Passive Neutron and Gamma Drum Monitor

User-friendly Windows software controls the semi-automatic loading of 220 litre (55 gallon) drums and the gamma ray and neutron data acquisition and analysis algorithms. The neutron instrument determines the ²⁴⁰Pu_effective mass and the high purity germanium (HPGe) gamma ray scanning instrument determines the Pu (and U) isotopic ratio data. This data is combined to determine the mass of all of the individual plutonium (and uranium) isotopes.

As with the Model N2221-220, the neutron detection system is based on 16 rectangular polyethylene modules. Each of these modules contains four 3He tubes that are connected to a high voltage junction box. Each junction box contains an Amptek A-111 charge sensitive amplifier/ discriminator circuit and connections for high voltage, low voltage and signal cables. The detector modules are housed inside a 210 mm thick polyethylene outer shield that is covered in stainless steel. The internal and external module surfaces are covered in cadmium to absorb re-entrant thermal neutrons. This cadmium liner can be removed to increase the detection probability for coincidence and total
counting.

Drums for measurement are loaded manually (drum lifter or forklift) onto a drum carrier which moves on slide rails. A drum is subject to a gamma ray measurement and then loaded into the measurement chamber for the neutron measurement in the following automatic sequence:

1. The drum is positioned on drum carrier, which is then rotated;
2. While the drum is rotating, the HPGe detector performs a helical scan of the entire drum during which time a gamma ray spectrum is acquired;
3. A gamma ray check measurement is made using the 356 keV line from a Ba-133 source installed in a shielded position below the drum carrier;
4. The Drum Monitor doors are opened using automatic linear actuators;
5. A slide rail “drawbridge” is automatically lowered into position linking the external slide rails (on which the drum carrier sits) to slide rails in the measurement chamber;
6. The drum on the drum carrier is moved into the measurement chamber by an electric motor driven “engine” which uncouples the drum carrier;
7. The drum on the carrier remains in the chamber and the “engine” withdraws to its original drum loading position external to the
   measurement chamber;
8. The slide rail “drawbridge” is automatically raised to its original position out of the chamber and clear of the doors;
9. The Drum Monitor doors are closed and the neutron measurement starts.

The ANTECH N2000 Universal Neutron Counter (UNC) is used for the neutron measurement data acquisition for both neutron coincidence and neutron multiplicity counting. For pair correlation counting in neutron coincidence mode the frequency histogram collected by the N2000 is used to generate the ‘Reals’ coincidence rates for each of the 16 modules. Each ‘Reals’ rate can be used with a multi-gate calibration function to determine the ²⁴⁰Pu_effective mass from the total plutonium mass. For a wide range of Plutonium Containing Materials (PCM) such as PuO₂ and PCM with unitary neutron multiplication (M=1), triple neutron correlation may be used in absolute multiplicity counting mode. In these cases total plutonium mass and ²⁴⁰Pu_effective mass are determined without the use of a calibration function and the matrix characteristics are determined from the measured detection efficiency.

A Safeguards HPGe detector with 20% detection efficiency (ORTEC SGD-GEM-5050P4) is housed in a stainless steel clad lead shield and collimator close to the rotating drum load position on a scanning mechanism. Digital DSPEC gamma ray spectroscopy data acquisition electronics is used for the Pu (and U) isotopic ratio measurement. The detector employs either liquid nitrogen (LN) or electromechanical cooling (ORTEC Integrated Cryocooling System (ICS)).