Integrating Photometer

Papers describing Astbury’s development of this apparatus, which measures the total X-ray intensity of a crystal reflection, were published in Proceedings of the Royal Society of London:

1927:    ‘A Simple Radioactive Method for the Photographic Measurement of the Integrated Intensity of X-Ray Spectra’
1929:    ‘A New Integrating Photometer for X-Ray Crystal Reflections, etc.’

An account of the finished result was also published in the Journal of Scientific Instruments in 1929, under the heading ‘A new integrating microphotometer for X-ray crystal reflections’.

The first paper explains the new method which Astbury had discovered, based on a photometer, which measures the intensity of light by directing a light beam through a photographic plate and onto light-sensitive cells. He had adapted this idea as a tool to estimate the intensity distribution in X-ray crystal photographs, replacing the photographic plate with a carbon print, the light with α-rays, and the light-sensitive cells with a simple α-ray electroscope (which measures the presence of electricity). Ionisation in the electroscope is produced which is in direct proportion to the original X-ray intensity which formed the carbon print.

Astbury realised that this new apparatus would be both cheap and easy to make, but he saw its main advantage in that it could be used to calculate the total X-ray intensity (regardless of its distribution over the photographic image) from the total amount of ionisation produced in the α-ray electroscope. Thus it can integrate the X-ray intensity of a crystal reflection, in the same way that the Bragg ionisation spectrometer does. In the publication in Journal of Scientific Instruments, hedescribes it as a ‘photographic analogue of the Bragg spectrometer’.

By 1943, Astbury could still claim that he had invented ‘the one and only truly integrating X-ray photometer’ [28].

In the early 1950s, the apparatus was still being used, although continually refined. Astbury’s letters to Professor George E. Cox give an account of the process of creating and acquiring new apparatus. [32]

The integrating photometer is not in use today, but it is the basis of several devices connected with autoradiography.

by Imogen Clarke