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significantly, indicating clearly the collimating
effect. The flat top at the peak was of the same
width as the collimator hole and represents the
resolution of the collimator when used with a point
source.
Gamma Scan of the MSRE Heat Exchanger
A gamma radiation scan was made on May 19,
1967, over the area in the reactor cell containing
the fuel-to-coolant-salt heat exchanger and fuel
salt line 102.
To accomplish the radiation scan, the portable
maintenance shield was located over an opening
in the reactor cell above the heat exchanger, and
the collimator and gamma ion chamber were in-
stalled in a hole in the portable shield. Measure-
ments were made by moving the portable shield
until the ion chamber was directly above the de-
OR N L- 0 WG 6 7- l I791
PORTAB1.E , [/ ION CIiAMBFii
/
48-in. COLLIMAl-OR TUBE
( 4V3. in. OD)
-.) ~~ --
INCREASING
Y
HEAT EXCHANGER
6 ft -3 in. FF 101
FF 102
I
18 it-9 in
I
13 ft-2 in. 1
Fig. 2.5. Source-Detector Geometry for Gamma Scan
of MSRE Heat Exchanger.
43
sired points in the cell. Figure 2.5 shows the
vertical distances from the tip of the ion chamber
to items in the cell.
The orientation of the various ion chamber
traverses of the cell along with lines of equal
radiation levels are shown superimposed over the
heat exchanger and line 102 in Fig. 2.6. It wilI
be noted that the maximum radiation levels were
3.5 r/hr above the heat exchanger center line
and 300 mr/hr above line 102. When the collimator
was removed, the radiation level at the top of the
hole in the portable shield was 100 r/hr.
These results show that the collimated gamma
ion chamber measures radiation levels with very
good resolution of position, even in the presence
of high background radiation levels. Therefore,
this method of radiation measurement should be
useful in locating accumulatiot~s of fission products
in reactor components and in planning maintenance
operations.
Gamma Energy Spectrum Scan of the
MSRE Heat Exchanger
On May 20, 1967, a gamma energy spectrometer
was set up over the same area in the reactor cell,
and measurements were made at several points
over the fuel-to-coolant-salt heat exchanger. This
exploratory experiment was made primarily to
evaluate the method.
The measurements were made by using a 3- by
3-in, NaI crystal and photomultiplier tube mounted
in a lead shield which had a collimating hole
'/32 in. in diameter and 7 in. long under the crystal.
The array was mounted on the hole in the portable
shield in the same manner as the collimator for
the gamma scan described earlier.
There were strong, well-defined peaks at about
0.48 and 0.78 Mev corresponding to Io3Ru and
"Zr-95Nb. The resolution of the NaI crystal was
not good enough to determine whether the second
peak was from g5Nb only or from the "Zr decay
chain. The energy resolution could be improved
by using a different crystal, since the collimator
did not appear to degrade the peaks. The problem
of assigning absolute disintegration rates to these
isotopes is complicated by the geometrical arrangement
of the heat exchanger and heaters, ihe
effect of gamma energy on absorption by the collimator,
the source distribution in the heat exchanger,
and the counter efficiency. It is believed,
however, that some measure of the relative