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National Advisory Committee for Aeronautics, Technical Notes - Coincidence Method Applied to Ion Beam Measurement

A coincidence Geiger counter system was constructed for the absolute
measurement of the activity of radioactive substances made in a cyclotron.
The average beam current in the cyclotron can be calculated from.the num—
ber of disintegrations per second observed, the'half-life of the radio-
active substance, and the reaction cross section.

A possible method for measuring the current of an iOn beam in an
accelerator would beItO'measure the absolute thick target yield for a
known reaction caused by the ions, and from this to calculate the
average current. Such a method has been applied to measurements of the
ion current in The Ohio State University cyclotron, and details of it
are contained in the following report.

The absolute measurement of the activity induced in the target
material cannot be made without complete quantitative information On the
manner by which the nucleus disintegrates; that is, full knowledge of
the quantum energies and intensities of the gamma rays emitted and of
those processes occurring in cascade is required. With this, the coin-
cidence method can be intelligently applied. The basis for computing
the source strength from measurements of coincidence rate and individual
counting rates is given below.

This investigation~was conducted at The Ohio State University Research
Foundation under the sponsorship and with the financial assistance of the
National Advisory Committee for Aeronautics.

The activated target foil is placed between two end—window Geiger
counters, which are connected to scaling circuits for measuring the
individual counting rates and to a coincidence mixer which is used to
measure the rates at which pulses occur simultaneously. A piece of
light plastic material may be placed in front of the window of one of
the counters in order to keep out all the beta rays. This counter
acts as the gamma counter, while the other is the beta counter.