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PUNITA

DESCRIPTION OF THE INSTRUMENT

The Pulsed Neutron Interrogation Test Assembly (PUNITA) is a versatile experimental tool. The instrument is designed to give the experimenter maximum flexibility with respect to detector and sample arrangements. When in the closed configuration the instrument forms a cube with a central void called the sample cavity (see figure). The top and bottom sides of the cube are part of a central structure, while the four vertical sides are located on movable trolleys. The trolleys move on a rail system to yield a perfect mating with the central structure. The size of the sample cavity is 50 cm by 50 cm cross-section and 80 cm height. The accelerator assembly of the (D-T) generator has a length of only 43 cm and can be placed without constraints anywhere inside the cavity. A thick graphite liner is located on all six sides of the sample cavity. On the vertical sides, the graphite liner is integrated in the trolley to give access to the cavity from all sides when in the open configuration. A total of 1,350 kg of reactor grade graphite is used in the liner.


The JRC Pulsed Neutron Interrogation Test Assembly (PUNITA)

A fast neutron detector module is integrated in each of the six sides immediately behind the graphite liner. The modules, nominated fission neutron counters, include sixteen 3He proportional detectors of 3040 torr and 1000 mm length each. The 3He detectors are embedded in a single row in a block of polyethylene with a 1 mm cadmium cladding on all surfaces. A neutron shield of 300 - 350 mm polyethylene is placed behind the fission neutron counters on all six sides. On each of the vertical sides eight 3He neutron detector of 3040 torr and 500 mm length, nominated source monitors, are embedded in the polyethylene shield. Other permanently installed instrumentation include bare 3He counters of pressure below 760 torr, nominated thermal flux monitors, located in the corners of the sample cavity.
The neutron generator, a model A-211 from Thermo Fisher Scientific Corp., is capable of emitting 14-MeV neutrons at a rate of ~2·108s-1. The generator can be pulsed at rates from 10 Hz to 150 Hz. The duration of the 14-MeV neutron pulse is about 5 µs resulting in a duty cycle as small as 0.5 per mille at 100 Hz pulsing. This feature permits a very good time separation of the fast and thermal neutron interrogation in PUNITA. Another important feature of the generator is the pulsing of both the Penning ion source and the acceleration voltage assuring that no neutrons are emitted between pulses. The 14-MeV neutrons emitted in each generator pulse are thermalised in the graphite liner in a period of about 200 µs following the pulse. After this time only thermal source neutrons persist in the cavity. After reaching a maximum value at about 280 µs after the fast neutron pulse, the thermal neutron flux in the sample cavity decays according to a single exponential function. In this time period a fissile sample in the cavity would undergo fission by thermal neutrons only. Also in this period, only the fast fission neutrons are detected in the cadmium covered fission neutron detectors. These features are essential in the projects concerning fissile material assay. The source monitors located in the shielding are used for normalisation of the 14-MeV neutron emission from the generator. Likewise, the bare 3He detectors in the sample cavity are used for normalisation of the interrogating thermal flux. The response of the various neutron detectors mentioned here are all recorded with Ortec MCS multi-channel scalers which are triggered synchronous with the pulsing of the neutron generator.

 

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Udapted : 14-Jan-2011