Measurement of absolute thick-target bremsstrahlung spectra

NUCLEAR INSTRUMENTS

AND METHODS

6I (1968) 34o-346;

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NORTH-HOLLAND

PUBLISHING

CO.

M E A S U R E M E N T OF ABSOLUTE THICK-TARGET B R E M S S T R A H L U N G SPECTRA* A. A. O'DELL, Jr., C. W. SANDIFER, R. B. KNOWLEN and W. D. GEORGE

EG &G, Inc., Santa Barbara Diviskm, Goh, ta, CaliJbrnia, U.S.A. Received 22 December 1967 Absolute energy spectra were measured for thick-target bremsstrahlung produced by incident electrons in the energy range of from 5.3 to 20.9 MeV. To obtain these spectra, a 0.2 radiationlength gold-tungsten target was bombarded by electrons from the EG&G/AECIinac, and the bremsstrahlung emitted in the forward

(zero-degree) direction was measured by use of a technique based on deuteron photodisintegration. Apparatus and experimental techniques are described, and absolute spectra are given for electron energies of 5.3, 6.8, 8.0, 10.0, 12.0, 14.0, 16.4, 18.0 and 20.9 MeV.

1. Introduction

target with electrons and measured bremsstrahlung emitted in the forward (zero-degree) direction. The EG&G/AEC linear accelerator (linac) provided a stable, high-current electron beam at the energies of interest, and a secondary heavy-water target placed in the bremsstrahlung field on the electron beam axis provided a source of photoneutrons. The neutrons produced by photons above the D(7,n)p reaction threshold of 2.23 MeV were energy analyzed by nanosecond time-of-flight (TOF) techniques 3) using an organic fluor scintillation detector. Theoretical cross sections were used for the deuteron photodisintegration process, and the neutron detector efficiency was calculated by use of a proton recoil model for neutron interactions. The experimental data were analyzed by digital computer techniques to provide absolute brems-

Electron linear accelerators have been used for a number of years as a convenient pulsed source of highintensity gamma rays for investigating various nuclear reactions and radiation effects mechanisms, and for measuring the energy sensitivity of radiation detectors. However, quantitative correlation of experimental results with theory has been limited due to lack of knowledge of the absolute photon energy distribution. In fact, although several relative spectral shape determinations have been reported for peak energies above 5 MeV 1), essentially all previous measurements of absolute bremsstrahlung spectra for thick targets have been made for energies of 3 MeV or below 2). Our objective has been to measure absolute thicktarget bremsstrahlung spectra for incident electron energies in the range from 5.3 to 20.9 MeV. To accomplish this, we bombarded a thick gold-tungsten

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* Work performed for the U.S. Atomic Energy Commission through the Nevada Operations Office.

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strahlung spectra in units of photons/MeV/sterad/ incident electron.

2. Description of apparatus The essential features of the experimental layout are shown schematically in fig. 1. A high-energy electron beam from the linac was deflected achromatically through90 ° by two 45 ° bending magnets, between which were placed a quadrupole focusing magnet and adjustable slits for selection of the electron energy spread A E. The electrons struck a watercooled bremsstrahlung target assembly consisting primarily of a 0.2 radiationlength gold-tungsten converter and a 1.125" thick aluminum filter. The entire assembly was electrically isolated to facilitate monitoring of the electron beam current. Bremsstrahlung emitted within a small solid angle about the beam axis (zero degrees) impinged upon a secondary target of heavy water (D20) contained in a thin-walled lucite vessel. Neutrons produced by the D(7,n)p reaction were observed at 90 ° with respect to the incident bremsstrahlung beam by a liquid organicscintillation detector placed at the far end of the flight path. Background radiation shielding included a 24" thick concrete bulkhead (linac cell) and lead-brick barriers placed near the detector. The 25 m long neutron flight tube was evacuated to approximately 10 -3 Torr and reduced to an effective 4" dia. by annular polyethylene and lead collimators.

INJECTOR PULSER

2.1. NEUTRON DETECTOR The neutron detector consisted basically of a 2" dia. by 2.5" long internally reflecting sealed container filled with NE 213 liquid scintillator and coupled to a single 56AVP photomultiplier biased at +2500 V. A "slow" output signal derived from dynode 10 via an emitter follower was used for pulse height analysis of detected neutron events. A "fast" signal obtained directly from the anode was used for T O F analysis of registered neutrons. 2.2. TIME-OF-FLIGHTMEASUREMENTSYSTEM The T O F measurement system, composed primarily of E G & G M-100 modular 100 M H z electronics, is shown in fig. 2. Primary timing was performed between the linac injector (start) signal (exhibiting low timejitter correlation with neutron creation) and the neutron detector anode (stop) signal which registered the traversal of a particular neutron over the fixed 25 m flight path. The time-to-amplitude converter (TAC) output, which is proportional to the start-stop time interval, was monitored by a TMC-404A 400 channel pulse height analyzer (PHA). A stop coincidence loop was also provided so that events recorded by the neutron detector prior to a preselected time could be disregarded. In general, however, adjustment of the start delay was found to be more satsifactory for this function. Time calibration of the T A C - P H A system was

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TIME MARK GENERATOR (TEK 180) I00 IO-Mc /.Lsec MARKER

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generator, provided TAC stop pulses at random 100 nsec intervals with respect to synchronized start pulses from the marker generator. In this manner, time calibration data for integral multiples of 100 nsec was accumulated and recorded simultaneously over the entire dynamic range of the T A C - P H A system. DELAY ] ~AMPLIFIER] ( LUMATRONt~ ~

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2.3. D 2 0 TARGET Preliminary tests performed to select an optimum D 2 0 target configuration showed that thin-walled spherical glass containers having diameters of 14, 25 and 45 m m produced considerable distortion in the neutron spectra. Further testing led to selection of rectangular shaped containers made of thin-walled (0.03") lucite, and having an edge width of 0.25" and square cross sections. To minimize variations in bremsstrahlung irradiation, cross-section dimensions ranged from 0.5 to 1.5", depending on peak bremsstrahlung energy. The target was oriented so the bremsstrahlung beam entered the 0.25" edge.

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3. Experimental techniques Fig. 3. Time calibration of neutron time-of-flight instrumentation.

The absolute bremsstrahlung spectra measurements were made with linac peak electron currents of from 0.3 to 1.2 A and a repetition rate of 360 pulse/sec. The nominal pulse width was 30 nsec except for the 5.3 and 6.8 MeV runs, for which 150 nsec pulses were used to improve the neutron counting rate. The electron energy spread was fixed at 3%.

accomplished by use of a Tektronix-180 time marker generator, as shown in fig. 3. A radioactive source was placed near the neutron detector to generate random pulses from the detector anode. These pulses, in coincidence with a 10 M H z pulse pattern from the marker 1400

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Fig 5. Comparison of measured and calculated efficiency for a 2" dia. by 2.5" thick NE 213 neutron detector biased at425 keV. 3.1. DETECTORSIGNALBIASING Low-level pulse height discrimination was required at the neutron detector anode output to reject photomultiplier noise. The effective electron energy associated with this bias level was determined by observation of Compton pulse height spectra from several gamma sources• These measurements showed that NE 213 has a linear pulse height response for electrons and that the half-maximum point on the high-energy portion of the pulse height distribution is equivalent to 1.05 _+ 0.01 times the Compton-edge energy. Fig. 4 shows a typical Compton pulse height distribution produced by gamma radiation from 6SZn. By use of the factor determined above, the energy at half maximum of the Compton edge is equivalent to 950 keV. This energy-channel calibration was then used to evaluate the electron energy of the low-level electron bias• The equivalent proton energy bias was obtained from experimental measurements of the relative light output of protons and electrons in NE 213• Neutron-proton scattering, the predominant neutron interaction in hydrogeneous materials, produces a proton energy distribution essentially constant from zero to the neutron energy• The detector pulses as-

sociated with these events are detectable for all bias conditions. For neutron energies below 10 MeV, carbon recoils and reaction alpha particles produce extremely small detector pulses, however, and are eliminated by any practical detector bias. Thus, the detector neutron energy bias is equivalent to the proton energy bias. 3.2. DETECTORNEUTRONEFFICIENCY Neutron counting efficiency of the TOF detector was measured in the energy range up to 20 MeV using timeof-flight techniques and a Van de Graaff accelerator as the source of monoenergetic neutrons. Fig. 5 shows the experimental results for neutron energies up to 8 MeV and for a detector neutron bias of 425 keV (31 keV electron bias). Calculated neutron efficiency, assuming that proton recoil interactions alone are registered by the detector, is shown as a solid curve for comparison. The calculated efficiency was obtained from the expression

= {n,~r n/(nnCr,, + ricOc) } • • {1 -- exp [---(nn~ n + nc~c)t']} {1 --(B/E)}, where

A . A . O'DELL, JR. et al.

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nu = hydrogen atoms per cm3; nc = carbon atoms per cm3; an = total n-p cross sections; a c = total inelastic carbon cross section; t = average scintillator thickness; B = neutron energy bias; E = neutron energy. The good agreement between measured and calculated efficiency using the single collision model for proton recoil interactions and the observed linearity of pulse height response for electrons provides a strong basis for applying the above formula to other bias conditions. In view of these results, the detector efficiency for neutron bias levels of 450, 850 and 900 keV were calculated and used for the reduction of experimental data in this work. 3.3. TIME-OF-FLIGHT SYSTEM CHECKOUT

For the T O F runs, the PHA baseline and overall conversion gain were adjusted to provide maximum linearity (___2%) for the T A C - P H A system over a timeinterval range of approximately 3 ysec. With zero startdelay, the bremsstrahlung flash channel was observed on the PHA and recorded. A calibrated start-delay was then inserted to shift the bremsstrahlung flash below channel zero. Zero time (neutron creation) was determined by use of this calibrated delay value and the calculated flight time for bremsstrahlung photons. A

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background run was taken at each electron energy by replacement of the D 2 0 with distilled H/O. Fig. 6 shows results of typical D 2 0 and H 2 0 runs at 10 MeV. 3.4. DATA REDUCTION Data reduction was performed by digital computation. The D 2 0 and H 2 0 data for each electron energy were individually smoothed by local averaging, normalized to equal integrated electron current, and then subtracted to obtain the net counts per channel. The corresponding neutron spectrum was determined by use of relativistic neutron kinematics. The photoelectric and photomagnetic cross sections for the D(y,n)p reaction were calculated from the closed form expressions of Evans 4) and from linear interpolation of the computations of Hulthen and NagelS). The agreement is good, especially for the photoelectric cross section which, except for photon energies near threshold (2.33 MeV), dominates the D(y,n)p reaction. For computational simplicity, the formulae of Evans were chosen for the work reported here. Corrections were included for: 1. Center-of-mass motion; 2. Nonuniform bremsstrahlung irradiation of the D 2 0 target; 3. Neutron attenuation in the D 2 0 target and the T O F tube entrance and exit windows.

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4. Results

The bremsstrahlung spectra measured for nine electron beam energies are shown in fig. 7. The overall experimental error varies from 5 to 10%, except for photon energies at the high energy limit where counting statistics increases the measurement uncertainty. The sources of error include : !. Effects of counting statistics ; 2. TAC-PHA nonlinearity; 3. Electron energy spread; 4. Timing jitter; 5. Start-delay calibration; 6. Measurement uncertainties in the integrated beam current, detector solid angle, DzO target solid angle, and D20 volume. The spectrum turnover at low energies is a result of neutron detector bias and the D(7,n)p threshold. A detector bias level of 450 keV was used for the runs at 5.3, 8.0, 10.0, 12.0, 14.0 and 16.4 MeV. A bias of 850 keV was used for the run at 18.0 MeV; a bias of 900 keV was used for the runs at 6.8 and 20.9 MeV. A separate experiment to measure bremsstrahlung energy spectra from the same linac target by means of a large Nal spectrometer is nearing completion. Data from this experiment will be used to complete the energy spectra shape below the D(~,n)p threshold.

Lent and Dickinson 6) recently calculated thick-target bremsstrahlung spectra for incident electron energies ranging from 1 to 10 MeV. Fig. 8 compares their predicted spectrum for 10 MeV electrons with the measured spectrum for the same target configuration. The spectra shapes are in good agreement; however, the theoretical spectrum is approximately 35°o higher in absolute value. A thorough review of the experimental technique has not, however, revealed a source of uncertainty with magnitude sufficient to account for this disagreement between the calculated and measured values for bremsstrahlung production efficiency. The authors wish to express their appreciation to Dr. M. Taher-Zadeh for his assistance in the computer analysis of measured data. References 1) N. Starfelt and H. W. Koch, Phys. Rev. 102 (1956) 1598. ") L. L. Baggerly, W. E. Dance, B. J. Farmer and J. H. Johnson, LTVO-71000/42-31 (1964). 3) B. V. Rybakov and V. A. Sidorov, Soy. J. Atomic Energy, Suppl. no. 6 (Engl. Transl. of Atomnaya Energiya, 1958) Consultants Bureau, Inc. (1960). 4) R. D. Evans, The atomic nucleus (McGraw-Hill, 1965). 5) L. Hulthen and B. C. H. Nagel, Phys. Rev. 90 (1953) 62. 6) E. M. Lent and W. C. Dickinson, Stanford Meeting of Am. Phys. Soc. (Palo Alto, Calif., Dec. 1966).