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1

JUL 20 1969:
..
2
.
1
PARALLEL-PLATE MULT ISECTION IONIZATION CHAMBERS
IN NUCLEAR SCIENCE ABSTRACTS
RELEASEDD FOR ANNOUNCEMENT
FOR HIGH-PERFORMANCE REACTORS
Dominique Roux
Oak Ridge National Laboratory
Oak Ridge, Tennessee U.S.A.
h. Introduction
A new model of parallel-plate multisection ionization chambers for reac-
tor control was developed to meet the requirements of high-performance research
and power reactors. The parallel-plate configuration was chosen to allow the
combination of different electrically independent active sections in a single
reactor penetration. Also, the use of parallel-plates has led to a new elec-
trode arrangement to achieve an electrically adjustable gamma compensation.
The insensitivity of this configuration to wide variations in intensity, energy,
and geometry of the gamma background permits a control of the compensation within
few tenths of a percent. The stability of the compensation eliminates the need
of frequent adjustments.
The penalty of a shutdown of a high-performance research or power reactor
is so high that an ionization chamber failure can hardly be tolerated. Accord-
ingly, during the design of this chamber model, much emphasis was put on
reliability performance. For underwater operation, the chamber and connecting
cables are contained in a hermetically sealed and welded metallic housing
assembly. Furthermore, provisions were taken so that the chamber and connecting
cables can be used for a minimum of five years without failure caused by radia-
tion damage.
Missions
Research sponsored by the U.S. Atomic Energy Commission under contract
with the Union Carbide Corp.

hiy... 11....
WOTER
BONTA
2.
Chamber design and construction
2.1 Sensitive volume
The sensitive volume is composed of titanium parallel-plate electrodes,
each 0.012 cm thick, spaced 0.1 cm apart. The electrodes have two different
kinds of shapes, as shown on the bottom part of Fig. 1. syriangular" platos
are used as signal-electrodes, and "circular" plates are used as high-voltage
electrodes. These electrodes may be coated with boron-10 for use as neutron
sensitive chambers, with uranium-235 for use as fission chambers, or left
uncoated for use as gamma-sensitive chambers. Also, the use of boron-coated
and uncoated electrodes, appropriately coated and connected (see Section 3.2),
will give a gamma-compensated neutron sensitive chamber. Electrically inde-
pendent active sections are combined to assemble multipurpose radiation
detectors as required for the different reactor applications.
The middle part of the Fig. 1 is a view of a typical sensitive volume
composed of two neutron-sensitive active sections. The active sections are
• separated and supported by alumina ceramic discs used to anchor the electrode
tie rods. The electrical connections are fed through the aluminum base of the
sensitive volume to a multipin header. The overall maximum electrical inter-
action between two adjacent sections is less than 1%. The interactions are
primarily caused by the passage of lead connections from the front active sec-
tion in close proximity to the rear section.
The sensitive volume is contained in the aluminum can shown on the upper
part of Fig. 1. If required, the can can be welded to the sensitive volume base
so that the sensitive volume can be filled with its own gas filling. However,
in the majority of the applications, the sensitive volume shares the same gas
filling with the rest of the chamber housing volume. Depending on the different
active section combinations, the gas and its filling pressure are selected to
obtain the best compromise.
2.2 Chamber assembly
The chamber assembly (see Fig. 2) consists of four main parts: a sensi-
tive volume described above, a core assembly, a caule assembly, and a nickel
chamber housing. The purpose of the core assembly is to provide a highly
radiation resistant multicoaxial cable extension of 70 cm between the sensitive
volume and the cable assembly. The core assembly, after completion, is a plug-
in unit to which the sensitive volume multipin header and the cable assembly
are plugged. This core assembly has an aluminum body 5.5 cm diameter and 68 cm
long which surrounds a 1.9-cm diameter alumina multihole insulator. The multi-
hole insulator is perforated by seven axial holes. Four holes contain coaxial
assemblies of nickel wires inserted in alumina insulator tubings which are
2
coated with Aquadag on their outside surface. The three other holes are pro-
vided for unshielded nickel wire connections. A multicoaxial plug 18 mounted
on the rear end of the aluminum body. From the multihole insulator to the
multicoaxial plug, the connection insulations are made of preirradiated poly-
ethylene, and the shields are made with copper braids.
The cable connecting the icnization chamber to elec'ironic instruments
consists of seven individual coaxial cables shielded and jacketed together.
• The Insulation is preirradiated polyethy).ene. This cable is connected to a
multicoaxial hermetic ceramic receptacle that mates to the plug of the core
assembly. The cable and the receptacie are inserted in a flexible annular
metallic tubing welded at the chamber end to a receptacle housing. The external
shell of the receptacle is welded to the receptacle housing to provide a her-
metic seal. The cable assembly is also sealed at its other end.
When the chamber assembly is compieted, the chamber housing is welded to
the receptacle housing, and the degassing and gas filling operations are made
through the exhaust tube mounted to the receptacle housing.
After accelerated irradiation of the chamber assembly in gamma-ray fluxes
corresponding to a period of 5 to 10 years of normal operation in reactor
installations, there has been no noticeable damage to the cables and seal or
degradation in the performance of the ionization chamber.
3. Ionization chamber performances
3.1 Boron coated ionization chambers
In the majority of boron-coated neutron-sensitive ionization chamber appli-
cations, the current output range is limited by lack of saturation at high cur-
rents and by the gamma radiation background. Thus, this chamber design was
oriented toward good saturation characteristics and optimization of neutron-
to-gamma-sensitivity ratio.
Good saturation characteristics, even at high current outputs, was obtained
by selecting a close electrode spacing of 0.1 om and a low gas-filling pressure.
These two choices are required to minimize ion recombination. Typically, for
a saturation current of 150 HA, the collection is more than 99% complete at an
applied voltage of 150 V. In this example, the boron-coated active section is
filled with nitrogen at a pressure of 1.05 atm and the current density 18
0.5 kA/cm2.
The optimization of the neutronto-ganma -sensitivity ratio is accomplished
by the selection of the sensitive volume materials and of the filling gas and
pressure. Also, since in many reactor control applications, ionization chambers
are located in light water media, it is advantageous to use a short-length
sensitive volume having a relatively low sensitivity to neutrons and gamma rays.
Because the relaxation length for thermal neutrons is much shorter than that
for reactor leakage gamma rays, for a given neutron output current a low-
sensitivity chamber must be located near to the core surface where the ratio
of thermal neutron to gamma-ray fluxes is favorable. Table 'I mustrates this .
point. The neutron and gamma flux data are taken from Maienschein et al. [1]
In this example it is assumed that both ivaization chambers have the same
neutron togamma -sensitivity ratio.
For the reason previously discussed, the neutron sensitivities of the
individual borun-coated Active sections used in the different chamber configu-
rations are between 3 x 10-15 and 6 x 10-25 A/nv.
For activation consideration, the materials used in the sensitive volumes
are titanium, aluminum, and high-purity alumina ceramic. To achieve low gamma-
sensitivity performances, low-density materials are used and the amount of
material is kept to the minimum necessary for the ruggedness of the assembly.
Titanium is preferred to aluminum as electrode material for that reason. Also,
the boron-10 coating adheres better to titanium when applied with the boron:
deposit technique.2
Several tests were performed to measure the neutron and gamma sensitivities
of the active sections as a function of gas-filling pressure for five gases. The
results are summarized in Table II. An active section consisting of seven
signal electrodes and eight high-voltage electrodes was used. The boron coating
was 0.35 mg/cm2. For each gas, the pressure was adjusted to obtain a neutron
sensitivity of 6.0 x 10-15 A/nv.
It is interesting to notice that the figures of merit indicated in the
Table II vary very slowly as a function of the atomic number 2. Hydrogen has
not been considered, because a pressure of approximately 13 atm would have
been necessary to obtain & neutron sensitivity of 6 x 10-15 A/ny. As a conse-
quence, the increaseà thickness of the sensitive-volume can required to with-
stand such pressure would make the chamber more sensitive to the gamma radia-
tion, and the net gain by using hydrogen would be cancelled.
Nitrogen is the gas used as filling gas for boror.-coated neutron-sensitive
chambers. It is the most stable gas recommended for close electrode spacings,
and it represents a good compromise between different requirements.
The ratio of the neutron-to-gamma-sensitivities for the nitrogen filled
chamber presented in the Table II is higher than any presently commercially
available boron-coated ionization chamber of sensitivities between 5 x 10-14
and 2 x 10-25 Aliv.
-
".'
2 Boron-10 in collodial suspension in mineral oil 18 sprayed on the elec-
trodes, then the oil 18 baked out at 340°c.
-
..
.
...
3.2 Gamma compensated ionization chambers
The use of parallel plates has led to a new electrode configuration to
achieve an electrically adjustable gamma compensation. This configuration 18
shown schematically in Fig. 3. The signal electrodes have a smaller surface
area than the high-voltage electrodes. For equal positive and negative polar-
izing potential.s, the zero-equipotential surface (Fig. 3a) is an extension of
the signal electrode, and as a consequence, the electrical field lines are
straight. For unequal polarizing potentials (Fig. 3b), the field lines are
distorted and the effective collection volumes, which are determined by the
field lines drawn heavily in the figures, are changed accordingly. The satura-
tion characteristics of the two situations are essentially the same; thus, adjust-
ment of compensation by this method does not affect the saturation.
The gamma-compensated active section contains 16 signal electrodes, 9
positive high-voltage electrodes, and 8 negative high-voltage electrodes.
Figure 4 illustrates the electrode sequence. To reduce the influence of geome-
trical effects on the gamma compensation, the length of the electrode stacking
is nearly equal to the electrode diameters. For the same reason, the eight
ion chamber wits indicated in Fig. 4 are symmetrical relative to the negative
high-voltage electrodes. The active section is filled with nitrogen at 1.05 atm.
Without gamma compensation, the chamber sensitivities are 6.0 x 10-15 A/av and
1.4 x 10-12 A.hr/r.
Manufacturing tolerances are such that initially the gamma currents cancel
within 1.5%. Figure 5 shows the gamma compensation as a function of electrode
voltages. The slope of the curve corresponds to 1-3% compensation change per
100 V change in negative polarizing voltage, with the positive voltage equal to
+300 v. These measurements were made in a hot cell with a 60c0 source.
Experimental results show the insensitivity of the gamma compensation to
wide variations in intensity, energy, and geometry of the gamma source. Typical
gamma compensation results obtained with the same ionization chamber are given
in Table III.
The gamma compensation as a function of the chamber current was also
measured in a hot cell with a 60Co source. The experimental results are shown
in Fig. 6. The gamma compensation values for fixed electrode voltages (+300 V,
-250 V) are plotted as a function of the chamber uncompensated current. Over a
range of three decades of chamber uncompensated current, the deviation of the
compensation stays within +0.1%. This range of three decades, expressed as
• gamma flux, is from 2 x 106 to 2 x 103 r/nr.
The experience in reactor application indicates that this method of gamma
compensation is stable. For a given reactor gamma background condition, a varia-
tion of less than 0.4% in compensation has been measured over a one-year period.
3.3 Fission chambers
A fission chamber with thi.s type of parallel-plate arrangement was built .
and tested. The active section consists of 15 signal electrodes and 16 high-
voltage electrodes coated with 1.0 mg/cm2 of > 99% 2350. The total coating
surface 16 1000 cm2. The electrode spacing 1s 0.2 cm, rather than the standard
0.1 cm, to increase the gas volume required fo;• the fission fragment energy
collection.
When used as pulse fission chamber, the active section has a sensitivity
of 0.55 count per neutron per cm2. Figure 7 shows the neutron plateau and
the alpha and gamma pileup characteristics of the detector filled with a mix-
ture of 97% argon and 3% CO2 at a pressure of 3 atm. Discrimination against
gamaa pileup pulses in a flux of 3 x 103 r/hr can readily be achieved.
For application as an Average-Current ionization chamber, the active
section was filled with a gas mixture of 99% argon and 1% nitrogen at 0.79 atm.
The corresponding sensitivities are 7.2 x 10-14 A/nv and 9 x 10-22 A.hr/r; the
alpha residual current is 1.1 x 10-20 A. At a saturation current of 1 x 10-3 A,
the collection is more than 95% complete at an applied voltage of 200 V.
In a fission chamber used as an Average-Current ionization chamber, the
chamber current arises from two sources. The first source is ionization pro- .
duced by the particles emitted at the time of the fission: the fission frag-
ments, and the prompt beta and gamma rays. The second source, which is undesir-
able, is the ionization produced by the beta and gamma rays emitted by the
radioactive fission products. This current, which can be called residual cur-
rent, is related to the "neutron history" of the chamber: thermal neutron flux,
chamber irradiation time, and time after irradiation.
To evaluate this residual current, an experiment was performed with the
fission chamber previously described for application as an Average-Current
ionization chamber. For this experiment, the chamber was irradiated for 5 min
in a constant thermal neutron flux that was between 103 and 10% higher than
any other previous irradiation of this chamber, in order to override the effect
of prior residual currents and the alpha current. The current I was
1 x 10-3 A. After irradiation, the chamber was removed from the reactor flux,
and the residual current I was recorded for 3 hr. Figure 8 shows the ratio
of I versus I as a function of the time after irradiation. Figure 8 also
shows a family of curves of I/I for different times of irradiation, calculated
using the empirical formula of Vatermyer and Weills[aj for afterheat from beta
and gamma rays of irradiated 2350. Fitting of the calculated currents with
the measured current was made at 10 s after irradiation.
Considering the estimated accuracy of +50% of the Untermyer and Weills
formula, the experimental and calculated curves fit satisfactorily for an
irradiation time of 5 min. From these curves, l'ission chamber range limita-
tions for Average -Current application can be determined for other configura -
tions.
3.4 Gamma -sensitive ionization chambers
Although neutron flux is used universally as a parameter for reactor
control and safety, the use of gamma radiation from the core can be advan-
tageous, especially in a light-water shield which attenuates neutrons much
more strongly than gamma rays. In high-performance research reactors, the
many beam holes and experiments can result in flooding, by mistake or accident,
a part of the region between the core ard the detector. Thus, safety gamma
chambers are recommended, because in this type of accident the gamma flux is
much less reduced than the neutron flux and a better safety action against,
core meltdown is obtained.
On the other hand, gamma chanbers have a well-known limitation: the radio-
active decay of the fission products gives rise to a signal that is not propor-
tional to the reactor power, Experiments have been performed with a gamma
chamber to be described iater in this section to evaluate the fission-product
component of the chamber current. Results show that as little as 50 cm of
light water is enough to absorb the low-energy fission-product gamma rays;
however, the high-energy fission-product component cannot be significantly
reduced by additional lead as a shield.
In the ORR (Oak Ridge Reactor), for a gamma chamber at 50 cm from the core
surface, 13% of the total current at steady power was attributed to the fission-
product gamma rays. Other measurements show that 85% of the fission-product
gamma component buildup occurs within 10 min after the fission. As a result,
10 min after an hypothetical step increase from zero to full reactor power,
this gamma chamber would reach 98% of its total current at steady power.3
Also, 10 min after a scram of the ORR, after an operating period of two weeks,
the measured residual current was 2%. This behavior is perfectly acceptable
for safety applications at near reactor rated power if one compares errors of
several hundred percent experienced when using a neutron sensitive chamber
located behind a flooded beam hole.
Different gamma -sensitive active sections have been built and tested using
the parallel-plate configuration. The two major factors which affect the sensi-
tivity of a gamma chamber are the electrode material and the gas filling. For
maximum output of gama current, the electrodes should be thick and made of a
30.87 + (0.85 x 0.13) = 0.98
high-density material. Nickel appears to be the most prectical material 11
one considers density, activation cross section and radioactive life, and
availability. Calculations and experiments have shown that optimized gamma
sensitivity per unit of active section volume 18 obtained with nickel elec-
trodes 0.025 cm thick, spaced, 0.1 cm apart. By comparison, for a given Pulling
gas and gas pressure, this configuration 18 two times more sensitive than
0.012-cm thick titanium electrodes also spaced 0.1 cm apart.
Gamma sensitivities as function of different filling gases are given in
Table N Por an active section consisting of 37 signal and 38 high-voltage
nickel electrodes 0.025 cm thick each and spaced 0.cm apart. In each case
the gus pressure is 1.32 atm. Experimentally, the chamber sensitivity was
found to vary linearly with the gas pressure up to 5 atm. Also, the sensitivity
is almost proportional to the gas atomic number 2.
. Conclusions
To date, eisht different models of Lonization chambers have been designed
according to this concept and used in various reactor applications. One
particular model is a combination of four individual active sections in the
same sensitive volume. Two chamber models are commercially available.
More than twenty chambers have been fabricated and installed in different
ORNL reactors. Over a period of 20 months of reactor operation no failure has
occurred.
5. Acknowledgments
The author acknowledges the important contribution of J. C. Gundlach in
the early stage of this project. Also the author wishes to express his grati-
tude to S. H. Hanauer for his many helpful suggestions and to G. C. Guerrant
and C. B. Stokes for their devoted assistance.
Bibliographical References
(1) Matenschein, F. C., et al. , ORNL-2518.
(2) Untcrmyer, s., and Wells, J. T., "Heat Generated in Irradiated Urnalum,"
ANL4790.
Figure Captions
Fig. 1. View oť a Two-Active Section Sensitive Volume. The sensitive -volume
can is shown at the tor, and the two types of electrodes used in the
assembly are shown at the bottom.
Fig. 2. Ionization Chamber Assembly.
Fig. 3. Diagram of Adjustable Electrical Gamma Compensation.
Fig. 4. Electrode Sequence of Gamma Compensated Ionization Chamber.
Fig. 5. Gamma Compensation as a function of Electrode Voltages.
Measurements were made in a hot cell with a 60co source.
Fig. 6. Gamma Compensation as a function of Chamber Current.
Fig. 7. I.itegral Curve of the Parallel-Plate Fission Chamber.
Fig. 8. Calculated and Experimental Fission Chamber Residual currents Due to
Fission Product Activity as a Function of the Time After Irradiation.
Parametric curves for different irradiation times.
TABLE I. Gama Current Outputs for Two Different Chambers Located in a
Reactor Light Water Shield
Swimming pool 1 MW,
Sensitivities
H20 shield, 50 MA neutron current
neutron
gamma
(A/nv)
3 x 10-24
(A.hr/r)
1 x 10-11
Gamma flux
(r/nr)
1 x 106
gamma current
(MA)
10
high sensitivity
· chamber
low sensitivity
chamber
3 x 10-25
1 x 10-22
2.7 x 106
2.7
TABLE II. Ratio of Neutron to Gamma Sensitivities for Different Chamber Gas Fillings
Atomic
number
Gas
Gas pressure
in atm.
Figure of merit
2
4.80
1.15
1.10
Sensitivity
neutron
gamma
(A/nv)
(A.br/r)
6 x 10-15
1.25 x 10-22
6 x 10-15
1.45 x 10-12
6 x 10-15
1.57 x 10-22
6 x 10-15
2.4 x 10-12
6 x 10-15
2.6 x 10-22
1.0
Ar
0.76
0.90
Kr
0.53
0.60
Xe
0.34
0.55
Ratio of neutron to gamma sensitivities, normalized to 1 for N2.
TABLE III. Gamma Compensation Obtained for Various Gamma Sources with Fixed Electrode Voltages
(+300 V, -200 V)
Source Condition
Deviation from
Total Compensation
Gamma Source
Facility
Hot cell, 60co
Shield and
Geometry
2000 curie
+0.8
2000 curie
+1.3
Hot cell, 6000
BSR, fission products
BSR,' fission products
BSR, fission products
BSR, fission products
+0.9
10 cm air, source at
front of chamber
10 cm air, source at
side of chamber
20 cm of water
20 cm of water
20 cm of water
cm of water
10 min after reactor scram
90 min after reactor scram
1 week after reactor scram
1 week after reactor scran
+0.9
+0.6
+0.5
"The sign + means undercompensation; that is, I, >Iv
'Bulk Shielding Reactor.
TABLE IV. Chamber Gamma Sensitivity as Function of Different Filling
Gases. Gas pressure: 1.32 atm.

28
| He |
N, |
Ar
|
Kr
|
Xe
Gamma sensitivity
(A.hr/r) x 10-11
0.25
1.23
1.87
3.8
6.9
Gas atomic number (2)
2
| 14
18
L
36
54
11.42 11.0
1.18
1.20
1.45
Gamma sensitivity
Z
Normalized to 1 for N2.
PHOTO 679514
5


ELECTRODE TIE ROD

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ACTIVE
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ACTIVE
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10
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-
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----
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-
-
-
-
-
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BLANK PAGE
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VA

Dominique Roux, PARALLEL-PLATE MULTISECTION IONIZATION CHAMBERS FOR
HIGH-PERFORMANCE REACTORS
Fig. 1. View of a Two-Active Section Sensitive Volume. The sensitive-volume
can is shown at the top, and the two types of electrodes used in the
assembly are shown at the bottom.

ORML-DWG 65-5950
HERMETIC RECEPTACLE-
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SENSITIVE VOLUME
Dominique Roux, PARALLEL-PLATE MULTISECTION IONIZATION CHAMBERS FOR
HIGH-PERFORMANCE REACTORS
Fig. 2. Ionization Chamber Assembly.
ORNL-DWG 65-5849
DA EFFECTIVE COLLECTION VOLUME
208 COATING
+
V
+2V
ELECTRIC
FIELD LINES
Ott
-SIGNAL
ELECTRODES
-V
10) EQUAL + AND – POLARIZING POTENTIALS
(6) UNEQUAL + AND - POLARIZING POTENTIALS


Dominique Roux, PARALLEL-PLATE MULTISECTION IONIZATION CHAMBERS FOR
HIGH-PERFORMANCE REACTORS
Fig. 3. Diagram of Adjustable Electrical Gamma Compensation.
ORNL-DWG 64-4232R
ION CHAMBER UNIT (8 each)
----
(+------
S SS
-
UNIT CELL (16 each)
DATING

Dominique Roux, PARALLEL-PLATE MULTISECTION IONIZATION CHAMBERS FOR
HIGH-PERFORMANCE REACTORS
Fig. 4. Electrode Sequence of Gemma Compensated Ionization Chamber.
ORNL -DWG 63-3062R2
+3

FIXED POSITIVE ELECTRODE
VOLTAGE: +300 V
BASIC COMPENSATION: -1.2 %
DEVIATION FROM TOTAL COMPENSATION (%)
-3
-400
-350
-150
-100
-300 -250 -200
NEGATIVE ELECTRODE VOLTAGE (V)
.
....
Dominique Roux, PARALLEL-PLATE MULTISECTION IONIZATION CHAMBERS FOR
HIGH-PERFORMANCE REACTORS
Fig. 5. Gamma Compensation as a function of Electrode Voltages.
Measurements were made in a hot cell with a 60CO source.
..
ORNL-DWG 63-3066R

CHAMBER
HV+: +300 V
CHV_: -250 VS
CHAMBER UNCOMPENSATED CURRENT (A)
_
10-9
OVERCOMPENSATION
– UNDERCOMPENSATION
-2 -1 0 1 2
DEVIATION FROM TOTAL COMPENSATION (%)
-3
3
Dominique Roux, PARALLEL-PLATE MUITISECTION IONIZATION CHAMBERS FOR
CRNL-DWG 63-3067R

...
SH
ELECTRODE SPACING: 0.2 cm
GAS FILLING: 3 atm ARGON +3 % CO,
NEUTRON SENSITIVITY: 0.55 counts
per neutron/cm2
THERMAL NEUTRONS
GAMMA PILEUP
# 3.0 x 100 r/hr
counts/s
_
ALPHA PILEUP
o
10 20 30 40 50
PULSE HEIGHT SETTING (arbitrary units)
60
• Dominique Roux, PARALLEL-PLATE MULTISECTION IONIZATION CHAMBERS FOR
HIGH-PERFORMANCE REACTORS
Fig. 7. Integral Curve of the Parallel-Plate F18sion Chamber.
1 min
5min
ORNL-DWG 65-5987
1 day
10 days
1 hour
NORMALIZED RESIDUAL CURRENT
4 day
10 days
NI
LW
EXPERIMENTAL VALUES
CALCULATED VALUES
109
106

t, TIME AFTER IRRADIATION (s)

.
Dominique Roux, PARALLEL-PLATE MULTISECTION IONIZATION CHAMBERS FOR
HIGH-PERFORMANCE REACTORS
Fig. 8. Calculated and Experimentai Fission Chamber Residual Currents Due to
Fission Product Activity as a function of the Time After Irradiation.
Parametric curves for different irradiation times.
END
DATE FILMED
11/ 17 /65






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