Long counter

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The Long Counter (ger .: "long count") is a common type of neutron detectors .

construction

The actual detector in the long counter is a counter tube that responds to thermal neutrons , usually a boron trifluoride proportional counter . The counter tube is located in the axis of a cylindrical moderator made of paraffin or plastic ( polyethylene ), which also contains a neutron absorbing material (see illustration).

Long counter, schematic. Length about 50 cm.

Several somewhat different designs of long counters have been published. With some, for example, the front of the moderator (the right side in the longitudinal section) is additionally surrounded by a concentric ring of axially parallel holes, the depth of which is approximately half the length of the counter tube. The outer paraffin layer can be surrounded by a metal housing open at the front.

Properties and use

The geometrical design of the moderator and shielding material ensures that the detection probability for neutrons entering from the front is approximately constant for neutron energies from a few kiloelectron volts up to several megaelectron volts . The long counter is therefore well suited, for example, to monitoring neutron production at accelerator facilities , where neutrons are alternately released at a constant point in space through various nuclear reactions with different energy ranges.

The first long counter was used and described in the USA as early as 1947, i.e. at a time when detailed spatial calculations of the energy-dependent neutron flux were not yet possible and the design had to be based on estimates and tests.

literature

Glenn F. Knoll: Radiation detection and measurement. 2 nd ed. New York: Wiley, 1989. ISBN 0-471-81504-7 . Page 520 ff.