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TRANSPORTABLE HIGH SENS'TIVITY ShI-iLL S-iJIPLE RADIOMETRIC CALORIMETER

J. R. Wetzel, R. S. Biddle, B. S. Cordova, T. E. Sampson, H. R. Dye, and J. G. McDow

39th Annual INMM Meeting Naples, FL

(FULL PAPER) July 26-30, 1998

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LA-UR-98-3045

TRANSPORTABLE HIGH SENSITIVITY SMALL SAMPLE RADIOMETRIC CALORIMETER

J. R. Wetzel, R. S. Biddle, B. S. Cordova, T. E. Sampson, H. R. Dye, and J. G. Mc3ow Los Alamos National Laboratory, MS-E540,

Los Alamos New Mexico 87544 USA Phone: 505/667-3285 FAX: 505/665-59 10

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presented at the Institute of Nuclear Materials Management

39th Annual Meeting Naples, FL

July 26-30, 1998

TRANSPORTABLE HIGH SENSITIVITY SMALL SAMPLE RADIOMETRIC CALORIMETER*

J. R. Wetzel, R. S. Biddle, B. S. Cordova, T. E. Sampson, H. R. Dye, and J. G. McDow Los Alamos National Laboratory, MS-E540,

Los Alamos New Mexico 87544 USA Phone: 505/667-3285 FAX: 505/665-5910

Abstract A new small-sample, high-sensitivity transportable radiometric calorimeter, which can be operated in different modes, contains an electrical calibration method, and can be used to develop secondary standards, will be described in this presentation. The data taken fi-om preliminary tests will be presented to indicate the precision and accuracy of the instrument. The calorimeter and temperature-controlled bath, at present, require only a 30411. by 20-in. tabletop area. The calorimeter is operated fkom a laptop computer system using a unique measurement module capable of monitoring all necessary calorimeter signals. The calorimeter can be operated in the normal calorimeter equilibration mode, as a comparison instrument, using twin chambers and an external electrical calibration method. The sample chamber is 0.75 in. (1.9 cm) in diameter by 2.5 in. (6.35 cm) long. This size will accommodate most 238Pu heat standards manufactured in the past. The power range runs fkom 0.001 W to <20 W. The high end is only limited by sample size.

INTRODUCTION Background. A request to design a calorimeter that could be used as another tool to measure small samples to characterize a larger batch of plutonium is the basis of this calorimeter system. The sample size was to be 0.75 in. in diameter by 2.5 in. long and would emit a power of 10 mW. The desired precision and accuracy was to be 0.2% relative standard deviation (RSD) for a 10-mW sample, which means the calorimeter must measure to f20 pW. The measurement time was not as important but should be capable of measuring a sample in less than 8 h. This design would allow the calorimeter to provide in-plant calibrations of the 238Pu heat standards now in use in most Department of Energy (DOE) plant sites. Either direct calibration with electrical tracability or comparison calibration using currently calibrated 238Pu heat standards would be possible.

With this design criteria as a starting point, it was decided that the system should be capable of operating in normal equilibration mode and also in comparison mode, which could be used without any calibration method since one sample could be used as the standard for the other sample comparisons. It would be convenient if the system could have an electrical calibration to be used in areas where heat standards were not available, but electrical calibration at low levels has always had bias problems. Very high sensitivity and a very stable reference bath would be required for the low- level measurements. The system should be transportable requiring a small footprint and laptop computer system. This would also require a multiple-channel signal measuring system capable of monitoring up to five channels of varying signal types.

System Design. The design chosen was the full twin water-bath calorimeter with the Wheatstone bridge configuration to meet the above requirements. The twin cells allow for the comparison

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were placed around the calorimeter cells and the Hart Scientific* bath stirrer/pump was connected to force water completely to the top of the calorimeter jackets. This configuration gave a quieter bridge signal and we could now see the measuring module lower in 0.1-pV steps from the bridge output with a peak-to-peak signal of less than 0.5 pV.

Electronics. The Keithley Instruments@ module was connected to a laptop computer using the serial port and running the Keithley Instruments* NetAcq operating software. This software allowed us to acquire the five channels of data at a rate as fast as 1.2 s per set. The module acquires all five channels and transfers them all at once to the computer. For the series of tests run with heat standards and electrical heaters, we chose 10 s per set so we could see the noise and room temperature effect. The data were entered into an Excel spreadsheet for analysis and data smoothing as well as charting. The data-acquisition system electronics are mounted on the water bath to improve the temperature stability (Fig. 1).

Fig 1. acquis

SmG ition

ill-sample and cona

caloi -01 sy:

?meter rtem.

with data

TESTING The calorimeter was-tested with both the 238Pu heat standards and the electrical calibration heater over a range of 1 mW to 0.4 W with the most extensive testing at the 4- to 20-mW range. Three runs each of the 238Pu heat standards were made at 0.7 mW, 4 mW, and 17.5 mW. Several runs were made with the electrical heater near the heat standard values. The electrical heater showed a much larger spread in the values than the heat standards. The power supply stability is the most likely cause of the measured values because we were using very low voltages and currents, and the reference for the power supply is a reference voltage produced by a diode configuration which is subject to small temperature changes with room temperature changes. Cycles were noted in the electrical heater power when plotted. If longer averages were used near the room temperature cycle, it may be possible to reduce the data spread for the electrical heater. An electrical heater with a much higher resistance would help reduce the spread if the power supply stability is the cause because it would operate at a voltage midrange for the power supply. The data for the 238Pu heat standards fall in the center of the data spread of the electrical heater indicating that there is no bias associated with the electrical heater run in this configuration at very low wattage (Fig. 2). This means that electrical calibrations can be used at the low-wattage level with no bias corrections.

2 1.5 I

0.5 0

-0.5 -1

-1.5 -2

-2.5 -3 + - ! ~

0 5 10 15 20

Power (rnw)

Fig 4. Measurement error (reference-observed) %.

CONCLUSION The transportable calorimeter performed as required, and with the self-contained water bath system can be used to provide both comparison and calibration measurements. Further development of a sealed-bath configuration will reduce the size and weight while reducing the power requirements since the cooling compressor will be eliminated. A new version of operating software being developed at Los Alamos will be used to operate the calorimeter on the laptop computer with the Keithley Instruments@ module system.

* This work is supported by the US Department of Energy, Office of Nonproliferation and National Security, Office of Safeguards and Security.