Ta = 0500.
Tb = 2200.
Assuming n = 1.2
Tb Ta
2200 0500
Using the mathematical formula T1
will produce T1
.
1
1
Ra
112 1.2
n
1
1
24
Rb
17
17
17
17
T1
6.54 or 6.5 hours or T1
6.54 or 6.5 hours.
4.67 1
yx.8333
.8333
1 2.6
2.6
4.67
NOTE: The yx is the power2 function on a scientific calculator.
Ra
Using the family of curves in Figure A-1, locate 4.67 on the
, read up to where 4.67 intersects
Rb
1
Ra n
the 1.2 decay rate slope, then read left to get the
value.
Rb
17
17
SOLUTION: T1
6.54 or 6.5 hours.
3.6 1 2.6
NOTE: Refer to Appendix C for rounding rules.
Since T1 is the time after H-hour at which Ra was made, then H-hour = Ta T1, which is 0500, 15 January
6.5 hours = 2230, 14 January.
(2) The ABC-M1A1 RADIAC calculator may be used in lieu of the above mathematical
procedure to determine H-hour. However, this is an expedient method and not as exact as the
mathematical technique. For example, use this method to determine H-hour for a particular location with
readings taken at 1600 (50 cGyph) and 1830 (40 cGyph) as follows:
NOTE: This calculation is made assuming standard decay (n = 1.2).
Step 1. Locate the first dose rate reading on the calculator's outer disk (Figure 4-1).
Step 2. Locate the second reading.
Step 3. Determine the time interval between the two readings (1600 and 1830). In this case, the
time interval is 2 ,, hours and starting with H + 1 hour at the first reading (50 cGyph), rotate the
intermediate disk clockwise until a time interval of 2 ,, hours fits between the 40 and 50 cGyph readings
on the outer disk.
CM2306
4-4
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