The BaF2 detectors from Karlsruhe


The detector consists of 42 BaF2 crystals of 15 cm length. Covering the full solid angle without any gaps requires two different crystal shapes, which can be seen in the figure. The shapes of the crystals are optimized in a way that they all cover the same solid angle, although they have different shapes.

Figure: The detector is composed of 12 crystals of type A (red), 30 crystals of type B (green).

Table: Dimensions of the different crystal shapes for a 4 array of unit radius.
See figure before for definitions of the linesegments.

            Line                   Segment Type
                              A                  B
            12              0.4329            0.3178
            23              0.4329            0.3178
            34              0.4329            0.4298
            45              0.4329            0.3178
            56              0.4329            0.3178
            61                 -              0.4298
            10              0.3682            0.3453
            20              0.3682            0.3820
            30              0.3682            0.3453
            40              0.3682            0.3453
            50              0.3682            0.3820
            60                 -              0.3453


The volumes described above can be arranged to form a closed sphere with an inner radius of 10 cm and an outer radius of 25 cm. In the present situation, however, the crystals were moved outwards by 0.335 cm and the resulting gaps of 2 mm between the crystals were filled 0.1 mm aluminum foil, 0.5 mm Teflon and 0.4 mm air surrounding. These materials are used to reflect the scintillation light at the surfaces and to isolate each crystal from its neighbors. The front of the crystal is also covered with aluminum and Teflon. The following picture shows the detector with all crystals in an opened position.

The neutrons reach the sample, centered in the BaF2 crystal ball, after 80 cm flight path. The crystal nearest to the Lithium target (neutron production) is replaced by lead  in order to absorb - rays as well collimate the neutrons. The opposite crystal contains a clyndrical whole of 5 cm diameter.


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