Nucleonics and Analytical Chemistry Symposinm
Instruments for Mea3 r
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1
Radioactivity
BORKC)WS&I Laborat ow,Oak Ridge, Tenn.
One of t h e ' m o s t widely used _-.-.Eoactive tracer studies is the Geiger-Miller counter, which has attained this widespread use primarily beeause of its simplioity, sensitivity, and availability. Less familiar is the use of proportional wunters for the detection of weakly ionizing radiations. Betaproportional wunters of the Row type which have counting yields over 50% and which are especially useful for low energy 6- and x-ray emitters are described. Permanently filled @-proportional ___i___ Il
M E A S U R E M E N T pf rfLdioactivityis ~enerallybased on the detection of iomeation produced in gases by energetic charged particles. The charged particles may be of nuclear origin--e.g., &particles, positrons, rrparticles-or they m y be electrons produced by the interaction of y- or x-rays with matter. The density of ionization dong the psth 0f.a charged particle is dependent on its velocity, charge, and the nature and pressure of the gas in the detector. A 0.5-m.e.v. &particle has a range of 1 meter in air at atmospheric pressure and will produce about 60 ion pairs per centimeter of its path. A 5-m.e.v. +particle under the same conditions produces about 25,000 ion pairs per centimeterandisstoppedin3.5cm. The detection of an rrprtrticle can he accomplished with a relatively shallow chamber with no gas amplification, because 25,000 ion pairs produce a voltage pulse well above the noise level of a pulse amplifier. However, the detection of a few ion pairs produced by p-particles requires a detector with high gas amelification. The measurement of j3-and y-radiation is made by either of two methods. The first method employs pulse counters in which individual ionizing particles are detected as single ionizing events or "counts." Geiger-Muller countem and proportional counters are representative of this class. In the second method the total ionization produced in an ionization chamber in a given time interval is measured with an electrometer-e.g., quartz fiher, vacuum tube, or. dynamic condenser type (7). The Lauritsen electroscope also falls in this category. 5. ,.,Pulse counters as a general rule are more sensitive than integrating ionization chamhers; however, for certain types of radiation--e.g., x-rays, y-rays, soft p-particles-the ion chamber sensitivity is comparable to the pulse counter. I
,munters ifter
have shown no change of characteristics
lox4 oounts.
Because of the simplicity of m a n y tracer experiments can he designed that the use of a y-emitting isotope in
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in some eases yield results in one tenth the time a n d effort that would otherwise be re+red if a pemitter were used. Ionization charnhers for C" measurements are especially useful fo. +ha I.sas.y - ~-f~ samples of low sp 1_
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asfollows: The electrons liberated in the sensitive volume of the counter by an ioniaing particle are attracted by the positive center wire. When the electrons reach the region of high field intensity close to the wire, sufficient energy is acquired hy them between collisions with gas molecules to ionize the gas further and release addit,ional electrons. Each of these electrons similar1 produces furher ionization, so that a large number of electrons &ally reach the m e . If the center wire potential is not too hish. the diseharre ~~~~~~~
,l~t.othe numherbf ion pairsproduced by the initial
