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NUCLEAR INSTRUMENTS AND METHODS 65 (1968) 8-25; © NORTH-HOLLAND P U B L I S H I N G CO.
CALIBRATION OF A N ORGANIC SCINTILLATOR FOR N E U T R O N SPECTROMETRY*
V. V. VERBINSKIt, W. R. BURRUS +, T. A. LOVE, W. ZOBEL and N. W. HILL
Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.
R. TEXTOR
Computing Technology Center, Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Tennessee, U.S.A.
Received 13 May 1968
The absolute differential efficiency of a 4.60- x 4.65-cm-dia. liquid organic scintillator, NE-213, was determined for nearly mono- energetic neutrons at 20 energies between 0.2 and 22 MeV incident on the curved side of the detector. These calibrations are shown to apply to an NE-211 scintillator as well. A 5-MeV Van de Graaff generator provided 2-nsec pulses of neutrons by means of T(p,n)aHe, D(d,n)~He (gas target), and T(d,n)4He reactions. With the aid of time-of-flight techniques and pulse-shape discri- mination to eliminate spurious neutron and gamma-ray events,
reliable pulse-height spectra were obtained for monoenergefic neutrons. The spectra were normalized to Monte Carlo calcula- tions of absolute differential efficiency by utilizing the proton- recoil plateau. The accuracy of the Monte Carlo calculation was verified in the region of the proton-recoil plateau by absolute experimental calibrations carried out at neutron energies of 2.66 and 14.43 MeV and by using a scintillator geometry more suitable for such tests.
1. Introduction
The present calibration combines the best features of a series of measurements and of Monte Carlo 1) calcu- lations of pulse-height spectra for monoenergetic neu- t rons incident on an organic liquid scintillator. The calculations provide accurate differential efficiencies at large pulse heights, where hydrogen collisions are pre- dominan t and where neutron reactions with carbon nuclei do not contribute directly. Consequently, nor- malization of the measured pulse-height spectra to calculations at the upper end of each spectrum converts the accurately measured spectral shapes to absolute differential efficiencies tha t are more accurate than those that could be obtained by measurement or calcu- lation alone. I t would be desirable, however, to perfect the calculations to produce results with comparable reliability, so that calculations could easily be made for other scintillator geometries. The calibrations presented here should prove useful in improving the input data for existing Monte Carlo response calculations, par- ticularly cross sections of various neut ron reactions with carbon, kinetic energy and angular distributions for each possible reaction product , and the light yields as a function of kinetic energy.
The measurements were made with the use of a pulse-shape discriminator circuit to reject gamma-ray events, and the calculations correspondingly ignored gamma-ray product ion following neutron reactions. The pulse-shape discrimination circuit, which is de- scribed in more detail in a related paper2), is a variant o f one o f For te ' s circuits3). When used with high-gain photomult ipl ier tubes operated with the proper amount o f saturation at the anode, it worked well over a pulse-
height range of 150:1 without correction and of 400:1 with correction for efficiency rolloff at low pulse heights. The correction functions were obtained with the as- sociated-particle method described in section 5, a method that is generally useful in evaluating pulse- shape discrimination circuits.
The subject matter of the remaining sections is as follows: section 2: the choice of scintillators, the proper- ties, and the optical coupling; section 3: the procedure for preparing the Monte Carlo calculations for direct compar ison to the measurements; section 4: the ac- curacy and limitations of the calculations and some support ing experimental checks of the calculations; section 5: the experimental arrangement and apparatus for calibrating the detector; section 6: the methods of gain checks and data taking; section 7: the data- handling and the final results; and section 8: several methods of utilizing these calibrations for neutron spectroscopy.
2. The scintillators
A small, 4.60- by 4.65-cm-dia., cylinder of NE-213 scintillator was chosen because it offers a good com- promise between efficiency and resolutions. NE-213 has an enhanced emission of delayed light which gives it
* Research jointly sponsored by the National Aeronautics and Space Administration and the Defense Atomic Support Agency and performed under subcontract for the Oak Ridge National Laboratory operated by Union Carbide Corporation under contract with the U. S. Atomic Energy Commission. Present address: Gulf General Atomic, P.O. Box 608, San Diego, California.
+ Present address: Tennecomp, Inc., P.O. :Box J, Oak Ridge, Telmessee.
CALIBRATION OF AN ORGANIC SCINTILLATOR 9
good pulse-shape-discrimination capabilities. Since NE-213 is noncrystalline, it is isotropic in response to neutrons and is not sensitive to mechanical or thermal shock. Carbon recoils and alpha particles from 14-MeV neutrons on NE-213 and its counterpart NE-211 pro- duce only about half the pulse height that is produced with stilbene scintillators, when normalizing to the largest hydrogen-recoil pulses. The pulses from carbon reactions can therefore be more easily biased out in NE-211 and NE-213 when desired.
The NE-213 scintillator* used for this work is made with xylene, activators, and POPOP as a wave shifter. Naphthalene is added to enhance the slow component of light emission. This is important to achieving a good pulse-shape-discrimination capability2'3). The scintil- lator was glass-encapsulated and bubbled with pure nitrogen to remove the undesirable oxygen which se- lectively quenches the slow component of light emis- sion. A shiny aluminum foil reflector and a 0.63-cm- thick plastic light pipe reduced the variation in response over most of the length of the scintillator to roughly 3%, or ½ of that with white paint and without light pipe, as determined with a narrow beam of 6°Co gamma rays. The greatest improvement was realized near the photocathode. These improvements apply only to our scintillator geometry, and were found to actually de- grade the performance of an 8-cm-dia. spherical scin- tillator with flat bottom.
3. Method of absolute calibration
With the experimental and data-handling techniques outlined in sections 5 and 6, we obtained 20 pulse- height distributions for nearly monoenergetic neutrons between 0.2 and 22 MeV. Assuming that the half height of the abrupt edge near the maximum pulse height corresponds to the maximum hydrogen recoil energy, a trial function, Lp(E), of proton light vs energy was constructed. Next, a preliminary series of Monte Carlo calculations was carried out at the 20 values of E, with the experimental spread of neutron energy factored in. The calculated pulse-height distributions were con- voluted with a Gaussian smearing function and inte- grated over the width of each pulse-height bin, so that they matched the measured pulse-height distributions. In this matching process the errors in the Lv(E) func- tion were determined, the function was corrected, and the Monte Carlo calculation was repeated with the correct Lp(E). Values of Lp(E) are given in table 1, with an estimated accuracy of 2% from 0.3 to 22 MeV. Values for similar functions for alpha particles and carbon-recoil ions, L~(E) and Lc(E), are also given in table 1. They are known much less accurately than
TABLE 1 Light output in pulse-height units. One unit is defined by 1.28- MeV 22Na gamma-ray Comptun edge presented in table 3. See also top of fig. 3. Estimated errors are 2% for protons from 0.3 to 20 MeV, 10% for alpha particles from 0.5 to 9 MeV, and
25 ~ for carbon ions from 0.5 to 4 MeV.
Energy Lp(E), Le(E), Le(E), (MeV) protons carbon alpha particles
0.1000 0.006710 0 . 0 0 1 0 3 8 0.001640 0.1300 0.008860 0 . 0 0 1 2 7 0 0.002090 0.1700 0.012070 0 . 0 0 1 5 7 3 0.002720 0.2000 0.014650 0 . 0 0 1 7 8 8 0.003200 0.2400 0.018380 0 . 0 0 2 0 7 6 0.003860 0.3000 0.024600 0 . 0 0 2 5 0 6 0.004900 0.3400 0.029000 0 .002793 0.005640 0.4000 0.036500 0 .003191 0.006750 0.4800 0.048300 0 .003676 0.008300 0.6000 0.067800 0 .004361 0.010800 0.7200 0.091000 0 .005023 0.013500 0.8400 0.117500 0 . 0 0 5 6 8 6 0.016560 1.000 0.156200 0 . 0 0 6 5 6 9 0.021000 1.300 0.238500 0 . 0 0 8 1 2 8 0.030200 1.700 0.366000 0 . 0 1 0 1 5 7 0.044100 2.000 0.472500 0 . 0 1 1 6 4 7 0.056200 2.400 0.625000 0 . 0 1 3 6 3 4 0.075000 3.000 0.866000 0 .016615 0.110000 3.400 1.042000 0 .018713 0.136500 4.000 1.327000 0 .021859 0.181500 4.800 1.718000 0 . 0 2 6 0 5 4 0.255500 6.000 2.310000 0 .032347 0.407000 7.200 2.950000 0 . 0 3 8 7 5 0 0.607000 8.400 3.620000 0 . 0 4 5 1 5 4 0.870000
10.00 4.550000 0 . 0 5 3 9 8 6 1.320000 13.00 6.360000 0 .071346 2.350000 17.00 8.830000 0 . 0 9 8 8 0 8 4.030000 20.00 10.800000 0 . 1 2 1 4 4 0 5.440000 24.00 13.500000 0 . 1 5 3 4 5 6 7.410000 30.00 17.700000 0 .206448 10.420000 3 4 . 0 0 2 0 . 5 0 0 0 0 0 0 .246192 12.440000 40.000 2 4 . 8 0 0 0 0 0 0 .312432 15.500000
Lp(E), but their accuracy has no direct bearing on the calibrations presented here.
After fitting the experimental data, the Monte Carlo calculations and Gaussian smearing were finally carried out at the mid-band energy of each "monoenergetic" neutron group and the experimental pulse-height spec- tra were normalized to the calculated curves in the proton-recoil plateau, the experimental data thereby being converted to absolute differential efficiency.
4. The Monte Carlo calculations and absolute checks
An accurate calculation of pulse-height distributions for neutrons up to 22 MeV incident on an organic scintillator is not possible without a complete know-
* Manufactured by Nuclear Enterprises, Ltd., Winnipeg, Canada.
10 V. V. VERBINSKI et al.
TABLE 2 Computed total efficiency and total cross sections for Monte
Carlo calculation and for analytic calculation.
Neutron Number Monte o't(1H) o't(P2C) Analytic energy of Carlo (b) (b) efficiency (MeV) neutrons total
detected efficiency
0.2015 10,000 0.8727 9.523 4.260 0.8728 0.3220 10,000 0.8212 7.719 3.857 0.8292 0.4265 10,000 0.7974 6.712 3.583 0.7958 0.6170 10,000 0.7353 5.532 3.174 0.7436 0.9035 10,000 0.6770 4.515 2.720 0.6816 1.205 20,000 0.6280 3.902 2.345 0.6309 1.613 20,000 0.5692 3.287 1.965 0.5706 2.234 40,000 0.5040 2.737 1.606 0.5058 3.329 40 ,000 0.4571 2.183 1.760 0.4690 4.236 40,000 0.4485 1.827 1.943 0.4502 4.919 40 ,000 0.3688 1.646 1.198 0.3704 6.017 40 ,000 0.3343 1.422 1.094 0.3358 7.038 40 ,000 0.2999 1.246 1.023 0.3082 8.029 40 ,000 0.3358 1.137 1.466 0.3380
10.00 40 ,000 0.2802 0.940 1.140 0.2825 11.98 40 ,000 0.2744 0.801 1.259 0.2773 14.01 40,000 0.2725 0.693 1.360 0.2741 16.06 40 ,000 0.2664 0.605 1.431 0.2703 17.81 40 ,000 0.2628 0.541 1.448 0.2640 19.91 40,000 0.2569 0.483 1.450 0.2566 21.81 4 0 , 0 0 0 0.2482 0.442 1.445 0.2508
ledge of the various neutron reactions with carbon. This includes light production functions and the correct "smearing" for alpha particles and carbon recoils, the cross sections, branching ratios to various levels of the excited residual nucleus, and angular distributions of the reaction products. Since, however, these reactions produce pulses much smaller than the largest proton- recoil pulses, use of the correct nonelastic cross section (the sum of all the cross sections for reactions which deplete the proton-recoil plateau) with the Monte Carlo code 1) accurately predicts the absolute differential ef- ficiency for the proton-recoil plateau. The absolute dif- ferential efficiency at neutron energy En as applied to the calibrations presented below is defined as the curve of counts per unit pulse height, normalized to 1 neutron incident on the cross-sectional area of the scintillator.
Each Monte Carlo calculation was checked with an analytic calculation in which the total neutron-removal probability [ 1 - e x p ( - 2 ; t T ) ] was averaged over all pathlengths T through the scintillator. The calculated Monte Carlo total (zero bias) efficiencies are compared with the analytic calculations in table 2. The total hy- drogen and carbon cross sections upon which these efficiencies depend are also given. The variation be- tween the Monte Carlo and the analytic results is due
to the finite number of histories in the Monte Carlo calculation.
The calculations performed for the 0.2- to 22-MeV calibrations reported here took into account proton- recoil leakage from the scintillator and nonisotropy (in the center-of-mass coordinates) of the recoil-proton angular distribution above 12 MeV. For these calcu- lations the density of NE-213 was taken as 0.867 g/cm 3 and the composition as CHI.zl as given in the manu- facturer's specifications. The hydrogen density is known to better than 1%, and the hydrogen cross section 4) to 1% from 0.2 to 22 MeV.
The estimates of the various cross sections for neu- tron interactions with ~2C and the method of carrying out the Monte Carlo calculations are described else- where'). The accuracy of these calculations in the pro- ton-recoil plateau region is indicated by some absolute experimental calibrations presented inS).
The results of the absolute measurements were com- pared 5) with Monte Carlo calculations for the same scintillators and the same neutron energies, and the agreement was better than 5~o in the proton-recoil plateau when the proton-recoil edges were carefully matched. These early Monte Carlo calculations as- sumed isotropic recoil-proton scattering above 12 MeV. The calculations have recently been repeated with the inclusion of the non-isotropic angular distribution 6) at 14 MeV, and the inferred agreement in the upper end of the proton-recoil plateau was improved to about 2½%.
Some additional associated-particle runs were made that were relative rather than absolute7). They were useful in determining the pulse-shape discriminator losses at low pulse heights, as described below, and for obtaining points on the L c and L= light production curves given in table 1.
An additional associated-particle run made with a 5-cm NE-211 scintillator and 14.4-MeV neutrons gave the same result as with NE-213 over a 700:1 pulse- height range, with each normalized to the same 6°Co and 1~3Sn gamma-ray pulse-height spectrum. This de- monstrated that the responses of the two scintillators are identical to within an estimated 3% matching ac- curacy for neutrons when the light pulses are integrated by a 1.5-#sec double-delayqine amplifier. The results of the two high-gain runs are shown in fig. 1, along with the 113Sn calibration spectrum.
5. Experimental arrangements for 0.2- to 22 -Meg calibration The Oak Ridge National Laboratory 5-MV Van de
Graaff generator was used in a pulsed mode to provide
CALIBRATION OF AN ORGANIC SCINTILLATOR 11
t0 ,000
~000
..A W Z Z < I (..) i'v" h i Cl
(.f) F-- Z
0 ~.)
t00
t0 0.00t
0
0
3Sn CALIBRATION - SPECTRUM
o NE 2t3 • NE 241
0.0t 0.t
PULSE HEIGHT (LIGHT UNITS)
Fig. 1. Pulse-height distribution at very low pulse heights (standard pulse-height units) for 14.43-MeV neutrons on 2" NE-211 and NE-213 scintillators obtained with associated-particle technique. This indicates that the two scintillators are equal in response to
neutrons. The carbon-recoil edge appears below 0.04 unit of pulse height. Below 0.01 unit, instrument bias effects are evident.
about 2-nsec bursts of protons or deuterons. Flight paths of 1-5 m were used. The T(p,n)aHe, D(d,n)aHe, and T(d,n)4He reactions were utilized to provide neu- trons in the respective energy ranges of 0.2 to 4.22, 4.92 to 8.03 and 12.0 to 21.8 MeV. The targets for the three ranges consisted, respectively, of a 0.40-mg/cm 2- thick 3H-Zr target on platinum backing, producing a neutron energy spread AE, of less than 30 keV; a deuterium gas target of about 2 a tm pressure and 0.65 cm thickness (AE n < 70 keV) with a 0.00025-cm- thick Havar* window; and a 4.3-mg/cm2-thick T-Er
target mounted on Cu(AEn ~ 0.3 MeV). We used a beam-energy calibration performed at 4.642 MeV by Kernell and Robinson of O R N L t, which was estimated to be accurate to better than 10 keV. Extrapolation to other energies was accomplished with the aid of fig. 1 of ref. 6).
In each case a horizontal beam of neutrons was in-
* Precision Metals Division, Hamilton Watch Co., Lancaster, Pennsylvania.
t We are indebted to R. L. Robinson for making this calibration available.
12 V. V, VERBINSK1 et al.
68t0A PHOTO MULTIPLIER •
300 k l ]
4.60-BY 4,65-cm NE215/
I LIQUID SCINTILLATOR
PHOTOCATHODE FOCUS
{DYNODE 1
(DYNODE 2
(DYNODE 5
(DYNODE
DYNODE 5
DYNODE 6
DYNODE 7
DYNODE 8
DYNODE 9
DYNODE 10
TIME-OF-FLIGHT SPECTRUM
RIDL 40D-CHANNEL ANALYZER ] ANALOG
I-- TRIGGER ~'X
PULSE HEIGHT SPECTRA
I TMC 400-CHANNEL ANALYZER
ANALOG I TRIGGER
I
ROUTING
t-CHANNEL •ZERO-CROSS
DISCRIMINATOR
COINC. I
0.0t WHITE II CATHODE I II
2 kv FOLLOWER 0.6 /~sec DYNODE t2 " /~l
FOCUS
DYNODE t5 TUNNEL DIODE II
0.01" 2 kv DISCRIMINATOR 50 ~ INPUT DYNODE 14
ANODE .j_O,01
1/4-in" ~ v
-2000 r 2 k
_z. I - - ' 2 o k
tN2t51
COINC.
/
:20k
t_CHAINE L ZERO -CROSS
• DISCRIMINATOR
TIME- } R T AMPLITUDE
CONVERTER
PULSE-SHAPE DISCRIMINATOR
WHITE CATHODE FOLLOWER
MODIFIED A-8 AMPLIFIER AND
FRONT -EDGE DISCRIMINATOR
/ 3.2 /~sec
9O 9. AD-YU
Fig. 2. Block diagram for 0.2- to 22-MeV calibrations, including details of photomultiplier tube base circuit.
C A L I B R A T I O N OF A N O R G A N I C S C I N T I L L A T O R 13
cident on the curved surface (perpendicular to the axis) of the 4.60- by 4.65-cm-dia. NE-213 scintillator. The scintillator was mounted on a 0.63-cm-thick, 5-cm-dia. Plexiglas light pipe, which was mounted on an RCA 6810A photomultiplier tube selected by the manufac- turer for about 100/~A/lumen photocathode efficiency and an anode sensitivity of 1800 A/lumen (manufac- turer's standard testing conditions) with light from a tungsten lamp at 2870 ° K color temperature. Scattering by the 1.5-mm-thick glass walls of the scintillator con- tainer and by other surrounding material introduced an error which was small, as estimated by a Monte Carlo calculation, compared with the overall error.
In fig. 2 is shown the circuit diagram for the photo- multiplier tube base with signals taken from dynode 11 for a linear signal, from dynode 13 for a fast timing signal, and from dynode 14 and the anode for two signals for the pulse-shape discriminator circuit. A neu- tron of the proper energy arrives at the scintillator at a time that causes the output or the time-to-amplitude converter to fall in the window of a single-cannel dis- criminator. A pulse from this discriminator and one
from the linear amplifier discriminator activate coinci- dence 1, which triggers a Technical Measurement Cor- poration pulse-height analyzer. Pulses from coincidence 1 activate coincidence 2 when the pulse-shape discri- minator identifies the pulse as a neutron event, which causes the pulse to be stored in the second half of the analyzer's memory. A gamma-ray pulse, or any other pulse which activates coincidence 1 but not coincidence 2, is stored in the first half of the analyzer. At neutron energies below 3 MeV, very few gamma rays are pro- duced with the 3H(p,n)3He reactions, and pulses stored in the first half are all neutron pulses rejected by the discriminator. When these data were added to the data in the second half of the analyzer, the correct pulse- height spectrum was obtained, for E n < 3 MeV, without the use of the correction factor presented in the next section; actually, these data provided an independent check on the correction factor.
An important point is that all leads in the photomulti- plier tube base were kept short and near the ground bus to reduce pickup at dynode 11 of the much larger pulses at dynodes 13 and 14 and the anode. Dynode 11 was
I 0 2 ~ ~ i
I 0 I
)ENTS r'RONS
i0 o
I 0 - I COUNTS PER UNIT PULSE iO_ 2 HEIGHT PER INCIDENT NEUTRON 10_ 3
10 -4
10 -5
I / 5
K I /5
I X I / 8
06 X 1/15
.81X 1/50
9.91X 1/50
21 .81XI /100
1 0 - 6 I I I t I
I0 -g 2 5 10 - I 2 5 I0 ° 2 5 I01 2
PULSE HEIGHT (LIGHT OUTPUT]
Fig. 3. Absolute differential efficiency at 4.60- by 4.65-cm-dia. NE-213 (and NI~-211) scintillator, axis vertical, in horizontal, plane- wave neutron field, normalized to 1 neutron incident on cross sectional area (h x 2r). The area of a given curve above a bias level thus represents the average efficiency for producing pulses above this bias. The 10-MeV response is an interpolation between the 8- and 12-MeV responses (see text). Use tables for good accuracy. The pulse-height scale is defined by the spectrum shown for incident gamma
rays from a ~2Na source.
14 v . v . VERBINSKI et al.
TABLE 3 Pulse-height distribution of Compton edge of 1.28-MeV 2ZNa gamma-ray line which defines one light unit for the 4.60- by 4.65- cm-dia. NE-213 (and NE-211) scintillator, at the pulse-height resolution implied here. For use with a system having worse resolution, the data here should be smeared to match the system's
resolution.
Average Counts Average Counts pulse height per interval pulse height per interval
0.740 2321 0.870 2106 0.750 2413 0.880 1815 0.760 2545 0.890 1551 0.770 2587 0.900 1268 0.780 2664 0.910 1008 0.790 2765 0.920 838 0.800 2839 0.930 705 0.810 2868 0.940 515 0.820 2886 0.950 378 0.830 2793 0.960 269 0.840 2795 0.970 221 0.850 2600 0.980 173 0.860 2400 0.990 79
chosen for the linear output because it was the highest dynode number f rom which a linear signal could be ob- tained without saturation under the operating con- ditions of the photomultiplier tube. With these tech- niques good data reproducibility was obtained for four 6180A photomultiplier tubes. Their photocathode sen- sitivities ranged f rom 95 to 120 #A/lumen.
6. Counting technique for 0.2- to 22-MeV calibrations Gain standardization has been found to be one of
the most troublesome problems in reproducing the experimental measurements and communicating the calibration procedures to other laboratories. The best approach is to ensure that the 22Na pulse-height spec- t rum, measured at the time of a neutron-spectroscopy experiment, has the same position on the pulse-height scale as the calibration shown at the top of fig. 3. Because of probable plotting inaccuracies, a similar calibration is given in table 3 for making detailed graphical comparisons. These data precisely define "one light unit" for the calibrations presented here. This unit is approximately equal to the light produced by 1.25- MeV electrons according to the prescription of Flynn et al.9), or 13% above the 1.28-MeV Compton edge half-height for our system.
In order to apply the gain calibration of table 3 to a system utilizing the same size scintillator but with poorer energy resolution, the Compton edge of the 1.28-MeV 22Na would have to be smeared correctly to match the poor-resolution spectrum.
All measurements were made with the TC-200 am- plifier time COnstants set at 0.8, 0.8 and 0.8 #sec for the first and second differentiation and for the integration. A precision pulser, adjusted to match the neutron pulse- shape output from the preamplifier, was used to opti- mize the system linearity and zero-channel setting.
Another important calibration consisted of a meas- urement ofthepulse-height distribution due to neutrons from a 241Am-Be source. The source was surrounded with a 0.25-cm-thick lead sheet to reduce the high count rate from 60-keV gammas. A typical pulse-height dis- tribution of this type is shown as the lower curve in fig. 4. The upper curve is the true Am-Be pulse-height spectrum obtained by correcting the lower curve for pulse-shape discriminator losses at low pulse heights. The pulse-shape discriminator loss as a function of pulse height was obtained from two associated-particle coincidence measurements of 2.66-MeV neutrons, one with and one without the use of the neutron-gamma pulse-shape discriminator. The two differ only by the losses of the pulse-shape discriminator since essentially no gamma-ray pulses occurred in coincidence with the detected 3He ions.
Before each measurement of a response function, a time-of-flight spectrum was obtained on the R I D L analyzer shown in the block diagram in fig. 2. This spectrum was used to set the amplitude window on the single-channel analyzer following the time-to-ampli- tude converter to accept only the peak in the time spectrum corresponding to monoenergetic neutrons. The time window had to be set to about a 15-nsec width because of the walk in the fast-timing tunnel- diode discriminator. In addition, the discriminator level had to be set low to prevent the pulse-height dis- tribution from being distorted because of some pulses being missed in the wings of the peak in the spectrum. The distance between the target and the detector was varied between 1 and 5 m to optimize the count rate and the separation of the monoenergetic neutrons from spurious neutrons arising from reactions with the target foils. Using this flight-time selection, the amplitude spectrum for monoenergetic neutrons was measured at from one to four different gain settings: one gain was used at low energies and four gains were used at 20 and 22 MeV, where a 1000:1 range in pulse heights had to be measured. For each gain setting, the " g a m m a " pulses were stored in the first half of the TMC-400 channel analyzer and the "neutron" pulses in the second half. A background measurement was also made in each case by using a 50-cm-long iron shadow shield located about half-way between the neutron source and scintillator. This measurement was used to correct for
CALIBRATION OF AN ORGANIC SCINTILLATOR 15
I05
8
6
to
8
6
g 4
2
t03
8
6
4
lO 2
I0 -2 2 4 6 8 t0 - I 2 4 6 8 t0 ° PULSE-HEIGHT UNITS (I ~- 1.25 MeV electron)
Fig. 4. Pulse-height distribution for small 2~aAm-Be neutron source in 0.25-cm-thick lead shield. Lower curve shows losses due to pulse-shape discriminator. Upper curve shows corrected pulse-shape distribution, with corrections obtained from associated-particle
measurements made at 2.66 MeV both with and without pulse-shape discriminator.
neutrons produced on the sides of the beam pipe or for neutrons scattered into the scintillator by the photo- multiplier and other materials nearby. Backgrounds were typically 5 to 10% and were not much different in shape from the total spectrum. During all runs a current integrator that gave a measure of total charge incident on the target was used to help achieve proper fore- ground-background normalization and gain-to-gain normalization for multiple gain runs at each neutron energy.
7. Data analysis and results
All data were normalized to a standard gain and
corrected for zero-drift, for a small analyzer nonline- arity, for background counts, and for pulse-shape dis- criminator losses. The multiple gain runs were com- bined by normalizing one to another in a predetermined overlap region, the data were grouped to a minimum of two bins per resolution width, and the results were plotted on a scale similar to that of fig. 3. The flat region, the recoil-proton plateau, of each measured pulse-height distribution was then normalized to the corresponding region of a Monte Carlo-calculated pulse-height distribution for the same neutron energy. This normalization is equivalent to equating the area of the experimental curve that lies above the lowest
TAB
LE 4
Abs
olut
e di
ffer
enti
al e
ffic
ienc
y ve
rsus
pul
se h
eigh
t (t
able
3)
of
a 4.
60-
by 4
.65-
cm-d
ia.
NE
-213
or
NE
-211
sci
ntil
lato
r ir
rad
iate
d o
n cu
rved
sid
e by
pla
ne p
aral
lel
beam
of
mon
oene
rget
ic n
eutr
on
s pe
rpen
dicu
lar t
o s
cint
illa
tor
axis
. T
ile
low
er e
dge
of
each
pul
se h
eigh
t bi
n L
is
give
n. T
he
bin
wid
ths
are
all
frac
tion
ally
equ
al,
or
AL
/L _
~ 4.
42~o
.
Ab
solu
te d
iffe
rent
ial e
ffic
ienc
ies f
or
neu
tro
n e
nerg
ies
of
Pul
se
0.20
1 0.
322
0.42
7 0.
617
0.90
4 1.
205
1.61
3 2.
233
3.23
8 4.
236
4.91
9 he
ight
M
eV
MeV
M
eV
MeV
M
eV
MeV
M
eV
MeV
M
eV
MeV
M
eV
0.01
540
30.5
9834
28
.453
00
16.3
9100
9.
3916
7 5.
8164
2 4.
3459
2 3.
3390
0 2.
6763
3 2
.10
11
1
1.39
750
1.25
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MeV
M
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MeV
M
eV
MeV
M
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M
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0.01
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0.99
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0.87
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1.3
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1.08
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1.55
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0.54
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0.29
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0.28
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0.27
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0.26
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0,26
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0.25
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0.24
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0.23
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0.23
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0.22
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0.22
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0.21
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0.21
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0.20
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0.20
282
0.19
877
0.19
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0.19
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0.18
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0.18
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0.17
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0.17
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0.17
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0.16
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O. 1
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0.51
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0.48
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0.46
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0.43
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0.41
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0.39
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0.38
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0.36
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0.35
387
0.33
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0.32
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0.30
669
0.29
924
0.28
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0.27
565
0.26
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0.25
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0.25
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0.24
560
0.23
939
0.23
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0.22
747
0.22
400
0.21
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0.20
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0.20
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O. 1
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0.
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0 O
. 152
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0.14
999
0.14
801
0,14
627
0.14
627
0.14
354
0.14
155
0.13
932
0.13
634
0.13
360
0.13
162
0.72
820
0.67
430
0.62
370
0.57
530
0.54
010
0.50
820
0.48
180
0.45
100
0.42
900
0.40
040
0.37
620
0.35
640
0,34
320
0.33
110
0.32
318
0.31
614
0.30
998
0.30
250
0.29
810
0.29
282
0.29
062
0.28
798
0.28
798
0.28
886
0.28
490
0.28
094
0.27
786
0.27
280
0.26
840
0.26
312
0.25
520
0.23
936
0.21
560
0.19
184
0.17
732
0.15
994
0.14
674
0.13
596
0.13
156
0.12
870
0.12
606
0.12
364
0.12
188
0.11
968
0.11
748
0.11
506
0.11
330
0.51
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0.46
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0.43
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0.40
417
0.38
638
0.36
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0.34
354
0.33
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0.31
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0.29
827
0.28
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0.26
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0.25
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0.24
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0.23
927
0.22
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0.22
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0.22
229
0,21
970
0.21
728
0.21
534
0.21
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0.21
178
0.21
017
0.20
887
0,20
726
0.20
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0.20
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0.20
176
0.19
982
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0.19
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0.18
527
0.18
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0.17
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0.15
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0.14
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0.37
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0.31
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0.27
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0.23
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0.23
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0.43
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0.37
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0.34
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0.31
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0.28
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0,25
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0.24
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0.22
525
0.21
165
O. 1
9890
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. 190
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0.18
156
0.17
476
0.16
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6269
0.
1550
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. 150
62
0.14
450
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4144
0.
1411
0 0.
1414
4 0.
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5 O
. 142
97
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. 160
65
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6405
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3 0.
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. 172
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7
>
>
o o >
o :Z
©
N
(co
nti
nu
ed o
n p
. 20
) ""
TAB
LE 4
(co
nt#z
ued)
A
bso
lute
dif
fere
ntia
l eft
icie
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s fo
r n
cutr
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f
Pul
se
6.01
7 7.
037
8.02
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11
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14
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16
.055
17
.805
19
.907
21
.809
he
ight
M
eV
MeV
M
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MeV
M
eV
MeV
M
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MeV
M
eV
MeV
0.15
849
0.15
372
0.12
913
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000
0.13
515
0.16
681
0.1
22
71
0
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90
1
0.10
675
0.08
950
0.08
433
0.16
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0.15
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0.12
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736
0.13
289
0.1
57
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0.07
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0.07
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0.10
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0.12
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0.09
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0.07
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0.21
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0.13
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0.09
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1
0.08
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0.06
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157
0.08
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0.06
633
0.06
483
0.30
290
0.11
798
0.10
033
0.08
492
0.06
176
0.08
168
0.08
687
0.11
143
0.08
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0.06
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0.06
417
0,31
623
0.11
392
0.09
784
0.08
272
0.06
079
0.07
973
0.07
573
0.11
127
0.08
577
0.06
677
0.06
333
0.33
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202
0.09
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11
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320
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245
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0.05
173
0.05
280
0.05
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154
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53
3
0.07
276
0.06
182
0.04
640
0.0
4•4
9
0.04
284
0.05
286
0.08
025
0.06
793
0.05
217
0.55
437
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22 V. V. VERBINSKI et a[.
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point on the proton-recoil plateau to the area of the calculation above the same pulse height. Each final experimental curve corresponds to a pulse-height dis- tribution f rom a plane parallel beam of monoenergetic neutrons incident on the curved face of a 4.60- by 4.65-cm-dia. scintillator, perpendicular to its axis, and normalized to one neutron incident on the NE-213. The resulting response functions are illustrated in fig. 3, along with a 22Na pulse-height spectrum used for gain calibration. The error bars shown at the lower left part of the figure indicate the degree of uncertainty in the pulse-shape discriminator corrections. For example, the correction function shown in fig. 4 was assumed to hold for carbon recoils and alpha particles that become important above 7-MeV incident neutron energy. The correction functions for these events are likely to be different from that for proton recoils.
The absolute differential efficiencies (response func- tions) for neutrons of 20 energies shown in fig. 3 are also given in table 4. In both cases the 10-MeV re- sponse does not represent the results of a measurement. In the region of the proton-recoil plateau this 10-MeV response is the result of a Monte Carlo calculation. Below this plateau the curve was obtained with a hand interpolation, using the measured 8- and 12-MeV re- sponses, since the calculation is not reliable below the proton-recoil plateau; the carbon cross sections, ob- tained mostly by extrapolation from other energies, do not match the data at 8 or 12 MeV.
For convenience in time-of-flight work, the detector efficiency versus bias (integral bias efficiency) is given in table 5. I f an NE-213 scintillator is used with a pulse-shape discrimination circuit of the type presented here, corrections below about 0.1 light units would have to be made for losses in the circuit. But for bias settings higher than this, a well-adjusted discriminator circuit should require no correction (fig. 4). For a NE-211 scintillator, with which good pulse-shape dis- crimination is not feasible, table 5 may be used as is. A small correction may be applied for detecting 4.43- MeV gamma rays from 12C(n,n')12C reactions in the scintillator. However, this correction is only about 2.5% at the 8-MeV peak in the inelastic cross section for this scintillator size. In any case, the discriminator must produce a sharp cutoff such as would be obtained from a discriminator operating on a slow amplifier out- put. I f a sharp cutoffis not produced, then the problem of an effective bias will make the use of table 5 more difficult.
8. Applications The present set of 20 calibrations and one inter-
TAB
LE 5
A
bsol
ute
inte
gral
cou
ntin
g ef
fici
ency
ver
sus
bias
set
ting
in
puls
e-he
ight
uni
ts d
efin
ed b
y ta
ble
3. T
he
effi
cien
cy is
obt
aine
d by
int
egra
tion
of
the
dat
a o
f ta
ble
4 ab
ov
e th
e bi
as l
evel
, an
d re
pres
ents
th
e co
un
tin
g e
ffic
ienc
y at
th
e gi
ven
bias
set
ting
, av
erag
ed o
ver
the
scin
tiIl
ator
's c
ross
-sec
tion
al a
rea.
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solu
te d
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rent
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or n
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on
ene
rgie
s o
f
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0.20
1 0.
322
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617
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205
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233
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8 4.
236
4.91
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eV
MeV
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te d
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251
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343
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0.22
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415
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24 v . v . VERBINSKI et al.
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d d d d ~ d d d d d d d d
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polated response for monoenergetic neutrons with energies between 0.2 and 22 MeV have been carried out to provide a reliable response matrix. The response matrix generally consists of a large number of response functions that can be obtained by interpolating between the 21 curves of fig. 3. This matrix must be accurately known when it is used with the method of neutron spectroscopy based on unfolding a pulse-height dis- tribution to obtain an energy spectrum 1°- 12), because small errors in the matrix can result in much larger errors in the unfolded energy spectrum. The responses presented in fig. 3 and table 4 should be a considerable improvement on past efforts 7'~3'14) because of the unique combination of measurement and calculation used here and because of the large range in pulse height achieved with the measuring techniques employed.
Integration of the calibrations and the interpolated response above some bias level Lb results in a set of efficiency values which are useful in time-of-flight measurementst 5,16). These values, presented in table 5, can be used directly with the present size of scintillator. Although the size and shape of this scintillator are far from optimum for most time-of-flight applications, the calibration results are nevertheless very useful: any other scintillator can be directly and rapidly cross- calibrated against the present one if a pulsed, broad- spectrum neutron source is available, such as from an electron linear accelerator with lead targetl6). With the resultant time-of-flight separation of neutron energies, the present spectrometer can also be used as an ab- solute neutron flux monitor in calibrating other de- tectors. The greatest accuracy is achieved by biasing the present "standard" detector in the proton-recoil pla- teau. In this way, Monte Carlo calibrations to which the present pulse-height distributions were normalized are utilized most directly.
We express our appreciation to J. R. Drischler for his tireless efforts in processing the data, to R. W. Peelle for many helpful discussions, and to F. C. Maienschein for unfailing encouragement, interest and support.
References 1) R. E. Textor and V. V. Verbinski, 05S; A Monte Carlo code
for calculating pulse height distributions due to mono- energetic neutrons incident on organic scintillators, Oak Ridge National Laboratory Report ORNL-4160 (1968). Available from Clearinghouse for Federal Scientific and Technical In- formation, National Bureau of Standards, U.S. Dept. of Com- merce, Springfield, Virginia 22151, U.S.A.
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