. Biophysical research methods. Biophysics -- Research. 612 ADOLF F. VOIGT trostatic chargers are available, a small power pack is much more re- liable for providing thi» potential. The scale of the electi'oscope is not linear throughout its length so that it should be calibrated, or only one part of it, preferably the first third, should be used. In the absence of any radioactive sub- stance, an electroscope fiber will still discharge at a slow background rate. The magnitude of this background will depend on conditions but is usually to scale divisions per minute. It is caused by co
. Biophysical research methods. Biophysics -- Research. 612 ADOLF F. VOIGT trostatic chargers are available, a small power pack is much more re- liable for providing thi» potential. The scale of the electi'oscope is not linear throughout its length so that it should be calibrated, or only one part of it, preferably the first third, should be used. In the absence of any radioactive sub- stance, an electroscope fiber will still discharge at a slow background rate. The magnitude of this background will depend on conditions but is usually to scale divisions per minute. It is caused by cosmic rays, minute amounts of radioactive substances near and in the instruments, and by leakage across the insulator. After the produc- tion of ion pairs by radiation, there is always a possibility that the ions will recombine and not be measured. This possibility increases Eyepiece of opticol system Adjustment knob optical system Fiber \ Window. Objective Ctiarging button Charging wire Fig. 3. Lauritsen electroscope. (Courtesy F. C. Henson Co.) with increasing density of ionization, so that as the intensity of radia- tion increases the electroscope no longer collects as large a fraction of the ions. There is then a limit above which the response of the elec- troscope will not be proportional to the amount of radioactive sub- stance. One should either staj' below this point or make a correction. For a particular case the point at which an error greater than 1% was introduced by this nonlinearity was found to be 60-70 scale divisions per minute. Numerous modifications of the chamber of the Lauritsen electro- scope have been made for different purposes (2, p. 119). The instru- ments come w4th a cylindrical chamber of sheet aluminum or a similar shell with a thin aluminum window, to permit the measurement of low energy j8 rays. Other modifications available from the manufac- turer include a brass chamber into which the sample can be put. A chamber, preferable to those on market,
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