SWRHL-502r
DATA ACQUISITION AND ANALYSIS SYSTEM FOR EMERGENCY ENVIRONMENTAL SURVEILLANCE
by
R. N. Snelling
Western Environmental Research Laboratory
ENVIRONMENTAL PROTECTION AGENCY
Presented at the
International Symposium on Rapid Methods
for Measuring Radioactivity in the Environment
Neuherberg, Germany
July 5-9, 1971
This study performed under a Memorandum of
Understanding (No. SF 54 373)
for the
U.S. ATOMIC ENERGY COMMISSION

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This report was prepared as an account of work sponsored
bv the United States Government. Neither the United States
nor the United States Atomic Energv Commission, nor any
of their employees, nor any of their contractors, subcon-
tractors, or their employees, makes anv warranty, express
or implied, or assumes any legal liability or responsibility
for the accuracy, completeness or usefulness of any infor-
mation, apparatus, product or process disclosed, or repre-
sents that its use would not infringe privately-owned rights.
Available from the National Technical Information Service,
U. S. Department of Commerce
Springfield, Va. 22151
Price: paper copy $3.00; microfiche $.95.

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INTRODUCTION
An environmental surveillance program, whether it be of a continuing
nature or an emergency action, can be represented as an integrated
system. Such a system, irrespective of its complexity can in turn be
described in terms of three basic functional processes:
INPUT, which is the basic component on which the system
operates;
OUTPUT, which is the end result of the operation; and
PROCESS, which is the activity which transforms input into
output.
In the context of a laboratory operation supporting an environmental
surveillance program, input may be explicitly defined as samples and
collection data. Output will be (in the case of radiological
surveillance) activity concentration or dose estimates. The process
is the laboratory function of analyzing samples and processing the
data generated therefrom. Such a system is shown below.
The system described herein is the "PROCESS" referred to above.
It is an integrated system for the raDid analysis of large numbers of
environmental samples.
SAMPLES
COLLECTION
DATA
INFORMATION:
DOSE
ACTIVITY
1

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GENERAL SYSTEM DESCRIPTION
Data acquisition and analysis is a production system designed to process
routine as well as special samples^ The predominant routine sample types
are: milk, water, air filters, and charcoal cartridges. Other types of
samples received periodically are: vegetation, animal feed, animal tissue,
soil, and gas. Sample load for gamma analysis and chemistry is typically
1000 samples/month. In addition, 100 air filters/day are received for
beta counting. During peak periods as many as 200 samples per day are
processed for gamma analysis and/or chemistry. The tvnes of analyses
performed on a sample (dependent unon the samnle type and program reauifo-
ments) are: gross and specific aloha and beta countinq, specific r^rlio-
chemistry and gamma counting with qualitative and Quantitative snectral
analysis.
For the routine continuing surveillance programs, a systematic procedure
has been established for processing both samples and data. Special
samples, notably those related to emergency situations require modifica-
tion of the basic system in order to provide quicker through-put of data.
A computer system is utilized in the analytical process in three ways:
it allows rapid conversions of data from one medium and format to another;
it performs calculations and presents its resulting output in a form
suitable for analysis by professional personnel; and it allows for the
storage and retrieval of accumulated data.
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Operationally, the processing of samples is complex. Many processes
occur simultaneously and are mutually dependent. Successful processing
of samples requires procedures that extend across operational unit
functions. Hence, description of the system is based on a general
breakdown according to processing function rather than operational unit.
This breakdown is developed as follows:
1.	Sample control system
2.	Gamma analysis system
3.	Air sample analysis system '
4.	Chemistry data analysis system
5.	Data Management system
All samples are processed through sample control. Air filters and charcoal
cartridges are then processed through the air system (possibly incorporating
a loop through the gamma and/or chemistry system). All other samples are
processed through the gamma and/or the chemistry system. Data from all
samples are processed through the data management system. These functional
sub-systems are shown schematically in Figure 1.
SAMPLE CONTROL
Sample control records, prepares, and distributes for appropriate analysis
all samples received. Each sample which is to receive gamma analysis or
chemistry is given a unique laboratory number. Collection information is
transcribed from the sample collection data form to a four-part log-in
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INPUT
PROCESS
OUTPUT
sample
Identifying
information
/sample
control
/processing
/ system
gamma
analysis
s.ys tem
*
air sample
analysis
system
chemistry
analysis
data
management
system
	c
FIGURE 1
Data Acquisition and Analysis Processing Functions
informatio
reports

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Collection Date
Milking Date (Milk Samples)
Off Date (Air Samples)
(Non-routine programs
only)
FIGURE 2
SAMPLE CONTROL LOG-IN FORM

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^SampT^e)
I Collection ;
i Data	j
Form	I
1
4.
5.
Assign, sample laboratory
number
Transcribe collection info
to 4-part log-in form
Complete missing items,
correct errors
Distribute 4-part form
Prepare and submit sample
for analysis
on-
J.
Sample
Analysis
J
4-part
log-in form
L.t-
\

j White
copy
I to keypunch
room
tor
! header
Yel1ow copy
accompanies
sample for
y analysis

Blue copy
to project
di rector
Chem. card
accompanies
sample for
chemi stry
Dnr/t
FIGURE 3
SAMPLE CONTROL PROCESS
6

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form (Figure 2). An appropriate location code, sample type code, and
program code are added. The sample is then forwarded with the appropriate
copy of the log-in form for selected analysis (Figure 3).
Air filters and charcoal cartridges are not immediately logged in.
They are posted as to date received and submitted for gross beta and
gross gamma counting, respectively. If the gross counts exceed pre-set
limits, they are returned to sample control for log-in and subsequent
isotopic analysis (see Air System).
Sample control is also responsible for contamination control in the
analytical laboratories. During emergency periods, all samples are
appropriately repackaged and sealed prior to submittal to "low-level
counting facilities.
GAMMA ANALYSIS SYSTEM
General Description
The "gamma system" encompasses the sample handling, counting, data
analysis and interpretation, and reporting procedures involved in
quantitative gamma spectral analysis. In processing large numbers
of samples by quantitative gamma spectral analysis, a number of
assumptions must be made prior to the analysis. The most important
are related to detector response characteristics, system reproduci-
bility, and radioactivity standards library. An analysis is made and
the results reviewed in order to validate the initial assumptions.
If the assumptions are found to be incorrect, they must be altered and
the analysis repeated.
1
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Instrumentation
The gamma counting facility consists of five analyzers each operating
in split mode with two thaiiurn activated sodium iodide detectors. The
analyzers are TMC Model 404C, 400 channels, with multiple input. The
detectors are 4-inch-thick by 4-inch-diameter crystals and are manu-
factured by the Harshaw Chemical Corp. The crystal housing, hermeti-
cally sealed, is of 0.019-inch Type 304 stainless steel. A 3 1/2-inch
diameter by 5/16-inch-thick Vycor optical window is coupled to a 5-inch-
diameter, RCA, Venetian blind, dynode multiplier phototube, Type 2065.
The detector assembly is seated on a lucite shelf in a steel shield of
6-inch-thick wall. The chamber within the shield is 20 by 20 by 24 inches,
lined with 0.1-inch lead, 0.03-inch cadmium and 0.015-inch electrolytic
copper.
Readout from each analyzer is by means of perforated tape and typewriter
(the perforated tape is processed by the computer for analysis and data
storage).
Calibration and Quality Control
Routine samples are counted in one of four standard geometries. Four
special purpose geometries are used for limited studies where sample
quantity or processing may require a non-standard configuration.
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TYRE
GEOMETRY
DESCRIPTION
Standard
01
2-inch-diameter planchet
Standard
02
4-inch-diameter planchet
Standard
03
400-ml polethylene container
Standard
06
3.5-liter Marine!li beaker
Special
12
250-ml polyethylene container
Special
15
1-liter cubitainer
Special
16
250-ml resin
Special
17
Soi 1
A radioactive isotope standard is counted on all detectors for each
geometry and nuclide to be analyzed. An effort is made to recalibrate
the more common long-lived isotopes on a yearly basis. A gamma efficiency
vs. energy curve is used for quantitation of nuclides that are not readily
available as standards.
A 400-minute background count is accumulated daily on each system.
Both the standard spectra and background count are processed through
appropriate computer programs and stored on disk for recall during data
analysis.
In quantitative and/or qualitative analysis by gamma spectroscopy, the
validity of the analysis depends on the satisfactory and reproducible
operation of the instrumentation. The first level of quality control,
then, is that applied to the instrument itself.
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System response is checked daily by counting a 207Bi
reference standard. This nuclide has a 30-year half-
life and two prominent gamma emissions at 0.570 MeV and
1.063 MeV. The source is counted for ten minutes and
read out on punched paper tape. The tape is then run
through a computer program (B1207)* and the following
parameters calculated:
1)	peak locations
2)	difference between peak locations
3)	sum of counts under photopeak
4)	resolution
5)	peak ratio
Peak location and interval are maintained within 0.5 channels
of the theoretical. If both peaks are shifted equally, a zero
shift is indicated. A gain change is indicated by a proportional
shift. Daily corrections are made to maintain the energy
calibration within the specified limits. Control charts are
updated daily to evaluate long-term trends. These are maintained
for each detector system.
*A name in capital letters designate the name of the computer code in the
process. Full documentation of all computer programs used by the Western
Environmental Research Laboratory is available at the Laboratory.
Documentation includes a complete index of programs, one-page summaries
identifying the nature of the programs, source language listing, flow
charts, data set-up information and operating instructions.
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The sun of counts within the photopeak provides a check on counting
efficiency. Resolution provides a measure of energy separation.
Charting of sum counts can detect long-term failure of the detector
while resolution charging can indicate gross detector failure.
Peak ratio is the ratio of counts in the two peaks. Although not
necessary as a quality control check, it does provide another
sensitive indicator of change in detector response.
Background data are accumulated daily to check abnormalities that
may occur on a long-term basis. After normal operations, the systems
are set for a 400-minute background count. The gross gamma count is
reported daily and plotted on a control chart. If the background is
unusually high, the spectrum is checked to determine the reason for
the increase. A background quality control chart is maintained for each
system to detect long-term trends and fluctuations.
Sample Counting and Data Flow
Figure 4 shows the routine sample and data flow through the gamma system. The
sample is received along with the yellow copy of the log-in sheet. Counting data
are added to the log-in form. The sample is counted from 10 to 40 minutes depending
on sample type, sample load, and desired data precision.
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Return to
Sample
Control,
Ihemistry
@
Gamma count
10-40 min
Paper tape
spectrum
P0N0
tape to cards
merge header
Type-out
(hard copy)
Gamma coding
record
Keypunch
header card
Yellow copy
of
Log-in form
Translate
info to
Jamma codinc
record
Add count
data
Merge
GMTRX
Disk
f i 1 e
std.,Bka
Review, post
results on
hard copy
Report
NRG EN
NRCOL
FIGURE 4
SAMPLE AND DATA FLOW GAMMA ANALYSIS SYSTEM
] 2

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Count Time (Minutes)
Sample Type
Air Filter
Charcoal Cartridge
Milk
Routi ne
40
10
Event-Related
20-40
10
10
Water
40
20-40
Feed
40
20-40
Vegetation
10
4-10
Count data are read out on punched paper tape and typewriter printout. A
gamma analysis coding record is used to identify the sequence of spectra on
the punched paper tape. The yellow log-in form is attached to the typewriter
printout to identify spectra. This becomes the hard copy of the raw data
that is retained for future reference as needed.
The gamma counting information is keypunched, producing a gamma header card
for each sample counted. The punched tape is converted to cards and merged
with the appropriate header card (P0N0). The data are then processed through
a gamma analysis program (GMTRX) which utilizes the simultaneous equation
technique to resolve the spectrumP^Radionuclide standards and background
information are stored on disk for access by the gamma analysis computer
program. The resulting data are reviewed and the results posted on the hard
copy. A plot of the spectrum can be generated if necessary. The results are
then keypunched onto cards for subsequent reporting.
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Data Analysis
Quantitative gamma spectral analysis is performed by the simultaneous
equation or matrix technique. Data relating to the interference co-
efficients among radionuclides are utilized in the matrix. Three files
of data are maintained for routine analysis, each containing a library
of eight radionuclides. These are grouped, according to whether the
predominant activity is of long, intermediate, or short half-life as
follows:
Intermediate
Long-Lived	Half-Life	Short Half-Life

Isotope
Peak Energy
MeV
Isotope
Peak Enerqy
MeV
Isotope
Peak Energy
MeV
1.
^"Ce
0.13
147Nd
0.09
ll+1Ce
0.14
2.
1 3 1 J
0.36
141Ce
0.14
1 31 j
0.36
3.
•106Ru
0.51
132Te
0.23
1 3 3 J
0.53
4.
137Cs
0.67
143Ce
0.29
137Cs
0.67
5.
9SZr
0.76
1 31 j
0.36
132Te
0.23
6.
5itMn
0.84
!03RU
0.50
"Mo
0.75
7.
l+0K
1.46
95Zr
0.76
1 3 5 j
1.28
8.
i^Ba
1.60
lt|0Ba
1.60
uok
1.46
The appropriate data set is utilized according to the circumstances
Knowledge of event characteristics allows special data sets to be
specified. These data can be assembled in any combination of up to
eight radionuclides.

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The program calculates the activity concentration of each of the nuclides at
the time of count and at time of collection. If an isotope is determined to be
absent, it is deleted from the matrix and a recalculation is executed. This
process continues until the matrix is exhausted.
System Performance
1.	Data Turnaround
On a routine basis gamma matrix output is available within 24 hours
of sample receipt. During an emergency situation, a batch of 50
samples can be logged in, counted, analyzed and reported within
6 hours. As many as 200 samples can be processed in a day.
2.	Sensitivity
It should be noted that in gamma spectral analysis, minimum
sensitivity is dependent upon both isotopic mixture and relative
isotope concentration as well as sample counting time.
Those values stated below for gamma isotopic results refer to low
level environmental samples.
Minimum Sensitivity, Gamma
Spectral Analysis
Minimum Sens.
PCi/L or kg.
Sample
Type
40 Min
Count
20 Min
Count
10 Min
Count
Milk (3.5L)
Water (3.5L)
Food (3.5L)
Feed
10
10
10
20
20
50
Veg.
Air (500M3)
50
0.1
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AIR SYSTEM
General Description
Approximately 100 air filters and 25 charcoal cartridges are received daily
for analysis. The air filters receive a sequence of three gross beta counts
and the activity is extrapolated back to the end of collection. Extrapolated
gross beta activity is used to document trends in long-lived airborne radio-
activity. Activity at time of count is used as a screen to detect sudden
increases in gross activity. If the beta activity is above a preset level
at time of count, the sample is submitted for gamma analysis. All the charcoal
cartridges receive a gamma scan. If the gross gamma activity is above a
preset guide, Isotopic quantitation is nerformed. Processing of
these samples follows the procedures established within the gamma analysis
system description.
Instrumentation
The counting systems consist of three Beckman (Sharp) Widebeta counting
systems. Each counter has a 5-inch-diameter thin window (100 mg/cm2)
gas flow, proportional detector which is incorporated in a 6-inch-thick
lead shield to eliminate background from environmental radiation. The
sample detector is operated in anticoincidence with a cosmic-ray guard
counter which removes the cosmic-ray component of background.
The systems use pure methane (99.99%) as counting gas and are operated in
the proportional region (3 KV) to provide for both alDha and beta
counting based on pulse height discrimination. Simultaneous alpha and

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beta accumulation and readout are provided. The systems incorporate an
automatic sample changer (60 sample capacity) and an automatic data readout
capability. Readout is by means of IBM Model 026 Hollerith card punch.
The air filters are counted on 4-inch stainless steel planchets. The
collection data pertaining to the sample are pre-punched on the Hollerith
card which is placed in the card punch in the same sequence as the samples
in the sample changer. Counting data for each sample are then automatically
punched onto designated fields on. the Hollerith card.
Calibration and Quality Control
Each system is calibrated over a range of beta energies and self-absorption.
Typically, using 90Sr/90Y in equilibrium, with an average maximum beta energy
of 1.40 MeV, a curve of beta counting efficiency vs. sample weight can be
developed. Using a weightless standard solution deposited uniformly on glass
fiber filters, a curve (Figure 5) of beta counting efficiency as a function
of maximum beta energy can be plotted.
For large scale processing of samples, calibration data used in data conversion
calculations must be selected based on assumptions made about the sample and
the nuclide composition. Typically a filter sample averages less than 10
milligrams of total solids (less than 1 mg/cm3) and therefore it is assumed
that self-absorption is negligible. The maximum beta energy for mixed fission
products, as a function of time after fission, averages approximately 1 MeV
at any time after two days post fission.
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FIGURE 5
BETA CALIBRATION-COATED FILTER

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A conservative efficiency value of *5% (corresponding to an average maximum
beta energy of 0.5 MeV) is used for data conversion.
A daily instrumental quality control check is made on each system. This
involves a 2-minute count of an alpha reference source (239Pu) and a beta
reference source (90Sr-90Y),as well as a 10-minute background count. Quality
control charts are maintained on each system.
Sample Handling and Data Flow
The routine sample and data flow for air filters is shown in Figure 6.
The filter is received by sample control along with its field data form.
The filter is removed from the mailing envelope and put in a clean glassine
envelope. Receipt of the sample is made on a posting form and obvious
errors are corrected on the data form.
The collection information is keypunched in the first 24 columns of each
of three-color-coded Hollerith cards. The sample along with its cards is
submitted for the first beta count. The samples are stacked in the sample
changer and the first count cards placed in the card puncher in the same
order. The filters are counted for 2 minutes each and the count data are
automatically punched onto designated fields on the card.
The first count card is checked and if the gross beta count is greater than
1000 counts, the filter is submitted for gamma scan (see Gamma Analysis System).
19

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Sample
received,
'repackaged,/
posted
Collection
Information
Assign No
Log in
Isotopic
Report

Gamma
'	
Scan
N

GMTRX
N
'

Keypunch \
Data Cards I
\

Beta
#
Count
1
	
( Collection
^Info. and
Counting Data
"A
\

Beta
recount
at 5,
12
days


Retai n
jn storage
Sample Flow
Data Flow
No
All capital lettered mnemonics
represent computer code
processes.
\No
Previous
y scan?
Yes
FIGURE 6
AIR FILTER, ROUTINE SAMPLE
AND DATA FLOW
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The filter is recounted five days after collection (after natural radon
arid thoron daughter products have decayed out) and at twelve days post
collection. The filter may be submitted for gamma analysis if the 5-day
count is unusually high.
Special (event-related) filters are handled in a similar manner with two
significant changes. First, the filter is submitted for gamma analysis,
and then for an initial beta count. If oamma analysis indicates that natural
radioactivity is negligible, the second beta count is made at 24 hours after
the first, and the third beta count at 5 days or less after collection. If
natural radioactivity is prominent, the normal 5-day and 12-day beta counts
are made. Second, a variety of algorithms are available for back extrapo-
lation of beta count data to end of collection.
The individual count cards are submitted for data processing. The computer
programs check for a variety of data errors, calculate the beta activity
concentration at time of count, and produce a report of these values. At
the end of each month the activity is back extrapolated to the end of
collection and a report of extrapolated data is generated.
A variety of computer programs are utilized to analyze and report surveillance
data.
System Performance
Routinely, 100 filters per day are received into the system, thus requiring
300 counts per day. As many as 300 filters per day could be handled. Turnaround
from time of sample receipt to daily beta report (for first count) is routinely
21

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24 hours. During event periods, a turnaround of 5 hours is possible for a
batch of 60 samples.
Sensitivity is calculated for each individual filter. Minimum detectable
activity is defined as that activity which produces a ±25% counting deviation,
at the 95% confidence level. For a typical routine sample, this is equal to
a net activity of 50 cpm.
50 cpm x 1.00 pCi x 1 •= 0 •15 pCi/m3
cpm 350 mJ
Charcoal Cartridges
Routine charcoal cartridges are received by sample control and are held
until 3 days post sampling before receiving a 10-minute gamma scan. The
gamma spectra are processed through P0N0. If the gross gamma count is equal
to or greater than 300 cpm above background, the spectra are processed through
GMTRX (see gamma analysis system). If the gross gamma is less than 300 cpm,
Ša gross gamma result is produced on the card and results reported weekly.
The spectra are reviewed in any case to confirm results.
Event-related cartridges are logged in, gamma scanned, and processed through
the normal gamma system. Figure 7 shows the routine charcoal cartridge
sample and data flow.
CHEMISTRY DATA. ANALYSIS SYSTEM
General Description
Radiological counting data generated for samples that require radiochemical
separation or preparation are processed by computer. Counting, calculations,
i!2

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ISample
/Received
/and Posted


Gamma
Scan


fReturn
'to ES)
- - 4
Sample Flow
Data Flow
FIGURE 7
CHARCOAL CARTRIDGE ROUTINE SAMPLE AND DATA FLOW
23

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reporting, and data storage and retrieval are described briefly in this section
Radiochemistry procedures and methods are fully described in Reference 2.
Instrumentation
Instrumentation described here is mainly for alpha and beta proportional
counting of heat-dried samples and soft-beta spectroscopy of liquid
scintillation solutions.
The proportional counter is a Beckman WIDEBETA II employing a 2 1/4-inch
diameter detector with an 80 pg/cm2 thin window. The gas flow system uses
pure methane counting gas (99.99% pure). Background is reduced by guard
detectors for cosmic radiation detection and 4-inch low-level lead shielding
lined with OFAC copper. A random access automatic sample changer
accommodates 100 sample planchets. Readout is by teletype printer. Three
systems are in operation to accommodate heavy sample load periods.
Four Beckman LS-100 Liquid Scintillation Systems comprise the counting
facility for soft beta spectroscopy. The systems operate at room temperature,
accommodate 100 samples on a conveyor, and have a full three channel capacity.
The systems have capability for automatic calibration (by the external standard
channels ratio method) with two separate and independent data channels for
external standard counts, and with automatic subtraction of sample counts from
standard counts. The output printer automatically displays data after each
count including channel number and conveyor number, elapsed time, 2o error and
counts per minute.

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Radon Gas analysis is described in the Southwestern Radiological Health
(2)
Laboratory Handbook of Radi ochenical Analytical Methods. There are two
separate systems utilizing the Lucas scintillation cell, an automatic and
a manual system. The automatic sample changing system is basically a
modified SHARP LOWBETA. The Lucas cell sets on a phototube which is coupled
to a preamplifier and amplifier/discriminator for straight pulse height
discrimination detection- There is no anti-coincidence circuitry. Readout
is via a line printer which identifies the cell number, counting time, and
the counts. The manual system is essentially the same with the exception of
the sample changing mechanism. Four phototubes are incorporated in a light-
tight box for simultaneous counting of four Lucas cells.
Plutonium and other alpha emitters amenable to electrodeposition are counted
by alpha spectrometry. The alpha spectrometer systems consist of 8 solid
state silicon surface barrier detectors, (450mm2 area and 300u deoletion depth)
under vacuum, connected through suitable electronics to four multichannel
analyzers. Two (2) detectors are operated in each vacuum chamber and two
sets of electronics are routed to each multichannel analyzer. Voltage to
the detector is provided from a bias power supply. The detector signal is
routed through a preamplifier, linear amplifier and bias amplifier before
being fed to the multichannel analyzer. The data are read out through a
switching box to a parallel printer.
Sample counting time varies with the activity level of the sample, usually
between 400 and 1400 minutes. Therefore, the weekly maximum output can vary
from 40 to 100 samples.


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Sample Handling and Data Flow
All samples pass through Sample Control. Those samples requiring
radiochemical separation or preparation are identified accordingly.
These are routed through the chemistry section after completion of
non-radiochemical analysis. For liquid scintillation analysis an
aliquot of the sample is removed so that processing of the sample
for various analyses can occur simultaneously.
Counting data generated by these systems are merged with other
information and submitted for computer processing.
Data Analysis
Several computer programs are used to process data, perform calculations,
and generate various reports relating to radiochemical analysis. These
report data are eventually merged with other radionuclide analysis data
for generating other summaries and reports as well as for storage for
future reference.
A sampling of computer programs in production is listed below.
RCHEM:
89Sr and 90Sr analysis
LIQSA:
3H and 14C from liquid scintillation counting
RADON:
222Rn in air analysis
GROAB:
Gross alpha and beta analysis
PLUTON:
Plutonium analysis
26

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DATA MANAGEMENT SYSTEM
Although our complete data management 5ystem is still under devplonment,
a conceptual outline is shown in Figure 8. Analysis results are enterf;d
into the system by Hollerith card and placed in an in-process file.
As analysis is completed a Tape File is created which in turn updates
historical tape files. Data may be retrieved at any time, sorted
according to sample type, location, time of collection, analysis,
program, or a number of other more specific samDle classifications.
A typical retrieval time for a summary listinq is two hours. A number
of report generators can list data in a variety of formats. A
complete description of the data management system is impossible in
the context of this paper.
27

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F1 E 8
data management system

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Bibliography
1. Lem, p. N., R. N. Sne111ng, Southwestern Radiological Health
Laboratory Data Analysis and Procedures Manual, SWRHL-21, March 1971
2. Johns, F. B., Southwestern Radiological Health Laboratory
Handbook of Radiochemical Analysis Methods, SWRHL-11, March 1970
29

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