NUREG/CR-3532

SAND83-2098

RV

Printed November 1983

Response of Rubber Insulation

Materials to Monoenergetic

Electron Irradiations

W. H. Buckalew, F. J. Wyant, G. J. Lockwood

This report documents a part of the Qualification Testing Evaluation (GTE)

Program being conducted by Sandia National Laboratories

Prepared by

Sandia National Laboratories,

Albuquerque. New Mexico 87185 and Livermore, California 94550

for the United States Department of Energy

under Contract DE-AC04-768P00789

I;

Prepa-ed for U". "'S, NUCLEAR REGULATORY COMMISSION

SF 2900-Q(6-82)
tNOTICE

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NUREG/CR-3532

SAND83-2098

RV

RESPONSE OF RUBBER INSULATION MATERIALS TO MONOENERGETIC

ELECTRON IRRADIATIONS

W. H. Buckalew, F. J. Wyant, G. J. Lockwood

Sandia National Laboratories

Albuquerque, New Mexico 87185

Operated by

Sandia Corporation

for the

U.S. Department of Energy

Prepared for the Electrical Engineering Branch

Division of Engineering Technology

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555

Under Interagency Agreement DOE 40-550-75

NRC FIN No. A-1051
r
ABSTRACT

The electron charge disposition in ethylene-propylene rubber

(EPR) insulation specimens irradiated with monoenergetic electrons has been investigated. Studied was charge. disposition in

slab and cylindrical (cable) geometries. For each geometry,

charge behavior was observed as a function of environment,

electron energy, and surface termination. Under certain conditions charge *was accumulated and spontaneous breakdown did

occur during irradiation in the vacuum environment; however, no

evidence of breakdown was observed during ambient air exposures.

Based on these experiments, it is concluded that electron charge

buildup and breakdown is not apt to occur in EPR rubber insulation exposed to electrons from a LOCA radiation environment

provided that the insulation is in contact with an ionized

medium. The results can probably be applied to other organics

and it appears that the LOCA beta spectra need not be precisely duplicated in insulation-radiation qualification tests.

iii-iV

TABLE OF CONTENTS

Page No.

EXECUTIVE SUMMARY . . . . . . . . . . . . . . . . . . .*. 1

1.0 INTRODUCTION. . . . . . . . . . . . . . . ... .. 3

2.0 APPARATUS AND PROCEDURES. . . . . . . . . . . . . 5

3.0 RESULTS . . . . . . . . . . . . . . . . . . . . 10

3.1 Slab Geometry .. ...... .. . . 10

3.1.1 Terminated surfaces--

vacuum environment . . . . . . . . 10

3.1.2 Unterminated front surface--

vacuum environment . . . . . . .

3.1.3 Terminated surfaces--

ambient environment. . . . . . . . 13

3.1.4 Unterminated front surface--

ambient environment. . . . . . . . 13

3.1.5 Effect of sample aluminization. . . . 14

3.2 Cylindrical Geometry . . . . . . . . 14

3.2.1 Ambient environment. . . . . . . . . 14

3.2.2 Vacuum environment. . . . . . . . . . 15

4.0 CONCLUSIONS ................... 15

REFERENCES . . o . . . . . . .. . . . . . . . . . . . 17

v
LIST OF FIGURES

Figure No.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

Page No.

Reactor Containment Volume Radiation

Environments. . . . . . . . . . . . . .. . .

Gamma and Beta Particle Time-Dependent

Average Energies. . . . . . . . . . . ....

Transmission Of Recoil Electron Particle

Number, Energy, And Absorbed Dose Ratio In Air

Electron Charge Deposition In EPR From LOCA

Source Electron Exposure. . . . . . . ....

Electron Charge Deposition In EPR From

Monoenergetic Electron Exposure . . . . ...

Maximum Electron Beam Deflection As A Function

of Raster Coil Current. . . . . . . . . ...

(a) Schematic Of Experiment Orientation In

Vacuum Chamber

(b) Schematic Of Experiment Orientation

Outside Of Vacuum Chamber. . . . . ...

Schematic Of Material Sample Holder

Configuration . . . . . . . . . . . . . ...

Schematic Of Cable Orientation In Sample Holder

Beryllium Stopper Current Versus Faraday Cup

Current . . . . . . . . . . . . . . . . . . .

Measured And Calculated Dose Rates Versus

Beryllium Stopper Currents. . . . . . . ...

Schematic Of Signal Measuring Configuration .

Measured Surface Currents Versus Bias Voltage

For 1.0 MeV Source Electrons. . . . . . ...

Measured Surface Currents Versus Bias Voltage

For 0.6 MeV Source Electrons. . . . . . ...

19

20

21

22

23

24

25

26

27

28

29

30

31

32

vi
Figure No. 

Page No.

15. Measured Surface Currents Versus Back Surface

Bias Voltage For 0.4 MeV Source Electrons . . 33

16. Measured Surface Currents Versus Front Surface

Bias Voltage For 0.4 MeV Source Electrons . . 34

17. Measured Front Surface Current Versus Front

Surface Bias Voltage (Expanded Scale) For

0.4 MeV Source Electrons. . . . . . . . . . . 35

18. Measured Surface Currents Versus Bias Voltage

For 0.2 MeV Source Electrons . . . . . . . . 36

19. Front Surface Equilibrium Voltage As A

Function Of Source Beam Energy. . . . . . . . 37

20. Back Surface Current As A Function Of Source

Beam Energy . . . . . . 0 . . . . . . . . . . 38

21. EPR Front Surface Voltage Versus Time For

0.2 MeV Source Electrons. . . . . . . . . . . 39

22. EPR Front Surface Voltage Versus Time For

0.45 MeV Source Electrons . . . . . . . . . . 40

23. Measured Surface Currents Versus Bias Voltage

For 1.0 MeV Source Electrons In Air . . . . . 41

24. Measured Surface Currents Versus Bias Voltage

For 0.4 MeV Source Electrons In Air . . . . . 42

vii
LIST OF TABLES

Table No. Page No.

1. Incident Beam Current Partition For 1.0 MeV

Source Electrons In Vacuum Environment ..... 43

2. Incident Beam Current Partition For 0.2 MeV

Source Electrons In Vacuum Environment. . . . . 44

3. Comparison Of Measured And Calculated Fraction

Of Incident Electrons Stopped Within EPR Volume 45

4. Incident Beam Current Partition For 1.0 MeV

Source Electrons In Ambient Air Environment. . 46

5. Incident Beam Current Partition For 0.4 MeV

Source Electrons In Ambient Air Environment . . 47

viii
ACKNOWLEDGEMENTS

The authors would like to gratefully acknowledge the valuable

technical assistance provided by S. M. Luker and L. E. Ruggles

during the course of this work. In addition, E. A. Salazar supplied us the EPR formulation used, as well as the benefit of his

expertise in organic material behavior.

ix

EXECUTIVE SUMMARY

As part of a study on the adequacy of Cobalt-60 sources to

simulate the radiation field accompanying a nuclear power plant

loss of coolant accident (LOCA), we have investigated the electron

charge disposition in ethylene-propylene rubber (EPR) insulation

specimens irradiated with monoenergetic electrons.

We studied the electron charge disposition in slab samples

irradiated by electrons with ranges both greater and much less

than the slab thickness, under terminated and unterminated sample

surface conditions. For terminated samples irradiated in a vacuum

environment with penetrating electrons, no evidence of charge

trapping was observed; however, we measured rapid charge buildup

on unterminated surfaces of specimens irradiated with electrons

with a range which was short when compared to the sample thickness. In this case the charge migrated to a terminated surface

within a matter of minutes.

Similar experiments were performed with samples exposed to

electron beams in an air environment. These samples exhibited no

tendency to accumulate charge on unterminated surfaces--this is

consistant with measured charge mobility in irradiated samples and

the presence of a highly ionized interface (due to air ionization

by the electrons).

Additional experiments were performed on short lengths of

cables similar to those used for nuclear power plant applications.

The results of this set of experiments were consistant with those

obtained for the slab samples.

Based on these experiments, we have tentatively concluded that

charge buildup is not likely to occur in EPR rubber insulation

exposed to electrons from a LOCA radiation environment so long as

the material is in contact with an ionized medium. Our results

can probably be extrapolated to other organics generally, and it

would appear that the LOCA beta spectra need not be exactly

duplicated in simulation tests.
I
1.0. INTRODUCTION

Sandia National Laboratories is currently conducting the

Qualification Testing Evaluation (QTE) Program [Reference 1] for

the U.S. Nuclear Regulatory Commission. This program, in part,

is concerned with the adequacy of gamma-radiation simulators

designed to test Class 1E equipment [2] under the radiation

environment characteristic of a loss of coolant accident (LOCA).

The radiation environment accompanying a LOCA is the result

of the distribution of electron and photon emitting fission

products throughout a reactor containment structure. It is assumed that the natural decay of these fission products will

create a radiation environment with these approximate specifications [3, 4]:

(1) An initial electron dose rate that is in excess of

10 Mrads/hr,

(2) an ultimate integrated electron dose that is on the

order of 300 to 400 Mrads,

(3) a time-dependent energy spectrum with an average

electron energy within the range of 0.2-0.8 MeV, and

(4) a photon component with dose rates about an order of

magnitude less than the electron values but with

average spectral energies comparable to those from

electrons.

Estimated values of the LOCA gamma and beta doses and dose

rates versus time after release are plotted in Figure 1.

Figure 2 shows how the in-containment gamma and beta particle

average energies are expected to vary with time after release.

The distribution of fission products throughout the containment will subject Class 1E components to combined electronphoton radiation effects. For our purposes we will define LOCA

radiation effects as energy and charge deposition in materials

and components exposed to the radiation environment described

above.

LOCA radiation effects are usually simulated using isotopic

sources, generally Cobalt-60. In contrast to a LOCA radiation

environment, Cobalt-60 decays by the emission of a 0.32 MeV beta

particle, and two photons with energies of 1.17 and 1.33 MeV.

Because of its low energy, the beta particle is (for all practical purposes) absorbed within the source. From the standpoint

of energy deposition, Co-60 irradiators can adequately simulate

the LOCA photon component in all aspects [5] (i.e., total energy

deposition, energy deposition profile, and deposition rate);

-3-
however, electron effects are in some instances less adequately

simulated and electron charge deposition probably not at all.

Cobalt-60 photon transport through air can result in the

development of a sizeable electron component. Photon transport

in air was calculated using the coupled electron-photon transport code, TIGER [61, to demonstrate the buildup of the recoil

electron spectrum. The results of this calculation are shown

in Figure 3, where we have plotted transmitted electron number

(charge) and energy, and the absorbed electron/photon dose ratio

as a function of air slab thickness. We note, (Figure 3),

simulation of a LOCA electron radiation environment by this

technique is probably not feasible when the LOCA electron/photon

dose ratio and the photon mean free path in air are considered;

i.e., the requirement of megacurie sources and an irradiation

cell with dimensions comparable to the recoil electron mean free

path in air (about 3 meters). In view of the above,. we chose

to investigate the electron effects with an electron accelerator

capable of providing electron energies and dose rates comparable

to those values associated with the LOCA beta spectra (0.2 to

1.0 MeV at dose rates between 1.0 to 40 Mrads/hr).

Specifically, we are concerned with the effects of electron

bombardment on the performance of organic cable insulations used

in reactor applications. Since there is a continuum of LOCA

electron spectra, we restricted this study to the effects of the

softest (4-day) and hardest (1-minute) spectra expected [3, 4,

5].

In Figure 4 we present the results of TIGER calculations,

which predict the electron charge distribution for the transport

of these two spectra through a representative thickness of a

typical ethylene-propylene rubber (EPR) formulation [7]. We

note the salient differences in the electron distributions

resulting from the two incident spectra. In one instance

(4-day spectrum) we observe that the distribution is

concentrated in the vicinity of the material front surface, in

the other case we see that the fraction of electrons deposited

in the first 0.05 cm of material is about a factor of 3 less.

The code used for this calculation predicts the electron

deposition, but not ultimate disposition; i.e., trapping,

mobility, etc. If the effects of electron bombardment on the

electrical performance of cable dielectrics is to be predicted,

then the ultimate disposition of deposited electrons must be

known. In other words, for an extended irradiation with

energetic electrons, is a charge buildup observed or does the

charge find its way to ground? Similar questions arise

regarding the electron distribution associated with low energy

electron transport.

-4-
We calculated the electron charge deposition in dielectric

samples irradiated with monoenergetic electrons, whose ranges

were both less than and greater than the sample thickness. For

comparison with the spectral results (Figure 4), we include

Figure 5, showing some representative electron distributions in

EPR slabs for several monoenergetic electron source energies.

As can be seen, the electron deposition profiles calculated for

both LOCA beta spectra, in Figure 4, are effectively bounded by

a superposition of the electron deposition profiles calculated

for the three monoenergetic beams (Figure 5).

Experiments were performed by exposing typical EPR materials

(slab and cylindrical geometries) to monoenergetic electrons

(0.2 to 1.0 MeV) in vacuum and ambient air environments. For

all incident electron energies (in vacuum), we studied charge

mobility for both voltage biased and unbiased conditions and the

effects of surface terminations on charge buildup in the material. Similar experiments, but with less extensive energy

ranges, were performed on the same materials and geometries with

the electron beam extracted into ambient air. By observing

surface voltages and the partition of the accelerator total beam

current, we were able to deduce the response of selected cable

insulation materials to LOCA-like electron radiation fields;

i.e., charge accumulation and surface potential buildup.

2.0. APPARATUS AND PROCEDURES

We used a Pelletron* electron accelerator to produce the

electron radiation fields used in our experiments. The accelerator is variable in electron energy and beam current. The

electron energy range is between 0.025 and 1.2 MeV, and beam

current is adjustable up to a maximum of 34 microamps. Uncertainties in the machine parameters (voltage regulation and

ripple) were carefully determined such that the electron beam

energy is known to within approximately 0.5% [8]. Total beam

current is determined with an "in-line" Faraday Cup, whose output is monitored with an electrometer. Most applications of

this accelerator (dose-depth determinations, transport code

verifications, etc.) [9, 10] require a tightly confined (i.e.,

small diameter) electron beam incident on the target. Our application, on the other hand, requires a diffuse beam pattern.

Accordingly, we designed a magnetic beam deflector system that

can produce rastered beams resulting in uniform rectangular

patterns with dimensions on the order of 15 centimeters. Preexperiment diagnostics allowed accurate prediction of beam

pattern size based on deflector magnet coil current, current

frequency, and electron beam energy. An example of these

*Manufactured by National Electrostatics Corporation,

Middleton, WI.

-5-
diagnostic data is presented in Figure 6, which depicts electron

deflection data as a function of magnet coil current for two

current frequencies. (A companion report [11], describing the

capabilities of the new accelerator-raster system and providing

all of the diagnostic results, will soon be published).

The beam deflection assembly is attached, in tandem, to the

Faraday Cup back flange. Integral with the accelerator is a

1.2 meter (diameter) stainless steel vacuum chamber, equipped

with several fixtures to accommodate in-vacuum experiments. In

Figure 7a the orientation of the accelerator (electron beam)

column, vacuum chamber, and the various components within the

vacuum chamber is shown. The electron beam is extracted from

the accelerator column and steered into the vacuum chamber

through the Faraday Cup mounted on the chamber interior wall.

A movable plate on the Faraday Cup permits either monitoring of

the total beam current (closed) or transmission of the beam into

the tank (open). Mounted on the Faraday Cup back surface is the

beam deflector (raster) unit. For in-vacuum irradiations, a

target mounting fixture (including a high-voltage feedthrough

insulator) is positioned near the vacuum chamber wall and diametrically opposite to the Faraday Cup. This configuration

allows maximum raster-target separation and hence maximum

electron beam pattern size.

For irradiations under ambient (air) conditions, the experiment package is moved in toto outside the vacuum chamber and

positioned behind the feedthrough insulator port. The following

modifications complete the transition: The high-voltage feedthrough insulator is replaced with a 0.003-cm thick mylar window

and, in order to define the electron beam prior to exit from the

vacuum tank, an additional mask is placed just ahead of the

mylar retaining assembly in the port. In Figure 7b, the experiment package orientation for ambient air irradiations is

shown with respect to the vacuum chamber.

Figure 8 shows the experiment (target) package configuration

used with slab geometry. It is essentially a three-element

unit. The lead element is an aluminum mask whose purpose is to

define the dimensions, or pattern, of the electron beam incident on the target surface. Additionally, the mask, whose

thickness is infinite to the most energetic source electron,

assures beam spatial uniformity across the front surface of the

sample in that the incident beam pattern is sized so that it is

always larger than the mask orifice. This condition assures

that the non-uniform electron distribution near the raster

periphery, due to the scanning magnet inductance, is masked

from the target. Adjacent to the mask element is the sample

holder. The holder, positioned approximately 1.3 centimeters

behind the mask, is a two-piece aluminum frame. This frame

-6-
allows rigid clamping of the test specimen samples. The frame

halves are secured with nylon screws so that the pieces are

electrically isolated and may serve as signal electrodes. The

entire frame and secured target are suspended in "free space"

via the frame support/signal bar. For in-vacuum experiments

the frame support fixture is connected to a high-voltage feedthrough insulator, located in a flanged port on the vacuum

chamber wall. Positioned 1.3 centimeters behind the sample

holder is an infinitely thick (to electrons) element consisting

of a 0.2-cm thick beryllium plate in intimate contact with a

0.3-cm thick aluminum plate. This stopper plate assembly allows

absorption of all electrons traversing the sample and the

beryllium component of the stopper assembly assures minimum

reflection of the transmitted electrons back into the test

sample's rear surface.

For the cylindrical configuration, the standard experimental

package was used to hold cable specimens during irradiation.

The cables were of sufficient length so that signal terminations

could be made outside the radiation environment. Orientation

of the cables in the experiment fixture is shown in Figure 9.

The two material sample configurations were studied--a slab

geometry and a cylindrical configuration. The slab geometry

samples are 0.15-cm thick generic ethylene-propylene rubber

sheets [7] comparable to EPR-type insulations currently used in

nuclear plant cable applications. Both surfaces of the test

specimens are coated with a five-micron layer of vacuum

deposited aluminum. The aluminum coating allows application of

bias voltage and/or detection of surface currents/potentials.

In some applications with terminated surfaces and either zero

or low applied voltages, samples with complete coating on both

surfaces were used. We assured by experiment that no alternate

electrical paths developed across the small sample dimension.

In charge buildup experiments involving unterminated surfaces,

we increased the surface-to-volume breakdown path ratio by removing a portion of the aluminum coating from one surface. This

procedure reduced the coated area to about 7.5 square centimeters, as opposed to a total, single-surface, coated area of

58 square centimeters. Electrical contact to the smaller aluminum coating was made by cementing a #20 varnished copper wire

to it. The modified sample geometry, with termination lead, is

shown in Figure 8. Contact to the other (fully-coated) surface

was made through the sample retaining frame. In applications

using samples with reduced surface coating, the fully-coated

surface was oriented toward the electron beam. A mask with an

orifice smaller than the reduced coating dimension was always

used in these applications.

-7-
The cylindrical configurations are representative of cables

used in nuclear plant applications. Both sample types have

0.08-cm ethylene-propylene rubber insulation, one encased in a

0.04-cm thick chlorosulfonated polyethylene jacket and the

other in a 0.04-cm thick neoprene jacket. The conductor is a

0.2-cm diameter tinned copper wire.

Extensive dosimetry measurements were made prior to the

actual experiments. The electron beam pattern size and uniformity were characterized as a function of raster parameters

using 0.005-cm thick dye-loaded nylon and polychlorostyrene

thin-film dosimeters. Film calibration (response) to ionizing

radiation was established by exposing equilibrated films to a

Cobalt-60 gamma source, positioned in a dry (air) cell.

Cobalt-60 dose and dose rates were measured using an NBS traceable air ionization chamber (Victoreen 550 Radacon III). Dosimeter response measured was the growth of absorption peaks in

the film, as a function of absorbed energy. The absorption data

was measured with a recording microdensitometer. In addition

to beam uniformity measurements, a correlation between total

beam current and specimen dose was established. For several

beam energies and currents, we used the dye-loaded plastics to

measure the resultant doses in the sample holder frame. The

corresponding beam currents detected by the beryllium stopper

were then used to estimate the doses using calculated current

to-dose conversion factors [11] appropriate for the geometry.

The results of the measurements and calculations are presented

in Figures 10 and 11. Figure 10 shows the relationship between

Faraday Cup and beryllium stopper currents for each beam energy.

The relationship between dose rates, as measured with the films

and calculated on the basis of stopper currents, are given in

Figure 11. The fall-off in dosimeter response at the higher

dose rates shown in Figure 11 is attributed to oxygen depletion

in the dosimetry films during these irradiations, which were

made in a vacuum environment. For confirmation, we compared the

response of dosimeters irradiated, to identical doses, in

vacuum and ambient air environments. The indicated dose for the

dosimeter irradiated in the vacuum environment was less than

indicated dose for the dosimeter exposed in the air environment.

Further, the electron dose measurements in air environments

agreed with doses calculated on the basis of beryllium stopper

currents.

Our study of electron effects in dielectrics has consisted,

thus far, of examining electron charge mobility and surface potential (voltage) buildup in slab and cylindrical geometries

for both vacuum and ambient (air) environments. Since reactor

accident radiation environments can be represented as quasi

steady state, all sample irradiations were in a steady state

radiation field. We studied charge mobility by observing surface and transmitted electron currents using slab geometry samples with terminated surfaces. Figure 12 presents a schematic

-8-
of the instrumentation used for the charge mobility measurements. The E-i's are electrometers used to detect and measure

mask, surface, and stopper currents. Mask, back surface, and

stopper current records are obtained with, the R-i chart recorders. Back surface bias is applied by a continuously variable bipolar DC power supply in the range of + 600 VDC. In

anticipation of possible charge trapping and voltage break-down,

we included the capability of capacitive coupling of an oscilloscope to the sample back surface. Charge mobility, with both

sample surfaces terminated, was such that no evidence of charge

breakdown were observed with the oscilloscope. For mobility

measurements we observed the I-i currents for bias voltages in

the range of + 600 VDC. Knowing the Faraday Cup current, we

were able to accurately estimate the total current incident on

the experiment package. Conservation of steady state currents

incident on the package, coupled with the absence of highfrequency transients was taken as evidence of charge mobility

within the material. For the surface potential buildup experiments, we replaced E-2 with an electrostatic voltmeter and

shunted the bipolar power supply; the sample back surface was

then terminated to ground through the electrometer, E-3. Front

surface voltage buildup was restricted to 12.5 kV--the voltage

rating on the high voltage feedthrough insulator. For the front

surface potential measurements, the sample was irradiated until

either breakdown occurred or a steady state surface potential

was established.

Charge mobility measurements on the cylindrical specimens

were more limited. The samples, representative of materials

used in nuclear plant installations, have no conductive coating

on the outer surface, thus barring front surface termination.

In this instance, charge mobility determination was restricted

to monitoring the central conductor current. For energetic

electrons, with a range greater than material thickness, we assumed establishment of a steady current as a measure of charge

mobility. Front surface potential buildup and discharge, on the

other hand, was assumed--independent of electron energy--if

sporadic current spikes were observed on the central conductor.

In addition, because of the capacitive coupling between the experiment package elements, spontaneous loss of charge (breakdown) in the test sample always resulted in induced spikes being

superimposed on the beryllium stopper current.

-9-
Ambient-environment experiments were similar to, but somewhat abbreviated versions of, those performed in vacuum; i.e.,

these irradiations were done at 0.4 and 1.0 MeV only.

3.0. RESULTS

3.1 Slab Geometry

3.1.1 Terminated surfaces--vacuum environment

Electron disposition in 7.6 by 7.6 by 0.15-cm generic EPR

rubber insulation samples was measured as a function of incident

electron energy and applied surface bias voltage. The circuit

shown in Figure 12, but without the oscilloscope, was used to

measure current disposition. Pressure in the vacuum chamber was

in the sub-millitorr range during all of the experiments.

Monoenergetic electrons with energies of 0.2, 0.4, 0.6, 0.8, and

1.0 MeV were used in these determinations. Thus the range of

bombarding particles varied from much less to greater than the

sample thickness. The electron beam pattern was approximately

9 by 9 cm square. An aluminum mask reduced the beam pattern,

incident onto the sample, to a 6.4 by 6.4 cm square area. Depending on the experiment, the radiation dose rate delivered to

the sample front face was between 1.0 and 40 Mrad(air)/hr. Bias

voltage was applied to the sample rear surface in all instances

except for one 0.4 MeV exposure, where the bias was applied to

the front surface. Each irradiation was of sufficient duration

to assure that steady state conditions had been achieved.

Table 1 gives a tabulation of the 1.0 MeV data. The mask

data (Column 1) have been corrected [10] for electron backscatter. As may be observed from the last column, the ratio of

partitioned current to incident current, the incident beam current is well accounted for. Similar data were obtained for the

other, lower energy, irradiations; i.e., a reasonable accounting

for the total current disposition.

Figure 13 is a graph of the 1.0 MeV data. The surface currents have been normalized on the basis of the incident (Faraday

Cup) current. Of interest is the indication of charge mobility

within the sample due to the presence of a biasing voltage.

Some changes in the observed back surface current are, no doubt,

influenced by the effect of the back surface bias voltage on

electron reflection from the beryllium stopper.

The 0.6 MeV data are shown in Figure 14. Since the 0.6 MeV

electron range is also greater than the sample thickness, the

-10-
data are similar to those observed for the 1.0 MeV irradiations.

Of note, however, is the increased normalized surface currents

compared to their 1.0 MeV counterparts. This observation is

consistent with calculated results which predict increasing

charge deposition with decreasing electron energy.

Figures 15, 16, and 17 show plots of the 0.4 Mev irradiation

data. Since 0.4 MeV electrons have a range less than the material thickness, we note the zero back surface current. Note

also the ineffectiveness of the back surface bias. The effectiveness of the front surface (negative) bias in preventing

secondary electrons from returning to the material front surface

is demonstrated in Figure 16; also observe the increase in front

surface current, due to collected secondaries when the applied

bias voltage is increased in the positive direction. (This effect is dramatically shown on the expanded surface current scale

in Figure 17). Some of the observed increase in front surface

current might be due to the collection of low energy mask recoil

electrons drawn through the mask aperature.

The data for the 0.2 MeV irradiations are plotted in Figure

18. These data are consistant with the 0.4 MeV back surface

biased data. Table 2 gives the current partition data for the

0.2 MeV irradiation. This table is identical in format to

Table 1. Of interest in this tabulation is the zero back surface and beryllium stopper currents.

Since, in all cases, both surfaces were terminated (to

ground) through instruments with internal impedance on the order

of one megohm, surface potentials achieved were on the order of

volts.

Table 3 is a tabulation of the charge disposition data for

each (energy) irradiation. The sum of normalized front surface

and back surface currents versus beam energy is tabulated. The

sums of the measured results are compared to the calculated

total (normalized) number of electrons stopped within the EPR

material for the incident energies. Agreement between measured

and calculated values may be seen in the last column. Note that

this table is for terminated surface cases only, but this table

and the data in Tables 1 and 2 taken together show that electrons stopped in the EPR are mobile enough (to go to ground)

such that no charge trapping occurs.

3.1.2 Unterminated front surface--vacuum environment

Front surface charge/potential buildup in the slab geometry

was studied with 0.2, 0.4, 0.45, 0.6, 0.8, and 1.0 MeV electron

beam energies. The circuit shown in Figure 12 was used with

-11-
the electrometer E-2 and the bias supply removed. An electrostatic voltmeter replaced the electrometer E-2. For these experiments, the back surface aluminum coating was reduced to a

2.9 by 2.9 cm square and, in addition, the mask orifice was reduced to a 2.5-cm diameter circular opening. In line with the

reduction in the mask orifice, the beam pattern was reduced to

4.8 by 4.4 centimeters. Because of the considerable capacitance in the total system, these irradiations were performed at

dose rates in the range of 10 to 40 Mrad(air)/hr. Since the

high-voltage components, the feedthrough insulator and electrostatic voltmeter, were rated at 12.5 kilovolts, our experimental

strategy was to irradiate the samples until an equilibrium front

surface voltage was achieved or breakdown occurred, unless it

was evident that the front surface potential would exceed the

equipment rating; in that case the experiment would be terminated as the equipment voltage rating was reached.

Using these guidelines, we studied charge buildup accompanying 1.0 MeV irradiation at dose rates of 10, 20, and 30

Mrad(air)/hr. No evidence of dielectric breakdown was observed

and only modest increases in front surface voltage accompanied

the increases in dose rate. Samples were then irradiated with

0.2, 0.4, 0.45, 0.6, and 0.8 MeV electrons. Sample behavior was

not always consistant. Charging times for electrons with ranges

short compared to sample thickness varied from tens of seconds

to the order of tens of minutes. These anomalies may have been,

in part, due to the high-voltage system. For example, upon

reaching a front surface voltage equilibrium we were unable to

account for the total incident current disposition.

The front surface voltage buildup results are presented in

Figures 19 and 20. In Figure 19, front surface equilibrium

voltage is plotted as a function of electron energy. The data

at 0.2 MeV are probably dependent on the "standoff" capabilities

of our system. Figure 20 shows the observed back surface current associated with the same beam energies. The decrease in

observed back surface current with increasing electron energy

is due to the increased range of the more energetic particles.

In general, we observed a voltage buildup on the unterminated (from ground) surface. Additionally, for irradiations with

electrons with a range greater than the sample thickness, breakdown did not occur. In all other cases, breakdown occurred some

where in the system.

During the course of these experiments, we observed evidence

of radiation damage. Since we intend to investigate this possibility further, we present the data without elaboration.

These data were obtained as explained earlier. Figure 21 shows

a plot of the time history of the front surface voltage of a

-12-
sample irradiated with 0.2 MeV electrons, electrons with a range

much less than the sample thickness. We observed a front

surface equilibrium voltage of approximately 12 kilo-volts.

The discontinuity indicates cut-off of the electron beam with

subsequent decay of the accumulated charge on the sample.

Following a short irradiation (approximately five minutes) with

1.0 MeV electrons we repeated the irradiation (at 0.2 MeV) and

observed an equilibrium voltage on the order of one half the

pre-l.0 MeV irradiation value. Similar data for 0.45 MeV electrons, range approximately equal to the sample thickness, are

presented in Figure 22.

3.1.3 Terminated surfaces--ambient environment

For these experiments, the electron beam was extracted

through a 0.003-cm mylar window and onto the standard experiment

package (shown in Figure 8). An additional mask was placed just

ahead of the mylar. Instrumentation was as depicted in Figure

12. For these experiments the small electron beam pattern and

2.5-cm mask aperature were used. Exposures were at 30

Mrad(air)/hr with 0.4 and 1.0 MeV electron beam energies. Surface currents as a function of back surface bias voltage were

again measured.

The 1.0 MeV data are presented in Table 4. The data presented are the same as in Table 1 except for the addition of a

second mask current (IM2). The point of interest in this tabulation is the presence of the large ionization currents existing

in the gaps between the experiment fixture elements and detected

by the enhanced front surface, back surface, and beryllium

stopper currents. The magnitude of these currents was sufficient to mask the primary surface currents. Similar data were

obtained for the 0.4 MeV irradiation, and are shown in Table 5.

Figures 23 and 24 are plots of the currents measured by the

front and back surface electrometers as a function of bias

voltage for the 1.0 and 0.4 MeV irradiations, respectively. As

may be observed in both cases, the onset of voltage saturation

of the air-generated ion current is not indicated (i.e., no

tendency for the back surface current to reach a limiting value

with increasing back surface bias voltage).

3.1.4 Unterminated front surface--ambient environment

Breakdown determinations were made with 1.0 and 0.4 MeV

electron energies at approximately 24 Mrad(air)/hr for each

energy. During the 1.0 MeV beam energy irradiation, the front

surface voltage was observed for approximately five minutes.

During this exposure, the front surface reached an equilibrium

voltage of 100 volts within 30 seconds and the back surface

-13-
current remained constant at 0.06 microamps. No current spikes

were superimposed on either the back surface or beryllium stopper current traces. This experiment was repeated with a 0.4

MeV beam . Front surface voltage equilibrium was again achieved

in 30 seconds, but at 200 volts. The back surface current remained at zero throughout the irradiation, with no evidence of

current discharge transients. Following these two irradiations,

the sample was again exposed in the vacuum environment to the

0.4 MeV energy beam to confirm that the sample could maintain a

front surface (high) potential. An equilibrium front surface

voltage of 2.4 kV was obtained within one minute.

3.1.5 Effect of sample aluminization

The effects of sample aluminization on charge mobility and

surface potential buildup were examined in both vacuum and ambient environments. For these experiments, the aluminum coating

was removed from the entire front surface of the samples. Potential buildup and breakdown were monitored by observing back

surface current disturbances and their induced effects on the

beryllium stopper currents. In addition, an oscilloscope was

capacitively coupled to the sample back surface.

No effects were observed that could be attributed to the

aluminization. A thirty minute ambient exposure to the 0.4 MeV

beam, at 25 Mrad(air)/hr produced no evidence of surface buildup

and/or breakdown. In the vacuum environment and at low beam energy exposures, however, breakdown occurred in seconds--

indicative of the magnitude of the external (measuring) circuit

capacitance.

3.2 Cylindrical Geometry

Experiments on the cylindrical geometry were similar to

those performed on the slab geometry, except that studies of

charge mobility and surface potential buildup were restricted

to beam energies of 0.4 and 1.0 MeV. Surface potential buildup

and discharge were detected by observing transients appearing

on the central conductor current trace and induced transients

appearing on the beryllium stopper.

3.2.1 Ambient environment

The two cylindrical, 23-cm long, samples were exposed to the

0.4 MeV beam energy at a dose rate of 25 Mrad(air)/hr for 10

minutes. Central conductor and beryllium currents were observed throughout the exposure. Central conductor currents

were zero and free of any transient spikes. The beryllium

stopper current was constant at 0.19 microamps and exhibited no

imposed induced transients.

-14-
3.2.2 Vacuum environment

These two samples were exposed to both the 1.0 and 0.4 MeV

beam environments at a dose rate of 30 Mrad(air)/hr. The 1.0

MeV beam energy was performed first. For this exposure, central

conductor currents were steady state with no evidence of current

breakdown pulses. In addition, the beryllium stopper current

trace was also steady state with no evidence of induced current

spikes. Cable behavior during the 0.4 MeV beam irradiation was

markedly different. Both cylindrical central conductor currents

were zero, consistent with the electron range compared to sample

thickness. Superimposed on the zero current traces were current

spikes indicative of current discharge to the central conductors. These spikes appeared within 10 seconds of the onset

of irradiation. The accompanying beryllium stopper current

trace detected these discharges via induced transient current

spikes.

4.0. CONCLUSIONS

We investigated the response of different EPR-type rubber

insulation materials to electron environments with electron

ranges both less than and greater than the sample thickness, and

for both vacuum and ambient environments.

Results from the vacuum environment experiments suggest that

charge trapping, if present, is insignificant. For electrons

with ranges short compared to sample thickness, a surface

buildup and discharge will occur for an unterminated surface;

however, if the surface is terminated, charge flow will inhibit

buildup and breakdown. For energetic electrons, buildup will

not occur regardless of termination.

In ambient environments, air ionization produced by the

electrons is of sufficient magnitude to neutralize any surface

charge accumulation by the irradiated sample regardless of the

incident electron energy; i.e., electron range less than or

greater than the sample thickness. These results were observed

to be independent of the sample geometry--slab or cylindrical.

Since the experiments were designed to approximate the reactor loss of coolant accident (LOCA) electron radiation field,

we believe that these data may be applied to predict the disposition of LOCA electrons incident on organic-insulated

electrical cables and the response of these insulations to the

electrons. Thus we conclude that so long as "EPR-like" insulated cables are in contact with an ionizable gaseous medium,

charge buildup and discharge in the cable rubber insulation will

not occur as the result of exposure to the LOCA electron radiation environment.

-15-
We observed radiation damage to the insulation material

after short exposures to 1.0 MeV electrons, in vacuum, with a

lowering of ultimate achievable surface potential buildup. This

lowering of ultimate surface potential was also accompanied by

an inability to account for the total incident electron beam

current. This damage phenomonon needs further study as does the

combined electron-photon effects on charge transport, mobility,

and ultimate surface voltage buildup.

-16-
REFERENCES

1. L. L. Bonzon, et al., "An Overview of Equipment

Survivability Studies at Sandia National Laboratories

(SNL)," SAND83-0759C, Sandia National Laboratories,

Presented at the International Meeting on Light-Water

Reactor Severe Accident Evaluation Conference, Cambridge,

MA, August 28-September 1, 1983.

2. IEEE Standard for Qualifying Class 1E Equipment for Nuclear

Power Generating Stations, IEEE Std. 323-1974, The Institute

of Electrical and Electronic Engineers, Inc., 1974.

3. L. L. Bonzon, "Radiation Signature Following the

Hypothesized LOCA," SAND76-0740, NUREG76-6521, Sandia

National Laboratories, Rev. October 1977.

4. L. L. Bonzon, et al., "Definition of Loss-of-CoolantAccident Radiation Source: Summary and Conclusions,"

SAND78-0091, Sandia National Laboratories, May, 1978.

5. L. L. Bonzon and W. H. Buckalew, "Evaluation of Simulator

Adequacy for the Radiation Qualification of Safety-Related

Equipment," SAND79 1787, NUREG/CR-1184, Sandia National

Laboratories, January, 1980.

6. J. A. Halbleib, Sr. and W. H. Vandevender, Nuclx. Sci.

Eng., 57, 94(1975).

7. E. A. Salazar, D. A. Bouchard, and D. T. Furgal, "Aging With

Respect to Flammability and Other Properties in FireRetarded Ethylene Propylene Rubber and Chlorosulfonated

Polyethylene," SAND81-1906, NUREG/CR-2314, Sandia National

Laboratories, March, 1982.

8. G. H. Miller and G. J. Lockwood, IEEE Trans. Nucl. Sci.,

NS-22, 1072(1975).

9. G. J. Lockwood, L. E. Ruggles, G. H. Miller, and J. A.

Halbleib, "Calorimetric Measurements of Electron Energy

Deposition in Extended Media - Theory vs Experiment,"

SAND79-0414, Sandia National Laboratories, January, 1980.

-17-
10. G. J. Lockwood, L. E. Ruggles, G. H. Miller, and J. A.

Halbleib, "Electron Energy and Charge Albedos -

Calorimetric Measurements vs Monte-Carlo Theory,"

SAND8O-1968, Sandia National Laboratories, November, 1981.

11. W. H. Buckalew, G. J. Lockwood, and F. J. Wyant,

"Capabilities and Diagnostics of the Sandia

Pelletron-Raster System," (Unpublished), 1983.

-18-
I-

'.0

0

rt,

0

(D1

0

0

H

rt,

Gama Dose/Dose Rote Beto Dose/Dose Rote

L

L

0

0)

Contaosnment Volume RodI~otLon EnvLronments
I~s

0

Tj

FJ.

(D

(D

to

I:1

Ct

m

0

Average Gamma-ýoy and Beta

PartLcLe Ener g Ln RLr versus

Reactor CooLLng TLme

L

U

Q..

0

CD

o

a

C-0)

LEGEND

o Gammas

A - Betas

ilo-" .1 UUUUIIj I I I 115111 U I * *I*..... .*.. .. . .' . . . . .. m

1O-I 10-3 10"L CooLLn9 10 10, be 10

T~me, do~js
ImT

tm 0

"~ IU)

wm

0

tzr0

0

p)rtii

I-a. 0

M 10

(DU.

m*

ELectron Number-Energy TransmLssLon

versus SLob ThLckness

1.25 MeV Photons LncLdent on

STP ALr SLobs
rLOI.J

ELectron OeposLtLon Ln EPR

(NormoLLzed to one LncLdent portLcLe)

versus Depth Into SLab

00 EPR SLob ThLckness - .1524 cm (60 miL)

0~r 0

00

Nr

0* m

0

m C2

0

W Ca

rt L

N LOCI

00

1-hII

c .UO 0.am 0.WO 0.WS a. 00 0.125 0.150

0 Depth, ca

0

CDuc

FR BETH SPECTRUMI
mz

r~a

ELectron Depos~tbon Ln EPR

(NormaLLzed to one LncLdent. partLcLe)

versus Depth Into SLob

EPR SLob ThLckness - .1524 cm (60 mLL)

I

0 0

rt ft

00

01*

0

0

I-l.

0

0

0

0D

rt

ca

.JC

0

0

ci

0

C4

0;.

L

la-

...................

----------

* I

:................ - -$ I - * 0

* a

.. I

* * I

MCJIENT ENERGY

1.0 Hey 0.4 MeV

3

I I I I I

0.000 0.025 0.050 0.075 0.1(3

Depth, cm

0.125 0.150 0.175 0.20
MaxLmum Beom DefLectLon versus

Raster CoLL Current

boom energu - 1 .0 MeV

. tor-get.-coLL sepoaratLon = 70 cm

OCO

0

0 I-i I M, a• .• 

a

H 10

CD w

0

-0 LEGEND ftC

H-0.0 0.2 0.4.012 . 1.6 182.0

O Peaik CoLL Current, ifmpere8 .

"I-
High-Voltage

Feedthrough Support and Signal Bar

Vacuum Chamber

Figure 7a. Schematic of Experiment Orientation

In Vacuum Chamber.

Aylar Holder

Al A MaskSample

Al Mask Be

Al

Rastered Electron Beam

Sample Holder

Vacuum Chamber Mylar Window

Figure 7b. Schematic of Experiment Orientation

Outside of Vacuum Chamber.

-25-

Bar

I?

Electron

Bearn

W.(tP)Alumn~fum 0 

EBeryfium I

Cdol* Sal*p

H&e 

ncoi

Rate ote

Akxri"n Msk Ah 25--cm YOreter
I-I.

Ii

a'

H

0

0

Ber~yLLLum Stopper Current versus

Forodoy Cup Current

I1

w

0

ft

.4

C-)

rt

a'

L)

E

L

3~

0)V

0.

.01

rCma

LEGEND

o - 1.0 MeV

V - 0.8 MeV

" - 0.6 Mev

A - 0.4 MeV

0 - 0.2 MeV

0

-i

0.0 0.2 0.4 I0.6 0.8 i.0 1.2

Faraodag Cup Current, plimperes

1.4 1.6 l.a . 2.0
I0

m~

rt. m

0 11

t

oC

Cti

'0

rt1 0

In

ct

rn

0)

(nw

CD

Bern

R

/// //

,," 

/

/

/

I

L

-c00

02 M

0

Dose Rote versus

,.LLum Stopper Current

Lnes are caLcuLated values)

0A0

LEGEND

- 1.0 MeV - 0.4 MeV - 0.2 MeV

BeriLLLum Stopper Current, pAmp
STOPPER

SAMPLE

v B

E- R-1E-2 E- 4

-3 E

I I

I SCOPE

Figure 12. Schematic of Signal Measuring Configuration

-30-
MI~

H

U)

(D M

0-9

(D(

AS

M

I.'

0)

H

CD

I-h

0

IQ

E

r I

0

L

S3

00-

Front and Back Surface Currents

(NormaLLzed to Frorodo Cup Current)

versus Back Surface BLas VoLtage beam energ9 - 1.0 MeV

LEG•ND

o- FRONT SURF'ACE

a - BRCK SURFRCF

...........

-flW.O -#AN.O -2B0.0 0.0 2o0.0

Bi~ae VoLtage,. voLts

86.O
MFront and Back Surface Currents

(NormoL.,zed to Forcdog Cup Current)

versus Back Surface BLas VoLtage

boom energg - 0.6 MeV

M C; •(D •"

OC/

0~ ~ LEGEND

(D l C

a a -FRONT SURFRCE

toi M BRCK SURFACE

ca-t

0 0

0 M

-600.0 -us0 -0.000•00. . O

.Ca

L

w L

w

(A

0-M0.0 -4(3.0 -200.0 0.0 200.0 4(LO0 G0.0

.Via. VoLtao9, voLtes
I-I.

Ii

(0

I-h

Lfl

IA

(A

rtom

I-h

0 En

(00

lrI,

0ow

:jDo

M 0C

I-'I*

CO

A

CL

9

4.C

2

Front and Back Surface Currents

(NormaLLzed to Faroadog Cup Current)

versus Back Surface BLas VoLtage

beam energq - 0.4 MeV

X AX

LEGEND

x - FRONT SURFAC

*- BACK SURFACE

B

-IO0. -4w0.0 -2d0.O BLoc 0.o

VoLtoge, 260.0 4dx.0 60.
I~J

I.J.

II

0

I~1 0~

I

I

0(DO

P-h

0W

00

0 M

ra :-I

0

rtzi

1-hl

(D

(to

0

aL

E

0

4.

L

00

o-.

4.-a;

La

(8

Front and Bock Surface Currents

(NormaLLzed to ForodaS Cup Current)

versus Front Surface BLas VoLtage

boom energ• - 0.4 M*V

LEGEND

N - FRONT SURFACE

* - BRCK SURFRCE

-'U V V V V V

-600.0 -400.0 -200.0

BLos

CI.O

VoLtage,

T

200.0

voLts

400.0 600.0
I-.

Front Surfoce Current

0 (NormoLLzed to rorodoS Cup Current)

versus Front Surface BLos VoLtage

boom energj - 0.4 MeV

,..j P-* m 00.W rt N

• °

0.0

00.

M. 0

om 0

(o

PI La

S000

rtU

110 L

m/

%0

<0

En rI

0

S-600.0 -100.0 -200.0 0.0 2C6.0 40.0 G0.0

0 BLO VoLtoge, voLts

0
mz

H~a

?0

Front and Back Surface Currents

(NormoL.zed to ForodoSl Cup Current,)

versus Back Surface BLas VoLtage

beoi energS - 0.2 MeV

I•o

o•

I

0 CA

mI

La

Ftl-

(rt

su

0D

C

Q.

i.O0.

L

*0

L

U

(n

C;"

8

U.

LEGEND

v - FRONT SURFACE

m - BACK SURFACE

L-.C :-.- :-._ ý ý ý m- ý

-Mo.0 -4w0.0 -20.0 0.0 20.0

BLos VoLtage, voLts

I 4W0.0 600.0
t.I.

I*1

CD

H

~0

Front Surface EquLLLbrLum VoLtage

as a functLon of

Beam Energy

LA~

-4

z0d

CD

M H~

~pi

0

L'i M

Ct'i

I-I.

0'

I"

0)

(D

0

(*t

0

0

0

0

0)

L

2

C!

0

L

S

a S.

S

S S

S S

-'I. I I

0.0 0.2 0.3 0.4 0.5 0.6

Beam Er•"g9j, MeY

0.7 0.6 I.0
I*D

N<

Io

CD

0

PV

0

t*1

0

CD

0

I*1t

En

0

M'

I.'

0

Back

as

IV

S.O0

soL

U0.

Ci.X

Surface Current

a functLon of

Beam Energy

II - I -

I m I I

0.0 0.1 0.2 0.3 0.4 CAS 0.6

Boom Energ~j, 11eV

I I l m

0.7 0.6 0.9 1.0
I-.

EPR Front Surface VoLtage versus

rT.me, Before and After IrradLotLon

NLth 1.0 MeV ELectrons

Ebom energ- 0.2 MeY

0 ftl"

II

•M BEOR

0

M

0 M

i-r1

0 >"

CC2

oU

LEEN

NC 1

0.0 100.0 200.0 300.0 4d.0 !&.0 ý.0 700.0

ft~wep seconds
I

192

(DM

0

Hrt

La 0

0

IC4

(D

to

CA

to

0

(D

C!

LEGEND

o - BEFORE

0' 0 a - AF'TER

EPR Front Surface VoLtage versus

TLme, Before and After Irrad~atLon

WLth 1.0 MeV ELectrons

boom energS - 0.45 MoV

L

C

U)

0

1&. L
Il..

m Front and Bock Surface Currents (NormoL~zed to rorodog Cup Current)

versus Back Surface BLos VoLtage .°~ beam energy 1.0 MeV, LrrodLotAon Ln oLr

em0 (A

LEGEND

o- FRONT SURF'ACE

I-' I"1I

t*m %%..BCKSRFC

A. I•..' ,. 0

C.)

0

H I

I-" _ _ _ _, ,__ _,_ _ _ ,_ _ ,

0I "-)O.O -400.0 -200).0 0.0 200.0 4100.0 600.0

BLos VoLtoge, volts
Front and Bock Surface Currents

m (NormaLLzed to Forodo9j Cup Current)

versus Bock Surface BLas VoLtage

"boom enrg9 - 0.4 1eV, LrrodLotLon Ln oLrI4

0 Enx

m00

00

IDC 0 0

(tz(

0 m

ow

Vrt

(D LEGEND

H~x - FRONT SURFACE

a~9 - BACK SURFACE

*U)3

W C

0

r1

Al

0-GW.0 -400.0 -200.0 0.0 200.0 dooo h Blas VoLtage, voLte

X
TABLE 1. Incident Beam Current Partition for 1.0 MeV Source

Electrons in Vacuum Environment*

VB

600

400

200

0

-200

-400

-600

IM

2.122

2.048

2.042

2.042

2.042

2.029

2.037

IFS

0.060

0.061

0.069

0.073

0.079

0.083

0.089

IBS

0.177

0.170

0.160

0.142

0.114

0.106

0.096

IBE

0.500

0.510

0.523

0.542

0.560

0.570

0.576

ISUM

2.859

2.789

2.794

2.799

2.795

2.788

2.798

IFC

2.850

2.825

2.830

2.835

2.840

2.835

2.830

RATIO

1.003

0.987

0.987

0.987

0.984

0.983

0.989

*All currents in units of microamps

I

VB

IM

IFS

IBS

IBE

ISUM

IFC

RATIO

=

=

=

=

Bias voltage on back surface

Mask current

Front surface current

Back surface current

Beryllium stopper current

Sum of IM, IFS, IBS, and IBE

Faraday Cup current

ISUM/IFC

(volts)
TABLE 2. Incident Beam Current Partition for 0.2 MeV Source

Electrons in Vacuum Environment*

VB

600

400

200

0

-200

-400

-600

IM

1.130

1.130

1.130

1.130

1.130

1.130

1.130

IFS

0.400

0.400

0.400

0.400

0.400

0.400

0.400

IBS

0.000

0.000

0.000

0.000

0.000

0.000

0.000

IBE

0.000

0.000

0.000

0.000

0.000

0.000

0.000

ISUM

1.530

1.530

1.530

1.530

1.530

1.530

1.530

IFC

1.510

1.500

1.500

1.500

1.500

1.500

1.500

RATIO

1.013

1.020

1.020

1.020

1.020

1.020

1.020

*All currents in units of microamps

!

I VB * Bias voltage on back surface (volts)

IM = Mask current

IFS Front surface current

IBS = Back surface current

IBE - Beryllium stopper current

ISUM = Sum of IM. IFS. IBSo and IBE

IFC Faraday Cup current

RATIO - ISUM/IFC
TABLE 3. Comparison of Measured And Calculated Fraction of

Incident Electrons Stopped Within EPR Volume*

EB

0.2

0.4

0.6

1.0

*e(calc)

0.998

1.004

0.904

0.222

*e(meas)

1.000

1.000

0.937

0.284

RATIO

0.998

1.004

0.965

0.782

*Fraction of incident electrons normalized to

one incident particle

EB . Electron beam energy (MeV) *e(calc) = Fraction of incident electrons in EPR

calculated by TIGER code

7 *e(meas) - Fraction of incident electrons in EPR

measured during experiment RATIO = *e(calc)/#e(meas)
TABLE 4. Incident Beam Current Partition for 1.0 MeV Source

Electrons in Ambient Air Environment*

VB

600

400

200

0

-200

-400

-600

IM1

3.167

3.167

3.167

3.200

3.167

3.167

3.167

IM2

0.040

0.060

0.090

0.110

0.013

0.150

0.160

IFS IBS IBE ISUM IFC RATIO

-0.115

-0.093

-0.062

0.008

0.069

0.103

0.128

4.700

3.400

1.700

-0.080

-1.930

-3.300

-4.600

-4.000

-2.900

-1.200

0.130

1.950

3.200

4.300

3.792

3.634

3.695

3.368

3.269

3.320

3.155

4.020

4.020

4.020

4.020

4.020

4.000

4.000

0.943

0.904

0.919

0.838

0.813

0.830

0.789

I

Ab

*All currents in units of microamps

VB 0 Bias voltage on back surface (volts)

IMI a In-vacuum mask current

IM2 = In-air mask current

IFS - Front surface current

IBS = Back surface current

IBE = Beryllium stopper current

ISUM = Sum of IM1, IM2, IFS. IBS. and IBE

IFC = Faraday Cup current

RATIO = ISUM/IFC
TABLE 5. Incident Beam Current Partition for 0.4 MeV Source

Electrons in Ambient Air Environment*

yE IMi IM2 IFS IBS IBE

600

400

200

0

-200

-400

-600

3.072

3.105

3.116

3.183

3.161

3.128

3.128

0.190

0.170

0.250

0.300

0.300

0.320

0.240

-0.039

0.000

-0.005

0.025

0.100

0.127

0.250

0.520

0.370

0.200

0.020

-0.215

-0.385

-0.540

-0.085

-0.066

-0.045

0.001

0.047

0.070

0.087

ISUM

3.658

3.579

3.516

3.530

3.393

3.260

3.165

3.920

3.950

3.950

4.080

4.020

4.000

3.980

IFC RATIO

0.933

0.906

0.890

0.865

0.844

0.815

0.795

I

-0.

*All currents in units of microamps

VB - Bias voltage on back surface (volts)

IMI - In-vacuum mask current

IM2 = In-air mask current

IFS = Front surface current

IBS - Back surface current

IBE = Beryllium stopper current

ISUM - Sum of IM1, IM2, IFS, IBS, and IBE

IFC = Faraday Cup current

RATIO - ISUM/IFC
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