ANTECH Series 2200 Passive neutron Multiplicity Drum Monitor (nMDM) is a compre-

hensive measurement system for the determination of plutonium mass in intermediate/

low level waste in 220 litre drums. The operation of the instrument is based on passive

neutron coincidence/multiplicity counting. User friendly Windows NT software controls

the semi-automatic drum loading, the neutron data acquisition and the analysis algo-

rithms. The system can be integrated with a germanium gamma-ray drum scanner for Pu

isotopic ratio determination using PC/FRAM or other isotopic analysis code.

The neutron detection system employs 64 He-3 tubes grouped in 16 rectangular polyeth-

ylene modules each with 4 tubes connected to a high voltage junction box containing an

Amptek charge sensitive amplifier/discriminator circuit and connections for high voltage,

low voltage and signal cables. The detector modules are fixed inside an outer shield of

polyethylene which is 210 mm thick and covered in stainless steel. The internal and ex-

ternal module surfaces are covered in cadmium to absorb re-entrant thermal neutrons.

The cadmium can be removed to increase the detection probability for coincidence and

total counting for very low level detection.

The basic design is flexible and can be modified to increase the number of tubes

(increase counting efficiency), to adjust the moderator thickness (vary shielding and die-

away time) or for gamma-ray active waste, lead shielding may be added. The nMDM is

optimised for use with the ANTECH 1000 Series neutron Time Correlation Analyser

(TCA) multiplicity counter.

For pair correlation counting in neutron coincidence mode the frequency histogram is

used to generate the 'Reals' rates for each of the 16 gates. Each 'Reals' rate can be used

with a multi-gate calibration function to determine the Pu-240eff mass and hence the to-

tal Pu mass. For a wide range of Pu containing materials (PCM) such as PuO2 and PCM

with unitary neutron multiplication (M=1), triple neutron correlation may be used in ab-

solute multiplicity counting mode. In these cases Pu-240eff mass and total Pu mass are

determined without the use of a calibration function and the matrix characteristics are de-

termined from the measured detection efficiency.

64 He-3 detector tubes, 25.4mm x 1.0m at ·

Estimated Sensitivity at sea level:

·

4 Atm.

-Multiplicity Counting (n triple

Detector efficiency: 20% (64 tubes).

correlation) - 50 - 100mg Pu-240eff

·

Die-away time: 68ms.

(Cd fitted)

·

Dimensions: (L x W x H) 3.3m x 1.8m x

-Coincidence Counting (n pair

·

correlation) - 10 - 50mg Pu-240eff (Cd

Operating Voltage: 1600V (4 Atm. He-3)

fitted)

·