IEEE TuzactZon6 on NucteaA Science, Vot.NS-25, No.4, Augu6t 1978EXPERIMENTAL RESULTS OF BREMSSTRAHLUNG SHIELDINGFROM DIELECTRIC TRAPPING WITH THE USE OF TQ-204*

W. R. Hanley, Jr., W. D. Hunt**, J. J. Wade***, D. L. HollisDepartment of Electrical Engineering

University of AlabamaUniversity, Alabama 35486

Received July 25, 1977

ABSTRACT

A method is described for investigating the effects on bremsstrahlung of charge buildup in a dielectric frombetas emitted by a radionuclide. Data is also reported of bremsstrahlung attenuation as a function of days ofirradiation with point of maximum attenuation depending on magnitude of charge current. Cyclic behavior isindicated. Effects of charge buildup in dielectric on bremsstrahlung have applications as shielding againstradiations from energetic electrons.

Hazards from energetic electrons exist not onlyfrom the particles themselves but also from brems-strahlung which is emitted from sudden stoppage ofthe electrons in whatever materials the particlesstrike. This is pertinent to spacecrafts such as theMariner vehicles in the high radiation environment ofJupiter and to those in synchronous earth orbit wherethe electron spectrum contains particles with energiesgreater than 1.9 MeV. 1 Even though the actual elec-tron particle dose rate inside of a spacecraft withaluminum wall thickness greater than 1.5 g/cm2 is neg-ligible at synchronous altitude, the bremsstrahlungdose rate has been calculated to be a few tenths of arad per day.2 However, this bremsstrahlung dose ratewas based on older electron spectral models with lesspenetrating capability than electrons of more recentmodels, and this dose rate might be an order of maani-tude too low.3 There has been renewed interestrecently in the synchronous earth orbit because of theconcept of locating large solar photovoltaic cells,among other applications, in this region. In additionto protection within spacecrafts, there would be apressing need to provide protection against energeticelectrons for astronauts during extra-vehicular activ-ities. Spacesuits will shield against low energyelectrons, but they will not be effective for moreenergetic electrons, especially those with tenths ofMeV and greater. Photovoltaic cells will also beaffected by energetic electrons. The problem occursin any environment, not just space, where high speedelectrons are present. Accelerators, in which ener-getic electrons are produced, must provide shieldingagainst the particle and bremsstrahlung. Layers ofhigh-Z materials can be used, but their large mass maybe undesirable, especially for space applications.

The need for a lightweight, and possibly flexible,shield against energetic electrons and bremsstrahlungled to the suggestion that trapping of incident elec-trons in a thin layer of dielectric material might beutilized for this purpose.4'5 A large electric fieldis produced inside the dielectric as trapped chargesaccumulate, and the potential energy of the field can

Support by a University Graduate College ResearchFellowship is gratefully acknowledged.

* Now at Harris E.S.D., Melbourne, FL 32091.

* Now at White Sands Missile Range, Los Cruces, NM88002.

exceed the kinetic enerqy of the incoming electrons.6The trapped field acts to reduce the amount of brems-strahlung from the incident electrons because of thelonger electron-field interaction distances in compar-ison to individual electron-atom collisions. Ineffect, some kinetic energy of the electrons is trans-ferred to potential energy of the trapped field with acorresponding decrease in bremsstrahlung. Thicknessof the dielectric shield is chosen to be just enoughto stop the most energetic electrons in the radiationfield. For example, if it is determined that 1.9 MeVis the maximum energy of electrons in sufficient numberto cause a radiation problem, then the dielectricshield would have a thickness greater than 0.9 g/cm2which is the maximum range of electrons with thatenergy. For Pyrex, with a density of 2.23 g/cm3, theshield would be at least 0.4 cm thick to shield againstthe 1.9 MeV electrons. Shielding from the trappingmechanism will be effective not only for the deep pene-trating electrons but also for all electrons with ener-nies less than the design maximum. Trapped chargeaccumulation is crucial to the shielding process, andit is necessary that the shielding materials have hinhresistivities. There are more than a few dielectricswith very large resistivities, including some trans-parent plastics. Retention of trapped charges can bemaintained for extended periods of time, especially atlow temperatures. Some plastics have maintainedtrapped charges for 30 days after irradiation at-78.5°C with no measurable decay.7 Shielding effectsdue to trapped charges would be cyclic in nature fol-lowing the charging and discharging patterns of charqeinjection and leakaae to the surface. Charge accumula-tion effects in the dielectric shield should be morepronounced under actual sDace conditions, with a loweraverage temperature, than in the laboratory, reqardlessof the time fluctuations of the space radiation field.

Initial theoretical and experimental studies ofbremsstrahlung attenuation from dielectric trappinghave been reported.8 Computational models in thisreport indicated that there would be considerableattenuation from the trapping mechanism, especially forhigher photonic energies, and experiments with elec-trons from an accelerator and Tz-204 beta source tendedto support that contention. However, the experimentalresults were of a preliminary nature. It is not feasi-ble to simulate the highly varying electron fluxesfound at synch,ronous Rititude, but it should not beabsolutely necessary to do so because of the effects ofcharge accumulation. Reasonable approaches to electroncurrent magnitude and representative electron energiescan be made in the laboratory. For the accelerator

0018-9499/78/0800-1043$OO.75 ® 1978 IEEE 1043

experiments of the above report the beam current wasabout 1 PA at 500 keV, and the 5 mCi TR-204 betasource was the equivalent of almost 15 pA to thetarget with a beta spectrum of 763.4 keV maximumenergy. In general, buildup of the internal electricfields in the dielectric depends on the chargingcurrent in opposition to the discharging current.Kinetic energies of the electrons act mainly tolocate the trapped charge layer at differing depthsin the dielectric. The accelerator current resultedin patterns of total bremsstrahlung count rate vstime which had a cyclic nature of charge buildup anddecay with a period of a few tenths of a second. Themuch weaker beta source did not produce cyclic pat-terns; in fact, only a slight bremsstrahlung reductionafter days of irradiation was observed with the thal-lium betas, but no correction was made for background.All of these experiments were conducted at room tem-perature; however, the average temperature was essen-tially the same over the course of the experiments.

Before optimum designs can be made of dielectricshields much more information is needed than thatalready reported. Since the shielding effect dependson charge accumulation, which should function despitevariations in electron fluxes, meaningful data isobtainable from laboratory experiments. This paperserves these two needs by: (1) reporting additionalinformation of bremsstrahlung attenuation from dielec-tric trapping; (2) describing a method which can beused to obtain more data about such effects as temper-ature, magnitude of charging current, and choice ofmaterials. Both are the results of the efforts, to bedescribed, to improve the earlier experimentation andanalysis of bremsstrahlung attenuation from TQ-204betas interacting with glass.

A major improvement is the correction for back-ground in the analysis of the measured radiationsusing TQ-204 betas. Background for this case includesnon-thallium radiation due to sources'external andindependent of the experiment plus radiations due tothe thallium betas but not due to bremsstrahlung fromthe target. The latter radiations consist of internalbremsstrahlung, x-rays, and external non-target brems-

strahlung.9 Internal bremsstrahlung is attributed tothe sudden change in nuclear charge occurring with theemission of a beta particle from the nucleus. Inter-actions of the beta as it passes through the atomicelectron cloud around the nucleus can result inx-rays. Once the beta is past its parent atom it caninteract with other atoms in the source material oratoms not in the source material (but not targetatoms) resulting in bremsstrahlung or x-rays. Deter-mination of the non-thallium background spectrum ismade by measuring the radiations present with thecomplete experimental setup except for the absence ofthe TQ-204 source. Thallium background is measuredindirectly. Since total bremsstrahlung is propor-tional to Z of the stopping material at a given energy,two different materials are used to determine the

thallium background for that energy.9 A 0.344 g/cm2aluminum plate and a 0.340 g/cm2 copper plate are usedfor this purpose. Thicknesses of these absorbers are

just enough to stop the most energetic betas from theTQ-204 source. With thallium betas striking a metalplate, a detector would measure bremsstrahlung fromthe metal plate, non-target radiations due to thal-lium, and non-thallium background. The last radiationcan be subtracted from the total, leaving only thatfrom the metal plate and thallium. Let K be the con-stant of proportionality, for a given energy, betweenbremsstrahlung count rate and Z of the target metalplate, then 13 K and 26 K represent bremsstrahlungcount rates at that energy from aluminum and copper.

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If Nso is the number of counts measured in a given time

from thallium source background, at the specific energy,the total numbers of counts measured from the two metalsat the given energy, not including the non-thalliumbackground, are NAl and NCu, where10

13 K + N =NA

26 K + N NCu

(1)

(2)

Simultaneous solutions of these equations for the thal-lium background Nso are made for each photonic energy

under consideration. The same thallium source and con-stant geometrical conditions are used throuqhout agiven experiment.

Basically, the experimental procedure consisted ofirradiating (5 x 5 x 0.25) cm plates of Corninq 7740

Pyrex (1016 Q-cm resistivity at 200C) with betas fromTQ-204 , and recording the resulting bremsstrahlungwith a scintillation-multichannel analyzer (MCA)system. Orientation of the absorber targets to thedetector was fixed in the head-on position. Countrates for each MCA channel over the range of 150-350keV were recorded each day along with energy calibra-tions from Cs-137 (661.64 keV qamma). Lower and uppervalues in this energy range were chosen to avoideffects from thallium x-rays while still maintainingsufficiently high count rates. Linearity between totalbremsstrahlung intensity and Z of the stopping materialimplies integration over all solid anqles, but forenergies < 0.5 MeV the bremsstrahlung spectral distri-

bution is almost isotropic.11 The chosen energy rangeis within this limit, and isotropic conditions wereempirically verified for the experiments reported here.Thus for these conditions the number of counts recordedby the MCA in an increment of energy dE at E with thecounting system oriented at a fixed angular location inthe bremsstrahlung field was a measure of the totalbremsstrahlung from the absorbers.

The first of several experiments using TQ-204 wasconducted with a 200 channel MCA-scintillation system.Calibrations for system drift were made for each of the24 days of continuous irradiation of Pyrex with Cs-137.Non-thallium background was measured at various ener-gies and was considered to be negliqible in comparisonto count rates due to thallium betas. Thallium back-ground data were measured with the two metal absorbersdescribed above. Count rates in unity channel widthvs channel number from the MCA were changed into countrates vs energy by means of a computer program whichcorrected for system drift from the daily cesium cali-bration checks. This was done by applying the ratio ofthe Cs-137 gamma energy to the MCA channel at which thepeak occurred to both the channel number and the numberof counts in that channel. For example, if the cesiumpeak occurred at channel number 670, then the channelswere converted to energy in keV by multiplying eachchannel number by (661.64/670). Linear interpolationwas used for the count rates, and the number of countsin each dE = 1 keV at E in keV was equal to the numberof counts in each channel divided by the ratio. Thal-lium background was computed by the simultaneous solu-tion of equations (1) and (2) for each energy. Thiswas subtracted from the daily glass count rates to getthe bremsstrahlung as a function of energy. Pyrexbrersstrahlung at a specific day was divided by thecorresponding count rate of the initial day of irradia-tion to obtain the ratio R. A linear least-squares-fitwas applied to the ratios vs energy for a given day toget line slope and ratio values at 151 keV and 350 keV.Ratio values at these energies were then plotted as

functions of days of irradiation as shown in Fig. 1.

1.0

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0.6

0.50 5 10 15 20

DAYS OF IRRADIATION25 30

Fig. 1 Ratio of bremsstrahlung from Pyrexrelative to day-zero as a functionof days of irradiation for photonicenergies of 151 keV and 350 keV.5 mCi of Tl-204.

Error ranges were due mainly to MCA drift and standarddeviation of the counts. These large errors indicatedthat improvements in experimentation and analysis wererequired.

Acquisition of a new 1024 channel MCA with papertape record of the data and a precision regulatedpower supply permitted much more detailed and accuratemeasurement of the effects of trapped charges on brems-strahlung. For the last of three experiments, de-scribed here, more than 200 count rates over the rangeof 150-350 keV were recorded and analyzed for each dataset. The extensive amount of data in perforated papertape form was transformed to digital computer cards bymeans of a RCA-llOA computer before being analyzed bya Univac 1110 computer with a program much more refinedthan the one used for the initial analysis. By thetime of the last experiment, the TQ-204 had decreasedto about half of its original activity. Continuousirradiation of the Pyrex plate was extended over 100

days to display any cyclic effects, and correctionsfor thallium decay were made for each day. This cor-rection amounted to several percent by the end of therun. Corrections for attenuations of the radiationsin the metals and glass were also made. A curve fit-ting subroutine replaced linear interpolation of thedata. This subroutine consisted of: (1) taking thenatural logarithm of count rates; (2) searching for athird degree polynomial equation to fit ln (count rate)vs energy in keV; (3) obtaining ln (count rate) inincrements of 1 keV for integral energy in 150-350 keV;(4) raising base of natural logarithm to ln (countrate) to get smoothed count rates in 1 keV vs E in keV.Three sets of data for each metal before glass irradia-tion and two sets after were averaged to determine NSO'Checks for accuracy of the analysis were made by com-paring the metal count rates and Nso vs energy beforeand after glass irradiation. These agreed within + 1%for the metals and ± 4% for NSO. The five data setsfor each metal were averaged in determining the Nsospectrum used for analyzing the irradiated glass, whichprobably was more accurate than that indicated above.Since the initial day of Pyrex irradiation, Day-¢ wasthe standard in computing R, the average of three setsof data were used for that day. Counts were taken onlyafter the detector system had 3-4 hours to stabilize

after any change in radiation intensity, such as place-ment and removal of Cs-137. Temperature and relativehumidity readings were recorded for each measurement ofa data set, and an average temperature of about 19°Cexisted for most measurements with no more than ± 30Cchange for the others. System drift was reduced con-siderably by decreasing the radiation intensity at thedetector, with an increase in target-detector separa-tion, and by a new photomultiplier tube (EMI-9856 B)for the 1 3/4" x 2" NaI(TQ) crystal. In reducing thetotal count rates, the relative significance of thenon-thallium background became greater, and correctionsfor it were necessary. Fig. 2 includes a plot of thenon-thallium background spectrum over the range of thethallium betas. To show its relation to data of theexperiment, full spectra of the 98th day of Pyrex irra-diation (GL-98) with and without Cs-137 are included.Thallium backqround was found to be approximately twiceas great as the bremsstrahlung emitted only from theglass plate. The non-thallium background was about onethird of the target bremsstrahlung at 150 keV, but itwas almost one and a half times greater than the glassbremsstrahlung at 350 keV. Before applying non-thal-lium background to the other data, it was first cor-rected for system drift and fitted to a curve of countsper 4000 seconds vs energy in keV. The non-thalliumbackground was then subtracted from the correspondinglyadjusted count rates for metals and glass. The anal-ysis continued as before to get the ratio spectrum fora given day of irradiation; however, correction forthallium decay was not applied until the non-thalliumbackground was removed. Fig. 3 is a plot of R vs daysof irradiation for 150 keV, 250 keV, and 350 keV.Error ranoes here are due mainly to standard deviationof the count rates and to MCA drift.

The method described in this paper of investigatingeffects of charqe buildup on bremsstrahlung with theuse of betas from a radionuclide resulted in data indi-cating a general reduction in bremsstrahlung with daysof irradiation, particularly for higher photonic ener-gies. This is in agreement with numerical models ofprevious studies. In the initial work of Fig. 1 largeerror ranges and insufficient number of irradiationdays made determination of the specific point of maxi-mum bremsstrahlung reduction to be uncertain. However,on comparison of Fig. 1 with Fig. 3, it can be deducedthat the larger charging current of Fig. 1 caused maxi-mum bremsstrahlung reduction to probably occur withless irradiation days than for the case of Fin. 3. Forthe more accurate results shown in Fiq. 3, there is agentle decline in the bremsstrahlunq ratio with irra-diation days up to about 70 days at which point thereis a sharp drop. All three curves for the differentenergies showed declines at about the same time, whichis consistent with theoretical considerations. Thecyclic behavior after the first sharp drop is to beexpected in view of the charging and discharqingeffects of dielectrics irradiated with energetic elec-trons. Determination of the backgrounds associatedwith the radionuclide by the method described in thispaper appears to be satisfactory, and the overallmethod of experimentation and analysis described hereshould be applicable to a number of investigations ofthe charge-trapping-shielding phenomenon.

REFERENCES

1. R. H. Hilberg, M. J. Teague, J. I. Vette, "Compari-son of the Trapped Electron Models AE4 and AE5 withAE2 and AE3", NASA NSSDC 74-13, Sep. 1974.

2. M. 0. Burrell, J. D. Wright, J. W. Watts, "An Anal-ysis of Energetic Space Radiation and Dose Rates",NASA TN D-4404, Feb. 1968.

3. J. W. Wilson, F. M. Denn, Nucl. Tech., 35, 178-183,

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Aug. 1977.

4. D. L. Hollis, "Electron Bremsstrahlung Shieldingat Synchronous Altitude by Electron Trapping inDielectrics", NASA TM-53954, Aug. 1969.

5. D. L. Hollis, Nucl. Tech., 10, 3, 325-327, Mar.1971.

6. B. Gross, S. V. Nablo, J. Appl. Phys., 38, 5,2272, 1967.

7. R. G. Brown, J'. Appl. Phys., 38, 10, 3904-3907,Sep. 1967.

8. J'. J'. Wade, D. L. Hollis, J'. Appl. Phys., 46, 12,5158-5162, Dec. 1975.

9. R. D. Evans, The Atomic Nucleus, New York, McGraw-Hill, 1955, Ch. 31.

10. J. J. Wade, dissertation, Univ. of Alabama,University, AL, 1975.

11. G. D. Magnuson, A. W. McReynolds, "Space ElectronRadiation Shielding - Bremsstrahlung and ElectronTransmission", NASA SP-71, 455-463, 1965.

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Walter R. Hanley, Jr. wasborn in Auburn, Alabama onNovember 28, 1957. He iscurrently working towardthe B. S. degree in Elec-trical Engineering at TheUniversity of Alabama, andis a member of the ComputerBased Honors Program. Hisinterests include micro-processors, graphics, anddigital design.

Daniel L. Hollis (Ph.D. inNuclear Engineering fromTexas A & M Univ.) is aorofessor in the Departmentsof Electrical Engineering andAerospace, Mechanical, andEngineering Mechanics at TheUniversity of Alabama wherehe is responsible for theNuclear Engineering coursesin the College of Enqineering.

William D. Hunt was bornTrn Jackson, Mississippi,on December 21, 1954. Hegraduated Summa Cum Laudefrom The University ofAlabama in 1976. Sincethat time he has been em-ployed by Harris Corpora-tion's Government Commun-ication Systems Divisionin Melbourne, Florida, and

will pursue a Masters degree in ElectricalEnqineering at M.I.T. beginning in the fallof 1978.

James J. Wade (Ph.D inElec-trT a-I Ininee ri nqfrom the Universityof Alabama)is currentlywith the US Army, Officeof Missile Electronic War-fare, White Sands MissileRange, New Mexico, wherehe is responsible for tech-nical analysis of electro-optical missile systems.

1047

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