Table B.3 summarizes the effect of distribution of sources around each of the underwater instruments.
Each column relates to a separate heavily-filtered radiation. For each instrument and for each radiation
are listed the responses corresponding to uniform 4 7 distributions of sources relative to the response to
the same sources concentrated at a point on a line normal to the instruments’ axes.
It can be seen here that calibration by exposure in one direction only is not sufficient when the instru-
ment is to be used underwater.
Table B.4 combines the information in Figure B.3 and Table B.3 so as to give the absolute response to
a distributed source predicted for one particular instrument, Mark Il. Response is given in pa/mr/hr of
each type of test radiation used.
Figure B.5 is a graphical plot of Table B.4 which wil! be useful in later computations. The ordinates
(m;) are the computed responses (in pa/mr/hr) to uniformly distributed sources having photon energies
listed on the abscissa. The photon energies given are, of course, effective energies since truly monochromatic beams werenotavailable.
B.2.6 Response of Mark If to Distributed Sources Comprised of Mixed Fallout Materials. An estimate
now can be made of the Mark II instrument’s response when it is submerged in water contaminated with
active material having any given spectral character.
Let mj represent the response to a monoenergetic source component of energy Ej and which is distributed uniformly around the detector. The ordinates of Figure B.5 approximate m, defined here.
Let D(Ej) represent the fractional dose delivered by the j, the component having energy Ej, that is the
fractional dose delivered by this component per mr/hrof total dose delivered by all components together.
Then in this nomenclature of the 502A Report, the response of the Mark II instrument to a source both
distributed in 4 Pi geometry and consisting of a number of constituents differing in photon energy would be
C= x mj D (Ej)
in units of pa/mr/hr of total dose.
Table B.2 showsthe final steps in deriving the overall response sensitivity C to fallout material distributed in the sea. The value of C is given for each of the four energy spectra of Table B.1.
It would appear that a mean value of C might safely be accepted here and applied to all Mark If measure-
ments made during the cruise, or Caye = 1.21 wa/mr/hr.
B.2.7 Derivation of Complete Response Curves for the Instrument Mark II WhenIt is Used Submerged
in Fallout Contaminants. The single number C is a solitary calibration factor pertaining to the single instrument Mark Il. It is a mean of the estimates of the responses to the four fallout source spectra supplied
by Scoville; and it strictly pertains only to one part of the instrument’s range as a dosimeter. It can be
seen in Figure 2.2 that the relationship between pa response on the instrument dial and dosage is not a
linear one even in connection with hard radium radiation.
The value of the Factor C given above pertains strictly to the use of the Mark II instrument near 19 pa
on its dial simply because the calibration experiments at the U. S. Bureau of Standards described in Figure
C.3 were carried out at or near this mid-scale reading only. Complete calibration at the Bureau atall
parts of the instrument’s scale range would have been expensive and was believed unjustified.
It may be seen on Curve A of Figure 2.2 that 19 wa on Mark II dial corresponds to 10.2 mr/hr of radium
rays, so that at this dose rate the radium calibration factor may be called C{ = 19/10.2 = 1.86 pa/mr/hr,
and by comparisonof this with C it can be seen that the instrument calibration made at Site Elmer against
radium must be increased by the factor C’//C = 1.86/1.21 = 1.5, whenever the instrumentis used in mixed
fallout underwater.
This correction factor was derived for points on the scale near 19 pa, but it would appear suitable for
approximately correcting the radium calibration curve at all other parts of the scale. This is because
there is reason to believe the shape of any of the characteristic curves such as seen in Figure 2.2 would
not be radically different for photon energies effective in fallout radiation.
The final calibration adopted for the Mark If instrument, therefore, was merely the calibration against
radium at Site Elmer (solid curves on Figure 2.2) but raised in numerical value everywhere by a factor of
1.5.
Thatis, the ordinates indicated by the solid curves must be multiplied by 1.5 whenever the instrument
was submerged in water contaminated by fallout debris.
87