Saturation effects of organic scintillators to nanosecond-pulses of low-energy x-rays

NUCLEAR

INSTRUMENTS

AND M E T H O D S

164 (1979) 4 9 1 - 4 9 9 ,

(~) N O R T H - H O L L A N D

PUBLISHING

CO

SATURATION EFFECTS OF ORGANIC SCINTILLATORS TO NANOSECOND-PULSES OF LOW-ENERGY X-RAYS* J.G. PRONKO, D KOHLER and L F CHASE Lockheed Palo Alto Research Laboratory, Palo Alto, Cahforma 94304, U.S A Received 19 March 1979 The nonhnear saturation effects of organic scintillators were investigated using low-energy X-rays from a laser produced plasma. The laser system consisted of a Nd glass faclhty operating at pulse widths between 2 and 6.5 ns at levels up to 15 J. The average energy of the X-ray spectrum was approximately 1.25 keV with a ( 1 - e -1) penetration depth of 10am. The organic scintdlators studied were NE102, N E l l l , and NEll'I doped with 296,596, and 10% benzophenone, 5% acetophenone, and 10% plpendme It was observed that for a 6 5 ns w~de pulse, the 10% nonlinearity point is at an irradlance of approximately 2 m J / c m 2 ns, while that for a 4 ns pulse is somewhat h~gher No saturation was observed for pulses < 2 ns wide at the upper levels of irradmnce ( ~ 5 rrd/cm 2 ns) achieved m this experiment

1. Introduction Organic scintillators are playing an ever increasing role in the detection of low-energy bursts of X-rays. Consequently, it is important that the response characteristics to low-energy X-rays be ascertained accurately. One of the properties which is of major importance in evaluating data taken at high intensities is that of nonlinear saturation effects. A study ~) of these effects was performed at the Lockheed Palo Alto Research Laboratory (LPARL) using a 200ps pulse of X-rays (Ex ~<2 keV). No nonlinear effects were observed in the response of NE102 and N E l l l at irradiances _<-_'15mJ/cm 2 ns. These measurements provide the only data available regarding nonlinear effects at high X-ray doses. Stevens and Knowlen 2) have conducted similar saturation studies using pulsed high-energy electrons with pulse widths > 6 ns. The point at which they observed nonlinearity was dependent not only on total dose but on dose rate as well. The extrapolation of their data to the 0.2 ns width used in the LPARL experiments, however, indicates an apparent inconsistency between the two sets of data. The results of Steven and Knowlen predict the occurrence of nonlinear saturation effects at irradiances two orders of magnitude lower than the 15 m J / cm 2 ns employed in the LPARL experiments. Other investigations 3,4) with electron-beam pulses > 6 ns support qualitatively the results of Stevens and Knowlen. The present experiment was therefore undertaken to uncover the source of the discrepancy between Supported by University of California (LLL) subcontract No P09451903, ERDA Prime W07405-ENG-48

the X-ray and electron-beam saturation measurements so that future X-ray diagnostic packages can be designed for optimum performance. The experiment involves measuring nonlinear effects on a number of organic scintillators at X-ray pulse widths equivalent to those used in the electron-beam measurements. The LPARL high-powered pulsed Nd:glass laser facility was configured into a long-pulse (<10 ns) mode for the present investigation. The preliminary procedures, such as configuring the laser cavity for long pulses, obtaining X-ray irradiance calibrations, and fabricating monitor systems are described in section 2. Section 3 describes the characterization of the X-ray spectrum used in the saturation studies through a series of spectrometer measurements. The actual performance of the saturation experiments using X-rays from a copper target is described in section 4. Measurements were performed, using a 6.5 ns wide X-ray pulse, on N E l l l and NE102 organic scintillators as well as on N E l l l doped with benzophenone, acetophenone, and piperidine. In addition, NE111 was investigated at X-ray pulse widths of 2 and 4 ns. A summary of the present study is given in section 5.

2. Preliminary experimental procedures Previous studies ~) conducted at LPARL on the saturation properties of organic scintillators to lowenergy X-rays were performed at a laser pulse width of 0.2 ns. To continue similar studies but at longer pulse widths (>__1 ns) it was necessary to reconfigure the laser cavity from mode locking to a giant pulse mode. A Pockel cell switching system

492

J G PRONKOet al

was set up to switch out prescribed segments of the main giant pulse using charged clipping cables. The temporal profile of the laser pulse was monitored by a photodiode that detected a small sample of the laser beam reflected from a pelhcle posiu o n e d j u s t before the entrance to the target chamber. This same detector monitored a sample of the unswitched portion of the laser oscdlator pulse. The latter pulse was delayed through the appropriate optical path such that both pulses could be displayed on a single oscilloscope trace. This system provided an online monitor of the laser pulses so that an optimization of the pulse switching system could be maintained as well as providing a record of the laser beam energy for each shot. The X-ray irradiance was monitored by two detectors positioned at 45 ° to the target on either side of the laser beam. One detector was an X-ray diode (XRD) the other was a scintillator photodiode (ITT FW114). For the N E l l 1 measurements, the scintillator on the monitor detector was N E l l l while, for the NEI02 sequence, the momtor scintillator was NE102. Both the XRD and scintillatorphotodiode monitors were calibrated against a "standard" tantalum foil calorimeter. During the

calibration measurements and actual saturation experiment, identical (12.7 or 25.4 ~zm) beryllium foils were used on all detectors. This ensured that all detector systems experienced the same incident low-energy spectrum. A typical laser output with a 7 ns clipping cable was approximately 10-15 J. For a copper target this translated into a 6.5 ns wide burst of X-rays at levels up to 10 mJ/sr; at a dtstance of 0.6 cm from the target this corresponds to an irradiance of 4.5 mJ/cm 2 ns. Using the appropriate clipping cable, similar irradiances were achieved for a 2 ns wide X-ray pulse. 3. Spectrometer measurements Intense bursts of X-rays can be generated from targets with a Z near 30; consequently, copper targets (Z = 29) were used in the present measurements. Unfortunately the X-rays emanating from a laser-induced copper plasma are not monoenergetic If one is to analyze the data in terms of dose rates, ~t is necessary to characterize the X-ray spectrum well enough so that an average penetratton depth for the spectrum can be determined. A spectrometer designed to fit inside the target chamber in the

X-RAY ENERGY (keV') 1.0

15

2.0

I

]

I

Cu TARGET 0.00254 cm Be FILTER

Z ~3

,Y. ©

5

Fig 1 Mtcrodensltometer analysis of typical spectrographic data taken with no screen film and a Cu target using curved-crystal (KAP) spectrometer Laser pulse characteristics are s~rndar to those used m collection of scintillator saturation data presented m th~s report

SATURATION

493

EFFECTS

I.C Cu TARGET 0.00254 cm Be FILTER

z, 0.5

] rr 1.0

0 0

I 1.5

I

|

5

10 DEPTH (~m)

/ 15

20

Fig 3 Plot of X-ray deposition depth in NE111 as function of total incident energy absorbed The curve was constructed from X-ray absorption integration of hlstographlc data of fig. 2

I 2.0

X-RAY ENERGY ~eV)

Fig 2 Histogram constructed from spectrum presented in fig 1 and corrected for geometric, film, and crystal efficlencles Relative intensity is proportional to units of photons/cm 2 keV Mean energy value of this distribution IS 1 25 keV close-coupled geometry was used with no screen film and a curved KAP crystal. A series of data runs were taken with 12.7 and 25.4 a m beryllium windows. Fig. 1 illustrates a densitometer trace of a typical spectrum taken with 25.4 ~tm beryllium foil. The low energy end is cut off by the beryllium foil while, on the high energy side, the drop off is due to the lack of radiation in this energy region. It is difficult to measure the spectrum above 2 keV with this spectrometer arrangement; consequently, a foil transmission experiment was performed using two calorimeters and various thicknesses of beryllium tbil on one o f the calorimeters. The results indicated that not more than 5%o of the total emitted X-ray energy lies above 2 keV. For the analysis of the spectral data, 12 energy bins were selected and an energy histogram was c,onstructed. This histogram is illustrated in fig. 2 and contains corrections for film and crystal effic,iency as well as all geometrical factors. The mean of this frequency distribution is at 1.25 keV. Using known cross-sectional data, a sum of energy deposition as a function of deposition depth was performed for the 12 bins in the histogram. The results of this investigation are illustrated in fig. 3;

as shown, 63% (1--e -1) of the energy contained in the spectrum of fig. 1 is deposited in the first 10/~m of the scintillator. In addition, the total energy contained in the spectrum (after efficiency corrections) agrees very well with the monitor data taken for that shot. This again is an indication that very little energy is contained in the spectrum above 2 keV. Nonetheless, to test how sensitive the average deposition depth might be to high-energy X-rays, a calculation was performed where a 13th bin was added and covered the region o f 2 - 8 keV. It was found that to increase the deposition depth by a factor o f 2, it is necessary to assume that 30°/6 of the total energy lies in this bin. As pointed out, such a percentage is not consistent with present measurements. Spectrograph data were also taken using 12.7 ~zm beryllium foil. The average energy of a photon in this case was found to be 1.22 keV.

4. Saturation experiments The saturation properties of NE111 were studied at X-ray pulse widths of 2, 4. and 6.5 ns. Those o f doped NE111 and NE102 were only investigated at 6.5 ns. In principle, the experiment involved the monitoring of changes in scintillator output with respect to systematic changes in X-ray irradiance. The scintillator samples were disks 0.32 cm thick and 4.13 cm in diameter, which were coupled to an ITT F W l l 4 photodiode through a light pipe and optional neutral density filter (OD1). The neutral

494

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P R O N K O e t al

density filter was used with the undoped scintillator, otherwise the scintillator light output might, at the highest irradiances, saturate the phototube through space-charge limitation. The purpose of the light pipe was to bring the scintillator (via a special holder through the chamber lid) closer to the target than was previously possibleS). Four factors determined how close to the target the scintillator could be placed: (1) blowoff damage to the beryllium foil, (2) partial occlusion of monitor detectors, (3) introduction of X-ray scattering surfaces that made it difficult to obtain reliable monitor data, and (4) interference with the incoming converging laser pulse. It was found that, at a target-spot-to-scintillator distance of 6 ram, any effects due to the above would be avoidable or at a negligible level. However, at distances so close to the target spot the majority of the X-rays would pass obliquely through the beryllium foil covering the scintillator and thus suffer a loss of intensity due to effective increase in foil thickness. To prevent this, a half angle of < 18° was ensured by fabricating a series of brass collimators (aperture diameters from 0.254 to 1.27 cm) any of which could be placed over the beryllium foil. The laser pulse energy was held at approximately a constant level for these measurements so that no changes in the spectral characteristics (described in section 3) would take place. The normal to the target was at 45 ° to the plane formed by the beam and monitor detectors. The scintillator-diode package was positioned above the target at 45 ° to the DIODE " ~

FAST OSCILLOSCOPE

S.IELOAND-~ ~ G H T COLLIMAr'OR ~ ¼ J

PIPE

_-i 'Nr'LLATOR "-'~1"~

SCINTILLATOR I PHOTO-DIODEMONITOR~FAST ~ (~MATORS / ~ j OSC,LLOSCOPE

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TAR E,

X,RD / MONITOR

~

~

ASERL,O r

""1 OSCILLOSCOPE l

F~g 4 Schematic representation of experimental arrangement used in the present experiment

normal (see fig. 4 for a schematic illustration). The diode package was positioned in a well in the chamber lid so that the scintillator and light pipe could be passed through the base of the well and form a vacuum seal. As was pointed out earlier, the closest scintillator-to-target distance was 6 ram. The amount of energy deposited on the surface of the scintdlator could be reduced by positioning the diode package at the end of evacuated pipes of different lengths positioned in the well. A series of different length pipes was available, the longest of which placed the surface of the scintillator at a distance of 148 cm from the target spot. To eliminate the possibility of X-rays being reflected into the scintillator off the walls of the pipes it was necessary to place apertures of the appropriate diameter in the four longest pipes. Using the system described above and assuming the irradiance varied inversely with the square of the distance from the target the scintillator could be exposed to levels of irradiance covering approximately 5 orders of magnitude. A series of data runs was taken at X-ray pulse widths of 2 and 6.5 ns for the NE111 scintillator sample. The scintillator used had been purchased from the manufacturer (Nuclear Enterprises) more than a year ago. In order to test the possibility of a surface dead layer having formed during that period, 6.5 ns wide data were first taken and then the sample was returned to the manufacturer and a 51 Ftm layer was removed from the surface. The sample was immediately placed in vacuum and returned to the laboratory whereupon saturation measurements were repeated at 6.5 ns and in addition, data were taken at 2ns. For some of the above measurements, data were collected using a 12.7~m instead of the 25.4/zm beryllium foil. From the onset of the experimentation, there appeared to be no noticeable difference between the data taken with the original and the repolished sample. Although a number of runs were made with different foil thicknesses and pulse widths, only the cases for which a continuous set of data points, covering the nearly five orders of magnitude in irradiance, will be presented for the two X-ray pulse widths. This range was determined, at the high end, by the maximum possible irradiance achievable at a target-spot-to-scintillator distance of 6 mm and, at the low end, by the maximum pipe distance that could be used before the signal was lost in the background noise.

SATURATION

Detector pulses from the XRD monitor were displayed on a 519 Tektronix oscilloscope and were recorded photographically using Polaroid 410 film. The signals from the sample and monitor photodiodes were recorded in a similar fashion but using 7904 Tektronix oscilloscopes. The photographic data were then digitized using a TI 980A computer system and two-dimension X - Y table. Using the appropriate computer program, areas under the digitized waveform could be obtained for the formal analysis. It was necessary to measure the target-spot-toscintillator distance quite precisely since a small error at such close distances could be quite serious. With the chamber and detector system under vacuum, the target-spot-to-scintillator distance for 108 NEll1

(a)/,

495

EFFECTS

the closest gap was measured by viewing the gap through a glass chamber port with a telescope having a calibrated prism translator. Since the NE111 detector system has a time resolution < 1 ns, it is not necessary to obtain data experimentally at different X-ray pulse widths in order to make an analytical comparison of nonlinear yields as a function of pulse width. The same analysis can be performed by taking data at a long pulse width (e.g. 6.5 ns) and then integrating over specific temporal portions of the recorded pulse. For the present analysis, the 6.5 ns pulse waveforms were integrated over the first 2 ns, first 4 ns and entire 6.5 ns. In each case, the time reference was the half height point on the leading edge. Fig. 5 illustrates plots of waveform areas over each of these time regions as a function of X-ray irradiance. The error bars on each data point were estimated to be approximately 10%. The solid lines through the data points are a least squares fit to the function

107

3

(1)

y = ~ akx k. k=O

The test for saturation was based on the search for evidence of a nonlinear relationship between yield and X-ray irradiance. Table 1 lists the resulting expansion coefficients from the least-squares fit to the data.

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,

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10-2 10-1 2 100 X-RAY IRRADIANCE (mJ/cm -nl)

t

t

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Fig 5. Plot of relative scintillator-diode y~eld vs X-ray irradmnce tor a given laser shot These data are (a) for a 6 5 ns wide X-ray pulse, (b) for the first 4 ns of the 6.5 ns X-ray pulse, (c) for the tirst 2 ns o f the 6.5 ns X-ray pulse. Solid lines through data points are best quadraUc fits to data while a dashed hne represents the best linear fit. Experimental error on each datum point is approximately 1096. The vertical axis is m arbitrary units and the mdwidual curves are not normalized to each other.

100 '-*1 -"10-4

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l l llllll

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10-2 10-1 X-RAY IRRADIANCE (mJ/cm2-ns)

100

Fig. 6. Plot of relative scintillator-diode yield, obtained from area o f pulse vs X-ray lrradlance for given laser shot Data are for 2 ns wide X-ray pulse. Solid line through data points is best quadratic fit to data. Experimental error on each data point ~s approximately 10%.

496

J

G

P R O N K O et al

TABLE 1 Results of scintillator saturation data analyses using X-rays from C u target and NE111 scintillator

Pulse width a (ns)

a b c d

a0

aI

a2

a3

Z2

2b

0 11 _+0 06 0 09 _+0 06 0 11 _+0 06

11 088_+159 11 192_+176 11 078_+ 186

d --70-----130 590--+490

d d 191_+ 119

0 82 0 81 0 79

4c

008 _+0.10 0.0 _+008 0 01 _+0 09

22 162_+258 22641-+240 22 690-+265

d -728-+172 -- 1031 -+644

d d 77_+ 157

067 055 0 55

65

0 456_+0 224 0 261 _+0.167 0 255_+0 175

38 329_+566 39 502_+470 39 536_+527

d - 1651 -+322 - 1839_+ 1242

d d 47+298

0 84 0 61 0 63

2

020 _+0 11 0 16 _+0 11 0 17 _+0 11

11382_+330 11 738_+372 11 6 2 2 _ + 4 4 0

o d -658_+1262

1 53 1 40 1 43

The values of the data points were based on pulse The values of the data points were taken from the The values of the data points were taken from the For this case the a k coefficient was assumed to be

d -940_+567 +116_+2106

area first 2 ns of the 6 5 ns data first 4 ns of the 6 5 ns data zero

I n the case of the 2 n s data, the quality of the fit (as indicated by the v a l u e o f ;(2) does n o t i m p r o v e w h e n t e r m s o f order higher t h a n k = 1 are included. H o w e v e r , in the case o f the 4 n s a n d 6.5 ns data, the fit q u a l i t y is i m p r o v e d slightly by the i n c l u s i o n o f higher-order terms. T h i s is particularly true o f the 6.5 ns data a n d is v i v i d l y d e m o n s t r a t e d in fig. 5 at the highest irradiances. T h e d a s h e d line is a c o n t i n u a t i o n o f the linear fit. I n the case o f the 4 ns data this d e v i a t i o n , a l t h o u g h there, is not as pron o u n c e d as in the case o f the 6.5 ns data. T h e implication is that s a t u r a t i o n is occurring at the highest irradiances for the 6.5 ns wide pulse b u t not as significantly as for the shorter pulses. A s a m e a n s o f further verifying this n o t i o n , the e x p e r i m e n t was repeated u s i n g a 2 ns wide burst of X-rays. T h e results are s h o w n in fig. 6 a n d table 1. A l t h o u g h the fit is n o t as good as the former case, there is n o e v i d e n c e for any serious nonlinearities. A further e v a l u a t i o n o f this s i t u a t i o n can be o b t a i n e d f r o m fig. 7. T h i s figure c o m p a r e s the m o m t o r a n d signal pulse for three levels o f X-ray irradiance. T h e solid c u r v e is the m o n i t o r pulse. while the d a s h e d c u r v e is that of the signal pulse. To m a k e this qualitative c o m p a r i s o n , the pulses were n o r m a l i z e d to each other by a visual overlay Ftg 7 Comparison of shape of momtor and signal scmtdlatorphoto-diode pulses for three levels of X-ray irradiance Sohd curve is momtor pulse while dashed curve is signal pulse Pulses were normahzed to each other by wsual comparison of leading edges Each set of pulses for a given ~rradmnce are plotted on a relauve vertical scale and are not normahzed to each other

NE[II

m D

Z Z i-

5

0

5

10

15 TIME ( qs )

20

SATURATION

497

EFFECTS

']'ABLE 2 Results of scmtdlator saturation data analyses using 6 5 ns wide pulse o f X-rays from Cu target Sample c

a0

aI

1 . 0 _ 0 36 0.8___0 34 0 8_+0 36

12 009_+243 12 396_+282 12 373-+333

b -715_+326 - 5 6 9 _ + 1068

0 3+0 2 0 2_+0 18 0.2_+0.2

5 243_+201 5 408_+229 5 412±290

(10B)

0 7_+ 1.0 0 2-+0.6 0 1_+0.6

NE111 (5A)

NElll (2B) NElll (5B) NElll

NElll (10P) NE102

a2

1 05 0 95 0 98

b - 2 2 5 _ + 177 -239_+701

b b 4 0_+ 195

1.98 1 86 1 92

2 937_+ 169 3 187_+122 3 217_+ 148

b -305_+86 -420_+318

b b 32_+86

2 77 1 64 1 68

- 17.9_+9 4 - 2 . 1 __,6 0 5.7_+5 4

3 281 _+ 135 2 850_+ 117 2 615-+ 122

b 235_+51 656_+ 147

b b - 101 + 3 4

1 58 0 83 0 66

- 1 6 2_+6.9 - 8 4_+5 1 - 4 0--+5 0

1 672_+75 1 516_+69 1 417-+77

b 82-+25 236_+80

12 559--+359 12 669-+476 11 905-----471

b -- 192-+530 3364_+1372

- 2 2_+1 1 --2 4_+ 1.2 --1 4-+1 0

106

~ 104

lOb) 103 I

~- 102

i

, JL 10-3

t

t

,,I I0 -2

I

t~LJI I0-I

t

X-RAY IRRADIANCE (mJ/cm2-n$)

,

,,I

, 100

,

b b - 2 7 - + 14 b b --1560_+573

1 82 1 19 1 04

1 48 1 52 1 21

B - benzophenone, A - acetophenone, P -

of the leading edges. The waveforms at 6.35 mJ/ cm 2 ns were obtained with the scintillator as close as 4 ram. However, as pointed out earlier, data collected that close to the target could not be included in the nonlinearity analysis because of normalization difficulties. At the lowest level of irradiance illustrated in fig. 7, the monitor and signal pulse have similar shapes. For intermediate levels, the trailing edge of the pulse suffers from what appears to be saturation. At the highest level of irradiance, it appears that a major portion of the pulse undergoes some form of saturation. For the case of NE102 and the doped N E l l l samples, data were collected only at an X-ray pulse

mJ

105

I

Z2

b b -74-+516

a T h e values o f the data points were based on pulse area b For this case the a k coeffioent was a s s u m e d to be zero. c T h e notation refers to the percentage of concentration and doping material plpendlne.

1001 10 .4

a3

J, 101

Fig 8 Plot of relative scmtdlator-dlode yield, obtained from pulse area vs X-ray irradmnce for given laser shot for NE111 doped w~th benozphenone Data are for 6.5 ns wtde X-ray pulse Sohd lines through data points are best quadratic fits to data whde a dashed h n e represents the best hnear fit Experimental error on each d a t u m point is approximately 10% T h e vemcal ax~s ~s m arbitrary units and the individual curves are not normalized to each other The notation m parentheses represent the percentage of dopant and type

498

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P R O N K O e t al

105

NEll1 ( 1 / :~1o4 E

NE102

o c~

3

)

0 I

~102 z

= ~ 101

100.4

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I

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I

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i,I

. . . .

i0-2

I

,

i0-)

I

1 , i

. . . .

i0O

X-RAYIRRADIANCE(mJ/cm2-ns) Fig 9. Plot of relatwe scmtdlator-dtode yield, obtained from pulse area, vs X-ray irradiance for a gwen laser shot Data are for 6.5 ns w~de X-ray pulse. Sohd lines through data points are best quadratic fits to data Expenmental error on each datum point ~s approximately 10% The vertical axis is in arbitrary units and the indtvidual curves are not normalized to each other The notation m parentheses represents the percentage of dopant and type

I

NEll| (10BI

100

width of 6.5 ns. The doped samples consisted of 2%, 5 % and 10% benzophenone; 5 % acetophenone; and 10% piperidene. These were all similar in shape and size to the undoped N E l l 1 described earlier in this section. It was known from the studies of N E l l l that it would take the highest available irradiance to cause saturation; consequently, data were collected mainly at the high end of the available five orders-of-magnitude range in irradiance. Figs. 8 and 9 illustrate the data collected for this portion of the experiment. The error bars on each data point were estimated to be approximately 10%. The solid lines through the data points are a least-squares fit to the function of eq. (1). Table 2 lists the resulting expansion coefficients from the least-squares fit to the data for NE102 and doped N E l l l samples. As is evident from these figures and table 2, there is no evidence of nonlinearity except at irradiances above 1 mJ/cm 2 ns. This is most evident in the data shown in fig. 8 where the highest irradiance achieved was approximately 4 mJ/cm 2 ns. The dashed line represents the best linear fit to the data. Since the monitor scintillator was N E l l l and not identical to the sample under investigation, a direct comparison between the line shapes of the monitor and signal FPD could not be made. However, a comparison between signal data taken at low and high irradiances can be made. Fig. 10 illustrates scintillator data taken at irradiances of 0.5 and 4.0 mJ/cm 2 ns. The XRD monitor associated with these two laser shots indicated that the temporal profiles of the X-ray pulse were nearly identical. As illustrated in fig. 10, there is obvious distortion in the trailing edge of the pulse taken at the higher irradiance.

5. Summary

0

5

TIME(ns)

10

F~g 10 Shapes of two s~gnal scmtdlator-photodiode pulses taken at different irradlances. Scintdlator is N E l l l doped with 10% benzophenone. Pulse heights are on a relative scale and not normahzed to each other

The previous section discussed a test that was performed over an extensive range of irradiance (approximately five orders of magnitude) to see to what extent the scintillators responded linearly with X-ray input. At the highest levels of irradiance achievable with the present experimental configuration, but only for pulses in the region of 4.0 ns or wider, a degree of nonlinear response was observed for some of the scintillators. The data suggest that at levels below 1 mJ/cm 2 ns it is safe to assume that for pulses 6.5 ns and shorter there is no significant level of nonlinearity to X-rays with the characteristic spectrum described in section 3. For the cases of undoped NE111 and NE111 doped with 10% benzophenone, experimental conditions were

SATURATION EFFECTS

such that it was possible to accumulate data stgnificantly above the 1 mJ/cm 2 ns level of irradiance. It is with these cases that a nonlinear response was observed. It is assumed that the same nonlinearity would have also been noticed had experimental conditions been such that equivalent levels of irractiance could have been achieved at the time the other samples were studied. On the basis of waveforms such as shown in fig. 7, and from the data in fig. 5, it would appear (in the case of undoped N E l l l ) t h a t a 10% nonlinearity level would be at an irradiance of approximately 2 mJ/cm 2 ns, while that for a 4 ns pulse is somewhat higher. The equivalent point for a 2 ns pulse lies higher than the irradiance achieved in the present experiment. For N E l l l doped with 10% benzophenone, it was found from fig. 8 that the 10% nonlinearity level is also at an irradiance level of approximately 2 mJ/cm 2 ns for a 6.5 ns wide pulse. It is assumed from the shape of the waveforms presented in fig. 10 that the level of irradiance at which nonlinearity occurs would increase systematically with a decrease in pulse width as was the case of undoped NE111. It is unfortunate that it was not possible to achieve equivalent maximum levels of irradiances at the time the other scintillator samples were studied. Nevertheless, it is assumed that a similar non-linear behavior would have been observed. Assuming an average X-ray penetration depth of 10/~m, the surface thermal loading and subsequent temperature rise can be calculated for a given value of irradiance. The melting point (75 °C) of NE111 can be reached at an irradiance of ~2.7 m J / c m 2 ns (assuming a specific heat of 0.33J/g°C). This corresponds to a level of irradiance at which a degree of non-linearity was observed. Such a temperature rise may be responsible for a form of transaent or permanent surface alteration and be the limiting factor in using scintillators in high-fluence X-ray environments. There have been studies 2-4) of the nonlinear response of the organic scintillator NE102 using high-energy electrons. These data suggest that the point at which nonlinearity occurs is dependent not only on total dose but dose rate as well. The effects

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observed in the present experiment also suggest that this dose-rate dependence does indeed exist. When converting the irradiance from the present experiment, at which 10% nonlinearity occurs, into a corresponding dose rate, one obtains a value of 2 × 10 TMRad/s. This calculation assumes a 10/lm ( 1 - e - t ) deposition depth as described in section 3. This dose rate is approximately two orders of magnitude greater than the equivalent level obtained from the electron data. However, it is consistent with what one would expect for a 6.5 ns pulse, based on previous LPARL X-ray saturation data ~) taken at 0.2 ns. The apparent discrepancy between the X-ray and electron data still exists. However, assuming that the nonlinearity effects are dose-rate dependent, the results of the earlier LPARL experiment t) and the present experiment are consistent. One possible explanation is that there is a significant deadlayer on the surface of the scintillator. As pointed out earlier, this possibility was tested by performing saturation tests before and after a 51/~m layer was removed from the surface. No change had been observed in the linearity over the region tested. Should there indeed be other surface effects not taken into account, they are probably the result of intrinsic properties of the scintillator. If this be the case, the measurements presented in this report empirically set the level of nonlinearity for scintillators being used under similar spectral and irradiance conditions. The authors would like to acknowledge the advice and help offered by P.J. Ebert, F. Roeske, W.C. Jordan and G.N. Biren in bringing this investigation to completion.

References 1) j G Pronko and L. F Chase, Nucl. Instr and Meth 164 (1979) 79 2) j Stevens and R. B Knowlen, IEEE Trans Nucl Sci. NS-15 (1968) 136 3) R C Powell and L.A. Harrah, J. of Chem. Phys. 55 (1971) 1878 4) A F Lauzon and M D. Panaro, UOPAD64-15 (1964) unpublished