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The recent arrival of low-power, digital-signal processing technology has now made it feasible to incorporate the high performance expected of a digital spectrometer into a small, battery-operated package. The portable DSP systems can show equivalent performance to the laboratory based instruments, as we shall see.

From a historical perspective, introduced almost 20 years ago, the LANL-developed Davidson PMCA2 with on-board cassette tape storage, local display and some limited “smart” capabilities has been the instrument of choice for safeguards inspectors across the world. It has been used for routine measurements such as uranium enrichment for many years; it still provides a unique combination of features in a single integrated and portable package. Many hundreds are currently in use. Its attraction is in its combination of useful features:

Battery Operated On Board Display and spectral data storage Full work shift operation (more than 10 hours) without change of battery “Good quality” spectroscopy (for a 1982 vintage portable system) Ability to do some calculations without an attached PC.

Until now, the recent-generation of small portable MCAs, have no on-board bulk storage, display, keypad or computing power and are “blind peripherals,” they require the use of an associated small

computer.

The new instrument reported here, with the trade-name digiDARTTM

is being produced by

PerkinElmer Instruments (ORTEC). It combines the high performance attributes of the digital signal processing in a small, low-power, battery-operated package. The instrument includes an on-board display and the ability to store multiple spectra without the need for an accompanying PC. It can be viewed as a state-of-the-art-technology successor to the PMCA. In addition, the instrument includes new “SMART-1” HPGe detector technology, which can monitor the system “state of health” and provide authentication of spectral data, very valuable in remote or unattended monitoring in a single instrument. Therefore, it can address a variety of existing and developing use needs. The wide range of applicability means that this instrument can help reduce the inventory of different MCA types required to support the spectrum of safeguards applications.

Circuit Description

The MCA hardware is shown schematically in Figure 1. The high voltage supply and detector monitoring are provided in a separate detector interface module (DIM). Following the front-end amplifier, a 14-bit, 10 MHz flash ADC samples the incoming pulse stream and converts it into a string of digital numbers. This is then filtered directly by a proprietary digital filter algorithm, implemented in a field-programmable gate array (FPGA). This also provides the functions of digital baseline restorer, fine gain, peak qualification, conversion gain, digital upper and lower discriminators, and spectrum stabilization. System control, keypad communications, spectral display and control of USB and/or RS232 communication is provided by the microprocessor. The microprocessor also controls the detector high voltage and provides the detector monitoring

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