Dual neutron flux/temperature measurement sensor

Simultaneous measurement of neutron flux and temperature is provided by a single sensor which includes a phosphor mixture having two principal constituents. The first constituent is a neutron sensitive 6LiF and the second is a rare-earth activated Y203 thermophosphor. The mixture is coated on the end of a fiber optic, while the opposite end of the fiber optic is coupled to a light detector. The detected light scintillations are quantified for neutron flux determination, and the decay is measured for temperature determination.

FIELD OF THE INVENTION 
The present invention relates generally to radiation interaction 
measurement devices and temperature measurement devices and, more 
specifically, to a dual neutron flux/temperature measurement sensor which 
utilizes a phosphor mixture having two principal constituents, one being 
neutron sensitive and the other being temperature sensitive. 
BACKGROUND OF THE INVENTION 
It is well known that emission properties of phosphors vary in accordance 
with temperature. This correlation has been used to devise various types 
of thermometry hardware. For example, surface temperature of a rotating 
flywheel has been measured by inducing fluorescence from a pulsed nitrogen 
laser in a material that includes lanthanum oxysulfide doped with 
europium. The temperature dependence of the phosphor emission has been 
shown both in amplitude and lifetime changes. With a pulsed laser as the 
stimulating source, either the ratio of two emission line intensities 
(amplitudes) or the lifetime of some selected line can be used to 
determine the temperature. 
In the field of nuclear reactor engineering, the interactions of neutrons 
with nuclei are important to the release of nuclear energy in a form 
capable of practical utilization. Inelastic neutron collisions do not 
occur below energies of about 0.1 Mev, but elastic collisions between 
neutrons and nuclei will be effective in slowing down the neutrons until 
their average kinetic energy is the same as that of the atoms of a 
scattering medium. This energy depends on the temperature of the medium, 
and is thus referred to as thermal energy. Neutrons whose energies have 
been reduced to values in this region are designated "thermal neutrons". 
Phosphors have been used to measure thermal neutron flux. A mixture of 
boron-containing plastic and ZnS(Ag) phosphor has been used to provide a 
slow-neutron scintillator. A slow neutron passing through the scintillator 
is captured by a B10 nucleus. The resultant energetic alpha and lithium 
particles reach a ZnS(Ag) granule with sufficient residual energy to cause 
a scintillation. Light from the scintillation travels to the 
photomultiplier photocathode and reaches it with sufficient intensity to 
cause a recognizable pulse at the anode. The slow-neutron scintillators 
have been made by using a transparent bioplastic mold cast from a negative 
steel mold. In use, the surface of the scintillator faces a 
photomultiplier, while the opposite surface is covered with aluminum foil 
or other light reflective coating. See, for example, "High Efficiency 
Slow-Neutron Scintillation Counters", NUCLEONICS, by K. H. Sun et al. 
(July, 1956). 
The extreme environment of some nuclear reactor cores, with temperatures in 
the range of 1,000.degree. C., presents a difficult problem for sensing 
both temperature and neutron flux. A need exists for an improved sensor 
capable of simultaneously measuring both neutron flux and temperature. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a single sensor capable of 
providing simultaneous measurement of both neutron flux and temperature. 
Another object of the present invention is to provide a sensor which is 
easy to install and relatively simple in construction. 
These and other objects of the invention are met by providing a dual 
neutron flux/temperature measurement sensor which includes a phosphor 
mixture having a first neutron-sensitive phosphor constituent and a second 
activated thermophosphor constituent coated on an end of a fiber optic, 
and means for detecting light generated by charged particles produced by 
neutron absorption in the first constituent. The first constituent 
produces the charged particles when neutrons are absorbed therein, and the 
charged particles produce scintillations in the second constituent. The 
scintillations of the second constituent are detected and correlated to a 
temperature value which varies in accordance with variations in the 
detected scintillations. The second constituent is preferably a rare-earth 
activated thermophosphor. 
Other objects, advantages and salient features of the invention will become 
apparent from the following detailed description, which, taken in 
conjunction with the annexed drawings, discloses preferred embodiments of 
the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1, 2 and 3, a dual neutron/flux temperature measurement 
sensor 10 includes a coating 12 made of a phosphor mixture homogeneously 
distributed within an optically transparent binder. The mixture is applied 
to and forms the coating 12 on the bare tip of a fiber optic 14. The 
mixture includes a first neutron sensitive phosphor constituent and a 
second activated thermophosphor constituent. The second constituent is 
preferably a rare-earth activated thermophosphor, but may also be a metal 
activated thermophosphor. When the sensor 10 is used to sense conditions 
in a nuclear reactor, thermal neutrons are detected in the first phosphor 
constituent via neutron absorption. When the neutrons are absorbed within 
the first constituent, charged particles are created which in turn produce 
scintillations in the activated thermophosphor. The ambient temperature 
surrounding the coating can be monitored by observing the ratio of two 
emission line amplitudes and/or the decay rates of the scintillations from 
the activated thermophosphor. 
Neutrons are neutral particles which normally are detected through nuclear 
reactions which result in energetic charged particles such as protons, 
alpha particles, etc. Conventional methods can then be incorporated to 
detect the charged particles. According to the present invention, a 
scintillation technique is preferable for detecting the charged particles 
created from the absorptions of a neutron. 
In one particular embodiment, the mixture forming the coating 12 includes 
as the first constituent 6LiF (95% 6Li) and rare-earth activated Y203. The 
6LiF (95% 6Li) has a high cross-section for thermal neutrons (940 barns) 
and, when the neutron is absorbed, produces an alpha and a triton. These 
charged particles are then detected by the rare-earth activated Y203, 
producing visible light scintillations which are guided to a light 
collector 16 by the fiber optic 14. The light collector 16 can be a 
photodiode or a photomultiplier tube, for example. 
The fiber optic may be made of quartz or sapphire or other comparable 
materials that are transparent to the scintillation light. The thickness 
of the phosphor material coated onto the tip of the fiber is such that the 
light pulses leaving the coating 12 are not significantly attenuated. 
Thermographic phosphors have a useful property in that the luminescence of 
the phosphor changes in emission line amplitude and decay rate with 
changes in temperature. As shown in FIG. 2, as the temperature of the 
surrounding environment is increased, the lifetime of the fluorescence 
induced in the phosphor decreases logarithmically. The graph shows 
lifetime verses temperature for europium-doped yttrium oxide. When the 
neutron is absorbed in the 6LiF, charged particles are produced which 
create scintillations in the rate-earth activated Y203. The lifetime and 
emission line amplitude of the scintillations will be determined by the 
characteristic properties of the rare-earth activated Y203, mainly the 
temperature of the phosphor. If the temperature of the sensor surroundings 
changes, this will be indicated by a change in the ratio of two emission 
line amplitudes and decay rate of the induced scintillations which are 
detected by the light collector 16 at the end of the fiber optic 14 
opposite the phosphor coated end. 
Light from the fiber optic 14 passes through a bandpass filter 18 before 
entering the light collector 16. Once the scintillations reach the light 
collector 16, the pulses are amplified. Amplification preserves the time 
emission peak characteristics of the pulse which is simultaneously 
directed to a discriminator-counter 20 for determining neutron flux and a 
waveform digitizer 22 or other suitable device to obtain the temperature 
dependent pulse decay constant or ratio of two emission line amplitudes. 
Other suitable means may be employed for performing the functions of the 
discriminator-counter 20 and the waveform digitizer 22. 
A practical use of the sensor 10 which incorporates a 6LiF phosphor mixture 
is to measure tritium production at a point in a reactor or zero power 
experiment and also simultaneously measure temperature. 
Many alternative coatings can also be made which serve as both a neutron 
absorber and scintillator. For example, Y203:Gd could be used as the 
neutron-sensitive activated thermophosphor. Using a single 
neutron-sensitive thermophosphor has advantages over mixing a 
neutron-sensitive phosphor and a thermophosphor, in that there is no 
concern over optimizing the ratio of the two phosphors. Moreover, there is 
no potential for inhomogeneity due to inadequate mixing. Advantageously, 
the large cross-section for gadolinium allows for thinner phosphor layers, 
thus reducing any gamma interactions. 
Measurements of other radiation interactions can be achieved by selecting a 
radiation-sensitive material in the phosphor mixture to be compatible with 
the type of interaction being measured, such as measuring fission 
fragments, beta particles, etc., with alternative versions used at 
reactors, fusion machines, or accelerators. 
The binder material can be of any suitable material which is optically 
transparent. Binders also exhibiting radiation resistance could be used, 
and would provide for measurements in high radiation fields. As an 
example, a colorless polyester can be used as the binder material. 
The sensitivity of the sensor 10 can be adjusted by varying the amount of 
reacting material in the coating 12. This feature may have particular 
significance where the sensor 10 is required not to significantly 
attenuate the radiation beam or production. Also, the nuclear reacting 
constituent mixed with the thermophosphor constituent can be varied to 
utilize reaction rates for other material while simultaneously measuring 
the temperature. 
If a high-temperature thermophosphor is selected, the temperature can be 
monitored in environments up to 1500.degree. C., depending on the 
thermophosphor used, the survivability of the binder/fiber, and on the 
temperature limit of the thermophosphor. Therefore, the probe can be 
customized to specific temperature ranges by choosing appropriate 
thermophosphors. 
The electronic components for processing the signal output from the light 
detector 16 are conventional. Each of the discriminator-counter 20 and the 
waveform digitizer can be provided with visual displays 24 and 26, 
respectively, indicating the respective measured values of neutron flux 
and temperature. A commercially available waveform digitizer suitable for 
use in the present invention is sold by Tektronix as model no. 7854. For a 
general description of similar components, see R. Stedman, "Scintillator 
for Thermal Neutrons Using Li6F and ZnS(Ag), Rev, Sci, Instrum., 31, 1156, 
and K. H. Sun et al., "High-efficiency Slow-neutron Scintillation 
Counters", Nucleonics, 14(7), 46(1956), both of which are incorporated 
herein by reference. 
While advantageous embodiments have been chosen to illustrate the 
invention, it will be understood by those skilled in the art that various 
changes and modifications can be made therein without departing from the 
scope of the invention as defined in the appended claims.