ASTM E721-94

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
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Designation: E 721 – 94
Standard Guide for
Determining Neutron Energy Spectra from Neutron Sensors
for Radiation-Hardness Testing of Electronics
This standard is issued under the fixed designation E 721; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope E 262 Test Method for Determining Thermal Neutron Re-
action and Fluence Rates by Radioactivation Techniques
1.1 This guide covers procedures for determining the
E 263 Test Method for Measuring Fast–Neutron Reaction
energy-fluence spectra of neutron sources used in radiation-
Rates by Radioactivation of Iron
hardness testing of electronic semiconductor devices. The
E 264 Test Method for Determining Fast-Neutron Reaction
types of sources specifically covered by this guide are fission or
Rates by Radioactivation of Nickel
degraded energy fission sources used in either a steady-state or
E 265 Test Method for Measuring Reaction Rates and
pulse mode.
Fast-Neutron Fluences by Radioactivation of Sulfur-32
1.2 This guide provides guidance and criteria that can be
E 266 Test Method for Determining Fast-Neutron Reaction
applied during the process of choosing the spectrum adjust-
Rates by Radioactivation of Aluminum
ment methodology that is best suited to the data that is
E 393 Test Method for Measuring Reaction Rates by Analy-
available and relevant for the environment being investigated.
sis of Barium-140 from Fission Dosimeters
For example, the data available from power reactor and
E 704 Test Method for Measuring Reaction Rates by Ra-
research reactor tests are expected to be different, and the most
dioactivation of Uranium-238
effective spectrum adjustment methodology may also differ for
E 705 Test Method for Measuring Reaction Rates by Ra-
each case.
dioactivation of Neptunium-237
1.3 This guide is to be used in conjunction with Guide E 720
E 720 Guide for Selection and Use of Neutron-Activation
to characterize neutron spectra.
Foils for Determining Neutron Spectra Employed in
NOTE 1—Although Guide E 720 only discusses activation foil sensors, 3
Radiation-Hardness Testing of Electronics
any energy-dependent neutron-responding sensor for which a response
E 722 Practice for Characterizing Neutron Energy Fluence
function is known may be used (1).
Spectra in Terms of an Equivalent Monoenergetic Neutron
NOTE 2—For terminology used in this guide, see Terminology E 170.
Fluence for Radiation-Hardness Testing of Electronics
1.4 The values stated in SI units are to be regarded as the
E 844 Guide for Sensor Set Design and Irradiation for
standard. 3
Reactor Surveillance, E706 (IIC)
1.5 This standard does not purport to address all of the
E 944 Guide for Application of Neutron Spectrum Adjust-
safety concerns, if any, associated with its use. It is the 3
ment Methods in Reactor Surveillance, (IIA)
responsibility of the user of this standard to establish appro-
E 1297 Test Method for Measuring Fast-Neutron Reaction
priate safety and health practices and determine the applica-
Rates by Radioactivation of Niobium
bility of regulatory limitations prior to use.
3. Terminology
2. Referenced Documents
3.1 Definitions: The following list defines some of the
2.1 ASTM Standards:
special terms used in this guide:
E 170 Terminology Relating to Radiation Measurements
3.1.1 effect—the characteristic which changes in the sensor
and Dosimetry
when it is subjected to the neutron irradiation. The effect may
E 261 Practice for Determining Neutron Fluence Rate, Flu-
be the reactions in an activation foil.
ence, and Spectra by Radioactivation Techniques
3.1.2 response—the magnitude of the effect. It can be the
measured value or that calculated by integrating the response
This guide is under the jurisdiction of ASTM Committee E-10 on Nuclear
function over the neutron fluence spectrum. For activation
Technology and Applications and is the direct responsibility of Subcommittee
reactions this would be the decay corrected activity. The
E10.07 on Radiation Effects on Electronic Materials, Components, and Devices.
response is an integral parameter. Mathematically, the re-
Current edition approved Sept. 15, 1994. Published November 1994. Originally
published as E 721 – 80. Last previous edition E 721 – 93.
sponse, R5( R , where R is the response in each differential
i i i
The boldface numbers in parentheses refer to the list of references at the end of
energy region at E of width DE .
i i
this guide.
3 3.1.3 response function—the set of values of R in each
i
Annual Book of ASTM Standards, Vol 12.02.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 721
differential energy region divided by the neutron fluence in that the device that will be tested is sensitive. For silicon devices in
differential energy region, that is, the set f 5 R /F(E )DE . For fission-driven environments the significant range is usually
i i i i
example, if R is the induced activity within DE , then f is from 10 keV to 15 MeV. Lists of suitable reactions along with
i i i
proportionate to the differential reaction cross section, s(E ). approximate sensitivity ranges are included in Guide E 720.
i
3.1.4 sensor—an object or material (sensitive to neutrons) Sensor set design is also discussed in Guide E 844. It is
whose response is used to help define the neutron environment. important that as many response functions as are feasible be
A sensor may be an activation foil. used, in order to minimize the uncertainties in the resulting
3.1.5 spectrum adjustment—the process of changing the spectrum as much as possible. This set should include even the
shape and magnitude of the neutron energy spectrum so that use of responses with thresholds outside of the ranges needed
quantities integrated over the spectrum (such as calculated for the DUT to aid in interpolation to other regions of the
activities) agree more closely to their measured values. Other spectrum. For example, knowledge of the spectrum below 10
physical constraints on the spectrum may be applied. keV helps in the determination of the spectrum above that
3.1.6 trial function—a neutron spectrum which when inte- energy.
grated over sensor response functions yields calculated re-
5.1.2 An example of the difficulty encountered in ensuring
sponses that can be compared to the corresponding measured
response coverage (over the energy range of interest) is the
responses. following: If fission foils cannot be used in an experiment
3.2 Abbreviations:
because of licensing problems, cost, or radiological handling
235 239
3.2.1 DUT—device under test. difficulties (especially with Uor Pu), a large gap may be
3.2.2 ENDF—evaluated nuclear data file.
left in the foil set response between 100 keV and 2 MeV—a
3.2.3 NNDC—National Nuclear Data Center (at
region important for silicon and gallium arsenide damage. In
Brookhaven National Laboratory).
this case two options are available. First, seek other sensors to
3.2.4 RSIC—Radiation Shielding Information Center (at
fill the gap (such as silicon devices sensitive to displacement
93 93m 103 103m
Oak Ridge National Laboratory).
effects (see Note 1)), Nb(n,n8) Nb or Rh(n,n8) Rh.
3.2.5 TREE—transient radiation effects on electronics.
See, for example, Method E 1297. Second, devote the neces-
sary resources to determine a trial function that is close to the
4. Significance and Use
real spectrum. In the latter case it may be necessary to carry out
4.1 It is important to know the energy spectrum of the
forward and adjoint transport calculations to generate a trial
particular neutron source employed in radiation-hardness test-
function which incorporates the use of uncertainty and cova-
ing of electronic devices in order to relate, in the most general
riance information.
way, radiation effects with device performance degradation.
5.1.3 Other considerations that affect the process of plan-
4.2 Since it is necessary to ensure that a satisfactory
ning an experiment are the following:
knowledge of the neutron spectrum is available for each test
5.1.3.1 Are the fluence levels low and of long duration so
environment, this guide describes the factors which must be
that only long half-life reactions are useful? This circumstance
considered when the spectrum adjustment methodology is
can severely reduce the response coverage of the foil set.
chosen and implemented. Although the selection of sensors
5.1.3.2 Are high gamma-ray backgrounds present which can
(foils) and the determination of responses (activities) is dis-
affect the sensors (or affect the devices to be tested)?
cussed in Guide E 720, the experiment should not be divorced
5.1.3.3 Can the sensors be placed so as to ensure equal
from the analysis. In fact, it is advantageous for the analyst
exposure? This may require mounting the sensors on a rotating
conducting the spectrum determination to be closely involved
fixture in steady-state irradiations.
with the design of the experiment to ensure that the data which
5.1.3.4 Does the DUT perturb the neutron spectrum?
will provide the most accurate spectrum is obtained. This data
5.1.3.5 Can the fluence and spectrum seen in the DUT test
may include portions of the following categories: (1) measured
later be directly scaled to that determined in the spectrum
responses such as the activities of the foils exposed in the
characterization experiment (by monitors placed with the
environment to be characterized, (2) response functions such as
tested device)?
reaction cross sections along with appropriate correlations and
5.1.3.6 Can the spectrum shape and intensity be character-
uncertainties, (3) knowledge of the geometry and materials in
ized by integral parameters that permit simple intercomparison
the test environment, and (4) a trial spectrum and its uncer-
of device responses in different environments? Silicon is a
tainties obtained from a transport calculation or previous
semiconductor material whose displacement damage function
experience. It is the accuracy, availability, quality, and cost of
is well established. This makes spectrum parameterization for
the data which determines the most efficient methodology to be
damage predictions feasible for silicon.
used in determining the spectrum.
5.1.3.7 What region of the spectrum contributes to the
5. Spectrum Determination With Neutron Sensors
response of the DUT? In other words, is the spectrum well
determined in all energy regions that affect device perfor-
5.1 Experiment Design:
mance?
5.1.1 The primary objective of the spectrum characteriza-
tion experiment should be the acquisition of a set of response 5.1.3.8 How is the counting system set up for the determi-
values (activities) from effects (reactions) with well- nation of the activities? For example, are there enough counters
characterized response functions (cross sections) whose re- available to handle up to 25 reactions from a single exposure.
sponses adequately define (as a set) the spectrum values where (This may require as many as six counters.) Or can the
E 721
available system only handle a few reactions before the ENDF/B-VI (5), have replaced the ENDF/B-V evaluation. The
activities have decayed below the counting sensitivity above IRDF-90 dosimetry library incorporates many of the early
background? release ENDF/B-VI materials and supplants this data with the
5.1.4 Once the experimental opportunities and constraints best available cross sections for reactions with specific impor-
tance to neutron dosimetry.
are understood and dealt with to optimize the experimental
design and to gather the most useful data, a spectrum adjust-
5.3.2 The code then compares the measured and calculated
ment methodology must be chosen.
responses for each effect and invokes an algorithm designed to
5.2 Spectrum Adjustment Methodology: alter the trial function so as to reduce the standard deviation of
5.2.1 After the basic measured responses, response func- measured and calculated responses. The process is repeated
tions, and trial spectrum information has been assembled, with code-altered spectra until the standard deviation drops
apply a suitable spectrum adjustment procedure to reach a below a specified value—at which time the code declares that
“solution” that is as compatible as possible with that informa- a solution has been obtained and prepares a table of the last
spectrum. This should not be the end of the process unless the
tion. It must also meet other constraints such as reasonable
smoothness and positive definite values. The solution is the initial trial was very close to the final result. The SAND II-type
code will alter the trial with each iteration most rapidly where
energy-dependent spectrum function, F(E), which approxi-
mately satisfies the series of Fredholm equations of the first the foil set has the highest response. If the trial is incompatible
with the measurements, the spectrum can become severely
kind represented by Eq 1 as follows:
distorted in a very unphysical manner.
‘
R 5 s ~E!F~E! dE 1 # j # n (1)
j * j
0 5.3.3 For example, if a trial function predicts an incorrect
gold activity, it may alter the spectrum by orders of magnitude
where:
at the gold high-response resonance at 5 eV while leaving the
R 5 measured response of sensor j,
j
trial spectrum alone in the immediate vicinity. It is unlikely that
s (E) 5 neutron response function at energy E for sensor
j
a real, thin foil will actually modify the spectrum by that
j,
amount, but SAND II cannot discern whether this is real or not.
F(E) 5 incident neutron fluence versus energy, and
The power of the iterative process comes in the next crucial
n 5 number of sensors.
step. The analyst must recognize that the trial must be changed
This equation is also discussed in Guide E 720. The impor-
in a manner suggested by the previous result. For example, if
tant characteristic of this set of equations is that with a finite
a peak develops at the gold resonance, this suggests that the
number of sensors, j, which yield n equations, there is no
trial spectrum values are too low in that whole energy region.
unique solution. With certain restrictions, however, the range
In fact, he will want to use a new trial drawn smoothly near the
of physically reasonable solutions can be limited to an accept-
spectrum values where the sensor set has high response. His
able degree.
direct modification becomes a part of an outer iteration on the
5.2.2 Neutron spectra gen
...