Page 253 - Radiochemistry and nuclear chemistry

Detection and Measurement Techniques 237

8.12. Exercises

8.1. A detector has a 1 cm 2 efficient area perpendicular to a 3,-particle flux produced by a source 7 m away. The
sensitivity for the 0.73 MeV 7-radiation is 8.2 %. (a) What must the source strength be for the detector to register
1000 cpm? Co) What fraction of the radiation is absorbed in the air space between source and detector?
8.2. A - 100 MeV fission fragment is stopped in a plastic plate with density -- 1 and an average atomic spacing
of 0.25 nm. Estimate (a) the range in the plate, and (b) the ionization density (ion pairs/Lm'l). If the ionization
along the track is spread out perpendicular from the track so that 1 in 10 atoms are ionized (c) what would be the
diameter of the track? From the track dimensions (d) calculate the average energy deposition to each atom within
the "cylinder', and, using the relation E = 3kT/2, (e) estimate the average temperature within the track volume.
In lack of basic data for the plastic material, use data for water.
8.3. Plutonium in an urine sample is soaked into a photographic emulsion so that the emulsion increases its
volume by 20%. The 12 #m thick emulsion is dried to original thickness and then left in darkness for 24 h. After
development, or-tracks are counted and an average of 2356 tracks cm -2 found. If the plutonium consists of 67%
239pu and 33 % Z4~ what was the plutonium concentration in the urine?
8.4. A ~Cm sample is measured in an ion chamber (Fig. 8.9). The voltage drop over a 3 x 1013 f/resistor is
measured to be 0.47 V. What is the activity of the sample if all c~'s emiued in the chamber (21, geometry) are
stopped in the gas?
8.5. In a proportional counter filled with methane of 1 arm the gas multiplication is 2 x 104-. What is the
maximum pulse size for a 5.4 MeV ~, if the ion-pair formation energy is assumed to be 30 eV? The capacitance
of the circuit is 100 pF.
8.6. In a GM counter, sample A gave 12630, B 15480, and A + B together 25147 cpm. (a) What is the resolving
time of the counter? (b) With the same counter, the distribution of radioactive samarium between an organic phase
and water was measured according to Dm= Ro~s/Raq. The measured Rorg is 37160 cpm, and that of Raq is 2965.
Co) What is the measured Din? (c) Using corrections for resolving time, what is the true D-value?
8.7. Assume that 109 alcohol molecules are dissociated per discharge in a GM tube of 100 cm 3 filled with 90%
Ar and 10% ethyl alcohol vapor at a pressure of 100 mmHg (25~ What is the lifetime of the tube in terms of
total counts assuming this coincides with the dissociation of 95 % of the alcohol molecules?
8.8. A 1 mm tick surface barrier detector of I0 mm diameter has a resistivity of 7000 fl cm and a capacitance
of 50 pF at 300 V reverse bias. Calculate the resolving time (time constant).
8.9. A plastic scintillation detector was to be calibrated for absolute measurements of ~-radiation. For this
purpose a 2.13 x 10 -s M :~*'I'ICI 3 solution was available with a specific activity of 13.93 #Ci ml-l; ~TI emits
~-particles with Ema x 0.77 MeV. Of this solution 0.1 ml is evaporated over an area of exactly 0.1 cm 2 on a
platinum foil. The sample is counted in an evacuated vessel at a distance of 15.3 cm from the detector, which has
a sensitive area of 1.72 cm 2. The detector registers 2052 cpm with a background of 6 cpm. What is (a) the surface
weight of the sample, Co) the backscauering factor, and (c) the detector efficiency for the particular/~'s?
8.10. A sample counted for 15 min gave 9000 total counts. A 30 min background measurement registered 1200
counts. Calculate (a) the count rate or the sample alone, with its standard deviation, and Co) with its probable error.
8.11. A certain sample has a true average counting rate of 100 cpm. What is the probability that 80 counts would
be obtained in a 1 min recording?

8.13. Literature

H. YAGODA, Radioactive Measurements with Nuclear Emulsions, Wiley, 1949.
G. B. COOK and J. F. Dur~cAr~, Modern Radiochemical Practice, Oxford University Press, 1952.
S. FLOGGE and E. CREUTZ (Eds.), Instrumentelle Hilfsmittel der Kernphysik II. Handbuch der Physik, XLV,
Springer-Verlag, 1958.
G. D. O'KELLEY, Detection and Measurement of Nuclear Radiation, NAS-NS 3105, Washington DC, 1962.
E. SCHRAM and R. LOMnAERT, Organic Scintillation Detectors, Elsevier, 1963.
W. H. BARKAS, Nuclear Research Emulsions, Academic Press, 1963.
W. J. PRICE, Nuclear Radiation Detection, McGraw-Hill, 1964.
P. C. STEVENSON, Processing of Counting Data, NAS-NS 3109, Washington DC, 1966.