Radiation Doses: The maximum permissible workday dose for occupational exposure to radiation is 11 mrem. A 77-kg laboratory technician absorbs 2.3 mJ of 0.5-MeV gamma rays in a work day. The relative biological effectiveness (RBE) for gamma rays is 1.00. What is the number of gamma-ray photons absorbed by the technician in a workday?
A. 3 × 1010
B. 3 × 109
C. 3 × 108
D. 1 × 109
E. 1 × 108
Answer: A
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An automotive catalytic convertor is a packed bed in which a platinum catalyst is coated on the surface of small alumina spheres. A metal container holds the catalyst pellets and allows engine exhaust gases to flow through the bed of pellets. The catalyst must be heated by the exhaust gases to 300°C before the catalyst helps combust unburned hydrocarbons in the gases. The time required to achieve this temperature is critical because unburned hydrocarbons emitted by the vehicle during a cold start comprise a large fraction of the total emissions from the vehicle during an emission test. A fixed volume of catalyst is required but the shape of the bed can be modified to increase the heat-up rate. Compare the heat-up time for a bed 5-cm-diameter and 20-cm-long with one 10 cm diameter and
5-cm-long. The catalyst pellets are spherical, 5-mm-diameter, have a density of 2 g/cm3, thermal conductivity of 12 W/(m K) and specific heat of 1100 J/(kg K). The packed-bed void fraction is 0.5. Exhaust gas from the engine is at a temperature of 400°C, a flow rate of 6.4 gm/s, and has the properties of air.
GIVEN
FIND
The heat-up time (t) for the pellet surface temperature (Tp) to reach 300°C
ASSUMPTIONS
The initial temperature of the bed (To) = 20°C
SKETCH
Briefly describe what you would see if your friend plunged into a black hole. Assume you are watching from orbit of the black hole and have a telescope powerful enough so that you can see him clearly as he falls.
What will be an ideal response?
In common transparent media, as the frequency of light increases, the index of refraction
A) increases. B) stays constant. C) decreases. D) varies as the sine of the incident angle.
For the multiplication of 2.817 m divided by 6.3 m, the result should be reported with how many significant figures?
a. Two b. One c. Three d. Four e. Seven