In 1927, Dr. Den Hoed uses the Ionomicrometer of Mallet and Danne [25, 26]. The detector of this dosimeter consists of a hollow aluminium sphere with an external diameter of 23 mm. The interior forms an ionization chamber of 1 cc in which is included a micro electrometer which can be read through the glass windows F.
A = aluminium wall of approx. 6 mm thick, B = metal holder with pliable gold strip C, D = pressure contact with opposing spring with which B and C are charged with 80 Volt, F = 2 glass windows for internal lighting and fixture of a telescope with which the results from the gold strip measurements can be read. B, C and D are insulated with amber and fixed.
In its discharged state, the gold strip lies alongside the rod B. After charging to 80 Volt, the gold strip bends out under the influence of static charge.
When exposed to the X-ray beam, the air in the sphere becomes ionized. The released electrons cause a slow discharge of the gold strip causing this to gradually return to its original position alongside the rod B. The change in the position of the gold strip is measured along a scale in the telescope. In this way the intensity of the radiation beam can be determined thereby allowing measurement of the beam characteristics.
Daniel den Hoed spends much time on the improvement of the dosimetry and the measurement of intensity of the ionising radiation. He is helped in this by co-workers from the Physics Laboratory of the University of Amsterdam. In collaboration with Mr. Stoel, a photographic method is developed for measurement of the dose .
With film, measurements can be made right up to the radium needles. In the measurement set up, measurements are made at various depths using rubber sheets as an equivalent of human tissue between the film sheets.
Den Hoed defines the sensitivity of a Kodak film for X-rays and he calculates the intensity of the radiation beam from the measured film blackening.
The results of this filmdosimetry are compared with the "iontometric" data and with the biological calibration methods, and good agreement is found. The advantage of the photographic method is that measurements can be made at small distances from radium needles. This was applied in the irradiation of tumours of the eye lid.