Ion chamber with a flat sensitivity response characteristic

An ion chamber exhibiting a flat response to a wide range of incident gamma energy is provided by a high-pressure fill gas mixture of a first major constituent, low atomic number gas which exhibits a reduced gamma response at low gamma energy levels, and a second minor constituent, high atomic number gas which exhibits an increased gamma response at low gamma energy levels. The preferred fill gas mixture is nitrogen as the major constituent and xenon as the minor constituent.

BACKGROUND OF THE INVENTION 
The present invention relates to ion chamber radiation detectors in which 
an electrical current is generated in response to the radiation field of 
the surrounding environment. Such ion chambers rely upon the interaction 
of the incident radiation with the fill gas within the chamber to produce 
an electrical current through the gas between voltage biased electrodes. 
The electrical current produced is a function of the radiation impinging 
on the chamber. Ion chambers are widely used as radiation monitoring 
devices, for example in nuclear reactor containment environments. 
It is desirable for detectors of this sort to have a response in amperes 
per roentgen per hour which is independent of the energy of the gamma-rays 
impinging on the chamber. If this is true then the chamber correctly 
indicates the health hazard associated with a gamma field regardless of 
the energy spectrum of the field. This is most easily achieved by use of 
air or tissue equivalent radiation monitoring chambers, which are 
constructed of low atomic weight organic materials to simulate either air 
or tissue. 
In ion chambers designed for post-accident nuclear reactor environments, 
the ion chamber must be capable of withstanding intense radiation, high 
pressure, high temperature, and even corrosive chemical reactants. These 
conditions eliminate the use of organic materials for construction of the 
detector, and dictate that high temperature resistant metallic members be 
used. However, metal walled ion chambers do not exhibit the same energy 
independent response attained with air and tissue equivalent detectors. It 
has been found possible in prior art metal walled ion chamber designs to 
satisfy the energy independent requirement when the fill gas was 
maintained at approximately atmospheric pressure. Such low fill pressure 
ion chamber designs however suffer from reduced sensitivity. The most 
widely utilized fill gas in such ion chambers is nitrogen. The more 
sensitive, high fill pressure, metal walled ion chambers of the prior art 
typically utilize nitrogen gas at a fill pressure of up to about 10 
atmospheres. Such high pressure, high sensitivity, walled ion chambers do 
not exhibit the requisite flat energy response characteristic. Recent 
regulations for such accident monitoring ion chambers call for an energy 
response which is flat within .+-.20% of the mean value. The high-pressure 
nitrogen fill gas ion chamber fails to meet this criteria because the 
response decreases significantly at low gamma ray energies. 
It has generally been recognized that for many ion chamber designs having 
low atomic number gas fills, such as nitrogen, a decreased response 
characteristic is observed at low energies of the gamma radiation field. 
This is particularly true where metal walled thick electrode structures 
are utilized. It is also known that for ion chambers with high atomic 
number fill gases such as xenon, the signal response increases 
significantly at low gamma ray energies. 
SUMMARY OF THE INVENTION 
An ion chamber which exhibits a flat sensitivity response to a wide range 
of incident gamma energies is provided by a selected fill gas mixture of a 
first major constituent, low atomic number gas, which exhibits a reduced 
gamma response at low gamma energy levels, and a second minor constituent, 
high atomic number gas, which exhibits an increased gamma response at low 
gamma energy levels. The major constituent, low atomic number gas, is 
preferably nitrogen, and the high atomic number, minor constituent gas is 
preferably xenon. The fill gas mixture is present in the ion chamber at a 
pressure of about 10 atmospheres and the preferred volume ratio of 
nitrogen to xenon is about 97.75:2.25 for the embodiment described. The 
sensitivity of gamma response within a .+-.20% range is obtained over a 
range of incident gamma energy from about 0.1 MeV to 3 MeV. The low atomic 
number, major constituent fill gas is selected from the group of nitrogen, 
neon, argon, helium, and mixtures thereof. The high atomic number, minor 
constituent may be xenon and/or krypton. The volume percent of the minor 
constitutent may be up to about 15 volume percent of the total fill gas 
volume in achieving the desired flat response in other embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention can be best understood by reference to the embodiment 
seen in FIGS. 1 and 2. The ion chamber 10 comprises a hermetically sealed, 
generally cylindrical outer electrode 12. A concentrically disposed first 
tubular electrode 14 is spaced within the volume defined by the outer 
electrode 12. A second tubular inner electrode 16 is disposed 
concentrically within the first electrode 14 and spaced therefrom. 
Suitable insulating hermetic end seals 20 and 22 are provided at opposed 
ends of the generally tubular electrode structures. The extending end 14a 
of first tubular electrode 14 extends through insulating hermetic end seal 
20 to serve as an electrical lead-in. The extending end 16a of the second 
tubular electrode 16 extends through insulating hermetic end seal 22 to 
also serve as an electrical lead-in. The respective volume between 
electrode members 12 and 14 and between electrode 14 and 16 following 
evacuation are filled with a mixture of nitrogen and xenon at a pressure 
of about 10 atmospheres. The volume ratio of nitrogen to xenon is 
preferably about 97.75% nitrogen to 2.25% xenon. 
A first gas-filled chamber 18 is defined between electrodes 12 and 14, 
while a second gas-filled chamber 24 is defined between electrodes 14 and 
16. 
The first tubular electrode 14 is biased at a high voltage, typically about 
1,000 volts, relative to the outer electrode 12 and the second inner 
electrode 16, which are typically at ground potential, to provide two 
separate current collection zones with the total current or response being 
summed from these two chambers 18 and 24. 
Table I below lists the calculated response characteristics of the ion 
chamber of the discussion over a gamma energy spectrum range of 0.088 MeV 
to 3 MeV and over a xenon fill gas percentage of about 1.5 volume percent 
to 6 volume percent with nitrogen as the remainder of the fill gas, and a 
10 atmosphere total pressure. The calculated mixture which exhibits the 
flattest sensitivity response, well within the .+-.20% range required by 
recent regulations, is best achieved at a 2.25 volume percent xenon level. 
The first column of Table I lists a calculated response for the same 
chamber design with a 10 atmosphere fill gas of nitrogen, with the 
sensitivity varying widely over the range of incident gamma ray energies. 
TABLE I 
__________________________________________________________________________ 
Gamma Ray 
Energy Sensitivity (10.sup.-10 A/R/h) 
(MeV) 10 atm N 
1.5% Xe 
2.25% Xe 
3.0% Xe 
6.0% Xe 
__________________________________________________________________________ 
.088 0.65 .+-. .03 
1.38 .+-. .08 
1.65 .+-. .09 
1.74 .+-. .10 
2.75 .+-. .13 
.1 0.93 .+-. .03 
1.62 .+-. .09 
1.87 .+-. .10 
1.92 .+-. .10 
3.29 .+-. .14 
.15 1.20 .+-. .04 
1.60 .+-. .11 
1.73 .+-. .13 
1.91 .+-. .12 
2.66 .+-. .15 
.25 1.40 .+-. .05 
1.69 .+-. .13 
1.77 .+-. .13 
1.76 .+-. .13 
2.15 .+-. .16 
.35 1.33 .+-. .05 
1.57 .+-. .11 
1.70 .+-. .11 
1.76 .+-. .11 
1.86 .+-. .13 
.5 1.51 .+-. .05 
1.63 .+-. .12 
1.78 .+-. .13 
1.84 .+-. .13 
1.86 .+-. .13 
1.2 1.66 .+-. .06 
1.82 .+-. .10 
1.85 .+-. .10 
2.07 .+-. .12 
2.20 .+-. .12 
3.0 1.66 .+-. .07 
1.93 .+-. .14 
1.97 .+-. .14 
2.08 .+-. .14 
2.38 .+-. .16 
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The tubular outer electrode 12 can serve as the ion chamber envelope, and 
can be a relatively thick-walled stainless steel member, which is 
high-temperature and high-pressure resistant. The tubular inner electrodes 
14 and 16, which are protected from the environment by the outer electrode 
or envelope, are typically thinner walled aluminum members. 
The respective electrodes 12, 14, and 16 are adapted to be connected 
externally to high sensitivity current measuring means having a 
sensitivity in the range of about 10.sup.-10 amperes per roentgen per 
hour. 
The calculated sensitivity for ion chambers of the present invention with 
the volume percentage of xenon varied from about 1.5% xenon to about 6% 
xenon is seen in Table I over a range of incident gamma energies from 
0.088 to 3 MeV. 
The specific details of the ion chamber structure can be changed. Thus, a 
single sensing chamber is all that is required between a pair of 
spaced-apart electrodes. The electrode materials can be varied. The 
teaching here is that a flat response to incident gamma energy over a wide 
range of gamma energy can be provided by a mixed fill gas, which comprises 
a low atomic number, first major constituent, such as nitrogen which has a 
reduced gamma response at low gamma energy. Other low atomic number gases 
which can be substituted for nitrogen in whole or in part are neon, argon, 
and helium. The second minor constituent, high atomic number gas may be 
xenon and/or krypton which both exhibit an increased response to low gamma 
energies. The high atomic number gas may be present in amounts up to about 
15 volume percent of the total. For mixtures with higher volume 
percentages of the high atomic number gas, the gamma response of this 
higher atomic number gas will have a greater effect and the desired flat 
response would not be had. The ratio of the high atomic number gas and low 
atomic number gas which produces the flattest response depends upon the 
specific structure and fill gas pressure of the ion chamber design.