Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

HEALTH PHYSICS INSTRUMENTATION 509
foil was counted immediately after the end of the irradiation period and found to
have an activity of 100 Bq (2.7 nCi). What was the thermal flux at the point where
the foil was irradiated? The activation cross section for gold is 98.5 b.
9.35. A thermal-neutron counter with dimensions 1 cm (diameter) × 10 cm (length)
is filled with BF3 gas at atmospheric pressure and 20◦C. What is the counting
rate when the counter is in a thermal-neutron flux (Emp = 0.025 eV) of
1000 neutrons/cm2 /s?
9.36. If an irradiated TLD chip is kept at room temperature, the probability that a
trapped electron will spontaneously drop to the ground state is 2 × 10−9 s−1 .If
the TLDs are read at the end of the month, 30 days after distribution, and if an
exposure occurred on the day of receipt of the dosimeter, what percent of the
original dose information would be lost?
9.37. A thin-walled air ionization chamber whose volume is 0.5 cm3 collects a charge of
1.65 × 10−9 C when placed into a plastic phantom whose relative mass stopping
power is 1.1. When manufactured, the ionization chamber was sealed when the
temperature was 25◦C and the atmospheric pressure was 770 mm Hg. What was
the absorbed dose, in Gy and in rads, at the point of measurement?
9.38. Twelve 1-minute counts on a check source gave the following results:
530 480 520 430 470 450 440 530 540 510 490 500
Using the chi-square criterion at the 96% confidence level, determine whether
the counter was operating properly.
9.39. A count-rate frisker whose time constant is 5 seconds reads 500 cpm. What is the
95% confidence interval for this measurement?
9.40. From the following results, which were obtained in a tracer study of the efficiency
of a solvent-extraction process, calculate the solvent-extraction efficiency and the
95% confidence interval:
SAMPLE cpm COUNTING TIME (min)
Mixture 200 5
Extract 180 5
Background 20 60
9.41. How many total counts are required if we want a 1% counting error at the 95%
confidence level?
9.42. A background count of 600 was recorded during a 30-minute counting time. How
long should a sample be counted in order to have a 5% precision for the net
counting rate, if the gross count rate is about 2000 cpm?
9.43. The radiation background on a 10,000 ft. (3048 m) high mountain is measured
with a 1-L capacitor-type air-wall ionization chamber whose capacitance is 50 pF.
The chamber was hermetically sealed, at a temperature of 27◦C and a pressure of
750 mm Hg. It was charged to 150 V, and was exposed for 30 days. After exposure,
the voltage across the chamber had decreased to 100 V. What was the radiation
exposure, in mR, over the 30-day period?