2
6OO
J
.~ 400 NE 1 1 ~ 91/Jgm/cm2-
z 300
=o
I
¢: 200 1100
60~
PILOT Y - 71 #gm/cm2
I
I
I
I
PILOT MY - 51/Jgm/crn2
i
~lsoo ==
40o
-
/
z 300
2
r(
~: 200 100
I 1 INCIDENT ENERGY(Me~
0
I 2 3 ,NCIDENT ENERGY(Me~v~
2OOO
1800~
PARTICLE BEAM TFD
1200
TFD z o
i S
S
D
I
~
~
~
~
E - AE ~
l-~
GATE I
10oo aoo
w 600 ,', u. 400 I- 2OO t 1
I 2
3
INCIDENT ENERGY(Me~
Fig. 2. Block diagram of apparatus used to measure TFD pulseheight distribution (response) as a function of particle type and energy. TFD: thin-film detector, SSD: solid-state detector, P.A.: preamplifier, L.A.: linear amplifier, D.A.: delay amplifier, D: discriminator, P.S.: pulse stretcher, P.H.A.: pulse-height analyzer.
Fig. 3. TFD response as a function of incident energy for protons (solid symbols) and alphas (open symbols) for various plastic scintillators. The ordinate error bars represent the fullwidth at half-maximum of the pulse-height distribution. The curves through the data points were added as a visual aid.
R E S P O N S E OF T H I N - F I L M
various TFDs as a function of incident energy for protons and alphas is shown in fig. 3. Note that the different materials differ also in thickness with a range of 50-130/~g/cm 2, with the exception of the 400 #g/cm 2 Pilot M. The electronic settings were the same for all to allow for direct comparison, again with the exception of the Pilot M. The vertical error bars indicate the fullwidth-half-maximum spread in the amplitude of the light output. One of the features to be noted is the similarity in the shape of the response curves for the different scintillator materials. At the lowest energies, protons give a greater light output than alpha particles. Then, somewhere between 400-600 keV, there is a crossover and alpha particles give a greater light o u t p u t - while the proton response levels off or even declines with increasing energy. Although the thicker Pilot M T F D has the same basic shape, the crossover point is extrapolated to ~ 1.2 MeV. The sudden drop-off in response MATERIAL NE 110 NE 111 PI LOT Y PILOT MY
THICKNESS 91/zgm/cm 2 132 71 51
SYMBOL ALPHAS PROTONS o-- • o. . . . . . • "~J¢
0.4 1
1
\
z
I
",o,
\.K'-
°.'~k k I ~t "...'-... 0
-
1
%\ 2
INCIDENT ENERGY (MeV) Fig. 4. A E in the T F D as a function o f incident energy for p r o t o n s (solid symbols) and alphas (open symbols) for various plastic scintillators. The solid line at the left represents the d E vs E for non-penetrating particles. Only the data points for penetrating particles are shown. The curves through the data points were added as a visual aid.
SCINTILLATORS
415
at the lower energies has been noted by Muga t) for NE102, and would seem to indicate the ionization quenching effect discussed by Birks6). It can be noted that the fwhm resolution is very poor for the thin TFDs and improves considerably for the thicker Pilot M TFD. It would seem that, even allowing for the thickness variations between the various films, the NE110 T F D gave a bigger response to protons and alphas than the other films. A word of caution is necessary, though. Although all conditions for testing the various films were made as identical as possible, some differences might occur which would be difficult to evaluate (e.g., quality of reflective coating on the different film holders, ageing of the plastic films, slight differences in the optical coupling, etc.). The non-linearity of the T F D response with energy loss (AE) can be seen by comparison with fig. 4, which presents the measured AE as a function of the incident energy for protons and alphas. The energy loss for the 394 #g/era 2 Pilot M T F D was not included because only a few points would fall within the limits of the ordinate. The data points shown for the various TFDs are for penetrating particles only. Because a limited number of data points in incident energy were taken, the shape of the curve near the point of juncture to the non-penetration curve and near the AE peak for alphas is not strictly defined. The lines shown through the points are only to make it easier for the observer to connect points. No comparison to calculated values was made. The heavy line to the left of the figure represents the values taken for non-penetrating particles. Note the fairly sharp drop-off in AE after penetration fol protons, while the AE for alpha particles continues to increase to a rounded peak at some higher value of incident energy before it begins to decrease. The thicker Pilot M T F D exhibits the shape of the proton curve for alpha particles also. The non-linearity of light output as a function of deposited energy is more obvious when presented as in fig. 5. This shows AL/AE, a measure of the efficiency of photon production in units of photoelectrons/MeV deposited, as a function of the incident-particle energy. The solid points are for protons and open symbols for alpha particles. A few sample error bars, indicating the fwhm spread in the T F D response, are shown. It can be seen, as pointed out by Mugal), that protons have a higher efficiency for useful photon production than alpha particles. Also, it appears that at the lower incident energies the value of AL/AE levels off to a constant value.
416
J.w.
Another presentation of the data is shown in fig. 6, which gives the specific luminescence (dL/dx) as a function of specific energy loss (dE/dx) for the various TFDs. Only the results for penetrating particles are shown. The data points of AL/Ax vs zlE/zlx shown here confirm the behavior suggested by Muga ~) that AL/Ax is double-valued for each zJE/Ax. It is apparent for the alpha-particle curves, but not for the proton curves which end at their maximum dE/dx value. The doublevaluedness of AL/Ax would become more obvious with the addition of the data for non-penetrating 160
1
I
INE 111 _~ 132 #gm/cm 2
NE 110 91 #gm/cm 2
140 120 c e 100
¥ 8 o
80 60
5£ 40 20
1
I .1
2
INCIDENT ENERGY (MeV)
INCIDENT ENERGY (MeV) 400
I 2
KOHL
particles, which would extend the curves toward the lower left of the figure. 4. Summary and conclusions Basically, the characteristics of the TFDs studied ( N E l l 0 , N E l l l , Pilot M, Pilot Y) were very similar. The curves of pulse-height distribution as a function of incident energy all had the same shape, depending on particle type. In all cases the response to protons was greater than alpha particles at low energy, with a reversal at the higher energies. All the TFDs exhibited a "saturation" effect at the higher values of specific energy loss. The Pilot MY, although a 50% mixture, seemed to behave more like the Pilot Y than Pilot M. The NE110 seemed to give a bigger light output for protons and alphas than the other TFD films tested, and might be the preferable plastic for heavy-particle detection, although any specific differences may be due more to things like age or optical coupling, etc., than to differences in the scintillator plastic used for the TFD. Due to the non-linearity of light output with AE,
500
I
PI LOT Y 71 .ugm/crn 2 400
MATERIAL
300
PI LOT Y PI LOT M PI LOT M, Y NE 110 NE 111
5~ 300
o ~d
20o 200
® - ----,0 b--=- - ~,. ~ o
51
91 132
10 o loo
SYMBOL PROiONS ALPHAS ~, ^
TIqlCKNESS 71 # cm/cm2 394
e I
I
I
.o
I
I
]
I
1
_ o
1
INCIDENT ENERGY (Me~
INCIDENT ENERGY (MeV) 2OO
1
i
PILOT M 394 .ugm/cm 2
180 160
~'~ :::L O
10-1
140
loo ~
so
~
40
-
O
120
vCL
x .._1 6 1
o"
,.o-
m
2
INCIDENT ENERGY (MeV)
Fig. 5. P h o t o n production efficiency (AL/AE) as a function o f incident energy for protons (solid symbols) and alphas (open symbols) for various plastic scintillators. N o t e that the ordinate scales are different. A l s o s h o w n are sample ordinate error bars which represent the full-width at h a l f - m a x i m u m o f the pulseheight distribution. The curves t h r o u g h the data points were added as a visual aid.
10-2 10-4
I
I
l
!
I
10 3 AE/AX (Ik],_V,ugm-1 cm2)
I
I I 10-2
Fig. 6. Luminescence (dL/dx) as a function o f the specific energy loss (dE/dx) for penetrating protons (solid symbols) and alphas (open symbols) for various scintillators. The curves through the data points were added as a visual aid.
RESPONSE OF T H I N - F I L M S C I N T I L L A T O R S
the use o f a T F D as the t r a n s m i s s i o n d e t e c t o r in a AE, E-AE technique to identify low-energy nuclear species does n o t seem to be feasible. H o w e v e r , because even a very thin T F D will give a recognizable light o u t p u t (using a p p r o p r i a t e electronic techniques) for transiting particles, it w o u l d seem that it w o u l d be suitable as a large t r a n s m i s s i o n d e t e c t o r in a time-of-flight a r r a n gement for species identification, p a r t i c u l a r l y in lowc o u n t - r a t e applications. The a u t h o r wishes to t h a n k M r Stephen A. G a r y for his assistance with the electronics t r o u b l e - s h o o t i n g a n d for the m a n y invaluable discussions on timing techniques. Especially m a n y t h a n k s to Stephen K. B r o w n o f N A S A / G S F C for his assistance a n d patience while using the V a n de Graaff. M r R o b e r t B r u b a k e r also deserves t h a n k s for assisting with the l o a d i n g a n d
417
u n l o a d i n g o f e q u i p m e n t the m a n y times we needed a n o t h e r d a t a set f r o m the N A S A - G S F C accelerator. T h a n k s also to the m a n y p e o p l e w h o m a d e the thin films for this p r o g r a m .
References 1) M. L. Muga and G. Griffith, Nucl. Instr. and Meth. 109 (1973) 289.
2) p. Braun-Munzinger and C. K. Gelbke, Nucl. Instr. and Meth. 114 (1974) 141. 3) M. L. Muga, D. J. Burnsed, W. E. Steeger and H. E. Taylor, Nucl. Instr. and Meth. 83 (1970) 135. 4) C. K. Gelbke, K. D. Hildenbrand and R. Bock, Nucl. Instr. and Meth. 95 (1971) 397. 5) M. L. Muga, D. J. Burnsed and W. E. Steeger, Nucl. Instr. and Meth. 104 (1972) 605. ~) L. C. Northcliffe and R. F. Schilling, Nucl. Data A7 (1970). 7) j. B. Birks, The theory and practice of scintillation counting (Pergamon Press Inc., New York, 1964) p. 187.
I2 5
([975) 413-417; © N O R T H - H O L L A N D P U B L I S H I N G CO.
R E S P O N S E OF VARIOUS T H I N - F I L M S C I N T I L L A T O R S T O L O W - E N E R G Y PARTICLES JOHN W. K O H L
Applied Physics Laboratory, The Johns Hopkins University, Silver Spring, Maryland 20910, U.S.A. Received l0 February 1975 The response of several different types of plastic thin-film scintillator detectors (TFD) to low-energy protons and alpha particles is presented. Film thicknesses vary from ~ 5 0 p g / c m 2 to ~ 4 0 0 # g / c m 2. Particle energies vary from ~0.05 MeV/ nucleon to ~ 2 . 5 MeV/nucleon and are both penetrating and non-penetrating.
The results extend previously published work and are consistent with it 1, 2). Response of the TFDs shows the effect of ionization quenching at low energies and the non-linearity of light output with deposited energy (dE). The double-valued nature of specific luminescence (dL/dx) as a function of specific energy loss (dE/dx) suggested by Muga and Griffith 1) is confirmed.
1. Introduction In recent years, the determination of the charge and isotopic composition of the low-energy (~< 10 MeV/ nucleon) interplanetary particle population has become of interest. Previous instruments for this investigation used the dE/dx-E technique for isotopic identification. However, the dE/dx measurement becomes unreliable for heavy particles at low energies, because the charge of the incident particle varies as it traverses matter. Hence the d E signatures are very difficult to identify. Also, present instrumentation has severe low-energy restrictions due to the limitations on thickness and area of present transmission detectors. It turns out that velocity and total energy (which is more easily measured at low energies) will work well, so that this method is probably dictated by the energyloss mechanism as a means of identifying low-energy charged particles. Such techniques have been described by Muga et al. a) and by Gelbke et alff) using thin scintillating films as the transmission detector. The advantages of this type of transmission detector are obvious: (1) they can be made very thin (,,~25 #g/cm 2 allowing measurement of particles down to -,~50 keV/ nucleon), and (2) they can be made in fairly large areas. The response of an NE102 thin-film detector (TFD) to light and heavy ions has been presented by Muga and Griffithl). The response of an N E l l l T F D to 160 ions has been presented by Braun-Munzinger and Gelbke2). In this work, the response of T F D s made from the plastic scintillators N E l l 0 , N E l l l , Pilot M, and Pilot Y* to protons and alpha particles will be pre-
sented. T F D thicknessses varied from ~,50 p g / c m 2 t o ,-,400 pg/cm 2. Incident particle energies varied from ,-~50 keV to upper limits of ,-, 1 MeV and ,,~3 MeV for protons and alphas, respectively. Basically this work represents this laboratory's initial investigation into the characteristics of TFDs and the feasibility of utilizing them in either a dE/dx-E or time-of-flight (TOF) low-energy heavy-particle spectrometer.
* Available from Nuclear Enterprises Inc., San Carlos, California, U.S.A.
Fig. 1. Exploded view of TFD assembly showing films and lucite light guide.
2. Apparatus and procedures The TFDs used in this study were made using the recipe by MugaS). Some difficulty was encountered with Pilot M plastic material in that no film <400/zg/ cm 2 could be obtained without tearing during removal from the water. For this reason, results will be presented
413
J
1.800 LUCITE
VYNS FILM
00
/
SCINTILLATOR FILM LUCITE DIMENSIONS IN INCHES
414
J . w . KOHL
only for a thick film of Pilot M compared to the other materials (50-130/Jg/cm2). Pilot Y, which seemed to have a greater material strength, was added, until a sufficiently strong film was obtained. The resulting mixture was approximately 50% each and will be referred to in the following discussion as Pilot MY. The thicknesses of the TFDs were determined from the energy loss of 5.477 MeV alpha particles from 241Am using the table of Northcliffe et al. 6) for polyethylene. The lucite film-holder and light guide is similar to that used by MugaS), except for some minor dimensional and geometrical modifications. A sketch o f the film holder is shown in fig. 1. A VYNS film acted as a bond and light coupler to one lucite segment, and silicon grease was used on the other half. The outside surfaces of the lucite were coated with a white reflective paint. Silicon grease was also used as the coupling agent between the lucite and the single RCA 8850 photomultiplier used to view the film. Because the l, 2, 3, etc. photoelectron peaks could be identified in the noise spectrum of the photomultiplier tube, some of the units related to light output could be calibrated in terms of photoelectrons and will be presented as such in the following discussion. The basic electronic arrangement which was used to measure T F D response and Ein-AETFD in the solidstate detector (SSD) is shown in fig. 2. As can be seen, it is a simple arrangement intended for initial study of T F D characteristics. Noise was eliminated, for penetrating particles, by relying on a signal from the SSD to provide a gating pulse. For non-penetrating particles, the gate was disabled, the resulting data being somewhat more ambiguous. Not shown in fig. 2 is an SSD, which is removable from the beam path, to provide information on the initial energy of the particles. Preliminary checks for proper operation and cali-
brations were made with 241Am and 22aTh radioactive sources which provide alpha particles ranging in energy from 5.4 to 8.8 MeV. The data to be presented here were obtained on one of the National Aeronautics and Space Administration/Goddard Space Flight Center Van de Graaff accelerators, which provides protons and alpha particles from 50 keV to 3 MeV. 3. Observations
The response (in units of channel number) of the I NE 111 132 #gm/cm 2
6OO
J
.~ 400 NE 1 1 ~ 91/Jgm/cm2-
z 300
=o
I
¢: 200 1100
60~
PILOT Y - 71 #gm/cm2
I
I
I
I
PILOT MY - 51/Jgm/crn2
i
~lsoo ==
40o
-
/
z 300
2
r(
~: 200 100
I 1 INCIDENT ENERGY(Me~
0
I 2 3 ,NCIDENT ENERGY(Me~v~
2OOO
1800~
PARTICLE BEAM TFD
1200
TFD z o
i S
S
D
I
~
~
~
~
E - AE ~
l-~
GATE I
10oo aoo
w 600 ,', u. 400 I- 2OO t 1
I 2
3
INCIDENT ENERGY(Me~
Fig. 2. Block diagram of apparatus used to measure TFD pulseheight distribution (response) as a function of particle type and energy. TFD: thin-film detector, SSD: solid-state detector, P.A.: preamplifier, L.A.: linear amplifier, D.A.: delay amplifier, D: discriminator, P.S.: pulse stretcher, P.H.A.: pulse-height analyzer.
Fig. 3. TFD response as a function of incident energy for protons (solid symbols) and alphas (open symbols) for various plastic scintillators. The ordinate error bars represent the fullwidth at half-maximum of the pulse-height distribution. The curves through the data points were added as a visual aid.
R E S P O N S E OF T H I N - F I L M
various TFDs as a function of incident energy for protons and alphas is shown in fig. 3. Note that the different materials differ also in thickness with a range of 50-130/~g/cm 2, with the exception of the 400 #g/cm 2 Pilot M. The electronic settings were the same for all to allow for direct comparison, again with the exception of the Pilot M. The vertical error bars indicate the fullwidth-half-maximum spread in the amplitude of the light output. One of the features to be noted is the similarity in the shape of the response curves for the different scintillator materials. At the lowest energies, protons give a greater light output than alpha particles. Then, somewhere between 400-600 keV, there is a crossover and alpha particles give a greater light o u t p u t - while the proton response levels off or even declines with increasing energy. Although the thicker Pilot M T F D has the same basic shape, the crossover point is extrapolated to ~ 1.2 MeV. The sudden drop-off in response MATERIAL NE 110 NE 111 PI LOT Y PILOT MY
THICKNESS 91/zgm/cm 2 132 71 51
SYMBOL ALPHAS PROTONS o-- • o. . . . . . • "~J¢
0.4 1
1
\
z
I
",o,
\.K'-
°.'~k k I ~t "...'-... 0
-
1
%\ 2
INCIDENT ENERGY (MeV) Fig. 4. A E in the T F D as a function o f incident energy for p r o t o n s (solid symbols) and alphas (open symbols) for various plastic scintillators. The solid line at the left represents the d E vs E for non-penetrating particles. Only the data points for penetrating particles are shown. The curves through the data points were added as a visual aid.
SCINTILLATORS
415
at the lower energies has been noted by Muga t) for NE102, and would seem to indicate the ionization quenching effect discussed by Birks6). It can be noted that the fwhm resolution is very poor for the thin TFDs and improves considerably for the thicker Pilot M TFD. It would seem that, even allowing for the thickness variations between the various films, the NE110 T F D gave a bigger response to protons and alphas than the other films. A word of caution is necessary, though. Although all conditions for testing the various films were made as identical as possible, some differences might occur which would be difficult to evaluate (e.g., quality of reflective coating on the different film holders, ageing of the plastic films, slight differences in the optical coupling, etc.). The non-linearity of the T F D response with energy loss (AE) can be seen by comparison with fig. 4, which presents the measured AE as a function of the incident energy for protons and alphas. The energy loss for the 394 #g/era 2 Pilot M T F D was not included because only a few points would fall within the limits of the ordinate. The data points shown for the various TFDs are for penetrating particles only. Because a limited number of data points in incident energy were taken, the shape of the curve near the point of juncture to the non-penetration curve and near the AE peak for alphas is not strictly defined. The lines shown through the points are only to make it easier for the observer to connect points. No comparison to calculated values was made. The heavy line to the left of the figure represents the values taken for non-penetrating particles. Note the fairly sharp drop-off in AE after penetration fol protons, while the AE for alpha particles continues to increase to a rounded peak at some higher value of incident energy before it begins to decrease. The thicker Pilot M T F D exhibits the shape of the proton curve for alpha particles also. The non-linearity of light output as a function of deposited energy is more obvious when presented as in fig. 5. This shows AL/AE, a measure of the efficiency of photon production in units of photoelectrons/MeV deposited, as a function of the incident-particle energy. The solid points are for protons and open symbols for alpha particles. A few sample error bars, indicating the fwhm spread in the T F D response, are shown. It can be seen, as pointed out by Mugal), that protons have a higher efficiency for useful photon production than alpha particles. Also, it appears that at the lower incident energies the value of AL/AE levels off to a constant value.
416
J.w.
Another presentation of the data is shown in fig. 6, which gives the specific luminescence (dL/dx) as a function of specific energy loss (dE/dx) for the various TFDs. Only the results for penetrating particles are shown. The data points of AL/Ax vs zlE/zlx shown here confirm the behavior suggested by Muga ~) that AL/Ax is double-valued for each zJE/Ax. It is apparent for the alpha-particle curves, but not for the proton curves which end at their maximum dE/dx value. The doublevaluedness of AL/Ax would become more obvious with the addition of the data for non-penetrating 160
1
I
INE 111 _~ 132 #gm/cm 2
NE 110 91 #gm/cm 2
140 120 c e 100
¥ 8 o
80 60
5£ 40 20
1
I .1
2
INCIDENT ENERGY (MeV)
INCIDENT ENERGY (MeV) 400
I 2
KOHL
particles, which would extend the curves toward the lower left of the figure. 4. Summary and conclusions Basically, the characteristics of the TFDs studied ( N E l l 0 , N E l l l , Pilot M, Pilot Y) were very similar. The curves of pulse-height distribution as a function of incident energy all had the same shape, depending on particle type. In all cases the response to protons was greater than alpha particles at low energy, with a reversal at the higher energies. All the TFDs exhibited a "saturation" effect at the higher values of specific energy loss. The Pilot MY, although a 50% mixture, seemed to behave more like the Pilot Y than Pilot M. The NE110 seemed to give a bigger light output for protons and alphas than the other TFD films tested, and might be the preferable plastic for heavy-particle detection, although any specific differences may be due more to things like age or optical coupling, etc., than to differences in the scintillator plastic used for the TFD. Due to the non-linearity of light output with AE,
500
I
PI LOT Y 71 .ugm/crn 2 400
MATERIAL
300
PI LOT Y PI LOT M PI LOT M, Y NE 110 NE 111
5~ 300
o ~d
20o 200
® - ----,0 b--=- - ~,. ~ o
51
91 132
10 o loo
SYMBOL PROiONS ALPHAS ~, ^
TIqlCKNESS 71 # cm/cm2 394
e I
I
I
.o
I
I
]
I
1
_ o
1
INCIDENT ENERGY (Me~
INCIDENT ENERGY (MeV) 2OO
1
i
PILOT M 394 .ugm/cm 2
180 160
~'~ :::L O
10-1
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-
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120
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INCIDENT ENERGY (MeV)
Fig. 5. P h o t o n production efficiency (AL/AE) as a function o f incident energy for protons (solid symbols) and alphas (open symbols) for various plastic scintillators. N o t e that the ordinate scales are different. A l s o s h o w n are sample ordinate error bars which represent the full-width at h a l f - m a x i m u m o f the pulseheight distribution. The curves t h r o u g h the data points were added as a visual aid.
10-2 10-4
I
I
l
!
I
10 3 AE/AX (Ik],_V,ugm-1 cm2)
I
I I 10-2
Fig. 6. Luminescence (dL/dx) as a function o f the specific energy loss (dE/dx) for penetrating protons (solid symbols) and alphas (open symbols) for various scintillators. The curves through the data points were added as a visual aid.
RESPONSE OF T H I N - F I L M S C I N T I L L A T O R S
the use o f a T F D as the t r a n s m i s s i o n d e t e c t o r in a AE, E-AE technique to identify low-energy nuclear species does n o t seem to be feasible. H o w e v e r , because even a very thin T F D will give a recognizable light o u t p u t (using a p p r o p r i a t e electronic techniques) for transiting particles, it w o u l d seem that it w o u l d be suitable as a large t r a n s m i s s i o n d e t e c t o r in a time-of-flight a r r a n gement for species identification, p a r t i c u l a r l y in lowc o u n t - r a t e applications. The a u t h o r wishes to t h a n k M r Stephen A. G a r y for his assistance with the electronics t r o u b l e - s h o o t i n g a n d for the m a n y invaluable discussions on timing techniques. Especially m a n y t h a n k s to Stephen K. B r o w n o f N A S A / G S F C for his assistance a n d patience while using the V a n de Graaff. M r R o b e r t B r u b a k e r also deserves t h a n k s for assisting with the l o a d i n g a n d
417
u n l o a d i n g o f e q u i p m e n t the m a n y times we needed a n o t h e r d a t a set f r o m the N A S A - G S F C accelerator. T h a n k s also to the m a n y p e o p l e w h o m a d e the thin films for this p r o g r a m .
References 1) M. L. Muga and G. Griffith, Nucl. Instr. and Meth. 109 (1973) 289.
2) p. Braun-Munzinger and C. K. Gelbke, Nucl. Instr. and Meth. 114 (1974) 141. 3) M. L. Muga, D. J. Burnsed, W. E. Steeger and H. E. Taylor, Nucl. Instr. and Meth. 83 (1970) 135. 4) C. K. Gelbke, K. D. Hildenbrand and R. Bock, Nucl. Instr. and Meth. 95 (1971) 397. 5) M. L. Muga, D. J. Burnsed and W. E. Steeger, Nucl. Instr. and Meth. 104 (1972) 605. ~) L. C. Northcliffe and R. F. Schilling, Nucl. Data A7 (1970). 7) j. B. Birks, The theory and practice of scintillation counting (Pergamon Press Inc., New York, 1964) p. 187.