The BF3 Counter

The in, a) reaction with B is probably the most useful reaction for the detection of thermal neutrons because 

The BF3 counter is a proportional counter filled with BFj gas, usually enriched to more than 90 percent in °B (about 20 percent of natural boron is B the rest is B). The BF3 counter detects the alpha and the lithium particles produced by the reaction 

With thermal neutrons, the Li nucleus is left in an excited state about 96 percent of the time. In that case, the Q value of the reaction is 2.30 MeV and the Li nucleus goes to the ground state by emitting a gamma with energy equal to 2.78 — 2.30 = 0.480 MeV. This photon may also be used for the detection of the neutron. 

The relationship between counting rate and neutron flux is derived as follows. Let 

N = number ofB atoms per unit volume V = volume of the counter 

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Proportional counters can also count neutrons by introducing boron into the chamber. The most common means of introducing boron is by combining it with tri-fluoride gas to form Boron Tri-Fluoride (BF3). When a neutron interacts with a boron atom, an alpha particle is emitted. The BF3 counter can be made sensitive to neutrons and not to gamma rays. 

Another technique for neutron detection uses a fission chamber. One design contains a stack of alternate anodes and cathodes, one of the electrodes being covered by a thin layer of uranium enriched in The fission fragments produce large ionization even though the gas multiplication is quite low. This detector is more sensitive to fast neutrons than the BF3 counter, and can be used for fast neutron fluxes up to 10 n s with a backgroimd of a few cps. 

Scintillation methods offer the possibility of high-efficiency detectors with a more rapid time response than the BF3 counter. As mentioned in the previous section, the basis of the scintillation detector is the conversion, in a suitable crystal, such as thallium-activated sodium iodide, Nal(Tl), of the kinetic energy of the charged particle to light, which can be amplified by a photomultiplier tube to provide an electrical pulse. Again, the neutron has to interact to produce either a charged particle or a 7 ray, the latter of which may in turn interact to produce ionizing particles. 

The exact stable period of the reactor is determined by the external BF3 counting system. The BF3 counter is placed at a desired location in access port number four. The counts per fraction of a minute are then taken periodically, A plot of the counting rate versus time is made on semilog paper. The plotting should commence when the first count is taken, this will enable the participant to determine whether or not the reactor is rising on a stable period. The stable reactor period will be determined from the semilog plot and recorded along with the control rod position and temperature on the data sheet. 

The MPC unit serves as the high-voltage source to the BF3 counter, and low-gain amplifier, and as a pulse inverter. The MPC switch is a convenient method of suddenly removing or restoring, the signal input to the multichannel analyzer for the time-versus-channel number calibration. 

B. Vertical Distribution The BF3 counter is connected to the same type of a drive unit described above and positioned with identical controls. 

The BF3 counter used is encapsulated in a Lucite tube. Why was this material used, and what other material could be used ... 

What could you suggest to eliminate the gamma counting registered by the BF3 counter ... 

Gas-filled detectors are used, for the most part, to measure alpha and beta particles, neutrons, and gamma rays. The detectors operate in the ionization, proportional, and G-M regions with an arrangement most sensitive to the type of radiation being measured. Neutron detectors utilize ionization chambers or proportional counters of appropriate design. Compensated ion chambers, BF3 counters, fission counters, and proton recoil counters are examples of neutron detectors. 

The BF3 proportional counter is used to monitor low power levels in a nuclear reactor. It is used in the "startup" or "source range" channels. Proportional counters cannot be used at high power levels because they are pulse-type detectors. Typically, it takes 10 to 20 microseconds for each pulse to go from 10% of its peak, to its peak, and back to 10%. If another neutron interacts in the chamber during this time, the two pulses are superimposed. The voltage output would never drop to zero between the two pulses, and the chamber would draw a steady current as electrons are being produced. 

The BF3/l,3j5-trioxane system is one of the few so far discovered in which there is a possibility that monomeric units add at the cationic end of a macrozwitterion. Fortunately, the cation seems to be stable in the presence of its counter anion. As a simple model system with which to study cationic propagation through zwitterion intermediates it is marred by its equilibrium nature and the insolubility of the polymer. Whilst kinetic termination seems to be absent, the authors report transfer to the solvent methylene chloride. Such a reaction would introduce non-zwitterionic chains. 

The detectors generally used for the y area monitors are GM counter and Nal(Tl) scintillator for lower dose rate regions and ion-chamber for higher dose rate regions. For the neutron, a BF3 counter with or without moderator is used. 

The ionic nature of these species was deduced from the very beginning on the basis of the identification of the anionic counter ion, [BFJ ", in the complex XeF6 BF3 116), but the first characterization of the [XeF5]+ cation itself was only attempted later 170). The full characterization of [Xe2Fn]+ was even more recent120). The two hexavalent xenon cations are related in much the same way as [XeF]+ and [Xe2F3] + in xenon(H) complexes i.e. the simpler cations are joined by a fluorine bridge. 

Because neutrons are essentially nonionizing when passing through matter, their detection is accomplished by first inducing nuclear reactions that produce ionizing particles. The gas counter is thus filled with either 3He or BF3 (enriched in 10B). In the nuclear reaction... 

The specifications of commercial BF3 counters consist of sensitivity, dimensions, composition of the filling gas, operating voltage, and maximum operating temperature. 

Descending coke profiles were observed experimentally by Van Zoonen in the hydroisomerization of olefins [1965]. Hughes et al. [1987] observed a decreasing coke profile when xylenes reacted over a silica/alumina bead catalyst around 460°C. The profile was measured by a noninvasive technique neutron beam attenuation coupled with a BF3 counter. 

PC-BF3 Counter - a Proportional Counter tube filled with B oron-Trifluorlde gas so that wnen a neutron enters the tube an n-oc reaction emits enough energy to form a pulse which will then activate a scaler. By combining amplification and a discriminator into the system a controlled portion of radiation can be counted. 

A Lucite blank is then placed at the center of the glory hole in place of the boron sample A. Caution is again stressed in this operation, since positive reactivity is being inserted. The coarse control rod is then raised to its previously recorded position. Once the reactor has reached its stable period, a period measurement should be made with the external BF3 counter as described above. The reactor period should, of course, be shorter since the boron absorber has been removed from the core. Record control rod settings, period and temperatures. 

With the equipment used in this experiment, pulsed-neutron generator AL-ZOl, BF3 counter, TMC analyzer, and the ambient natural background present, we can obtain the value of a for > 0.1 cm directly and for B < 0.1 cm" through harmonic analysis. 

What is the sensitivity of a BF3 counter Must it be known before the neutron-flux distribution can be determined ... 

Open the beam hole and rotate the chopper shutter by hand to get the maximum count rate out of the detector. The shutter slits are then aligned parallel to the beam. This open position of the shutter can be verified by the reflected light ray which should be now horizontal (parallel with neutron beam). Now realign the BF3 detector for the maximum count rate, using the monitoring MFC counter. 

This instrument uses a REM/n counter as a detector, and normally has 3 logarithmic ranges extending from 10 to 10 Sv/hr (1 mRem/hr to 100 mRem/hr). The basic detector is a BF3 counter surrounded by a moderator/attenuator assembly (rich in hydrogen) to provide a weighted dose response from. 025 eV to 14 MeV of neutron energy. 

An important difference between free radical and ionic polymerization is that a counter ion only appears in the latter case. For example, the intermediate formed from the initiation of propene with BF3-H2O could be represented as... 

Unlike radical chain polymerisation, initiation in cationic polymerisation uses a true catalyst that is recovered at the end of the polymerisation and is not incorporated at one end of the growing chain. Catalysts for cationic chain polymerisation are molecules able to withdraw electrons, mainly Bronsted (H2SC>4, H3PO4) and Lewis acids (BF3, A1C13, SnCh). The choice of solvent for cationic polymerisation is also important because it plays a major role in the association between cation and counter ion. A too tight association will prevent monomer insertion during the propagation step. However, the use of "stabilized"... 

B10 lined or BF3 gas-filled proportional counters are normally used as source range detectors. Proportional counter output is in the form of one pulse for every ionizing event therefore, there is a series of random pulses varying in magnitude representing neutron and gamma ionizing events. 

Fast neutron detection sometimes uses a hydrogenous moderator to slow down the neutrons and then employs a low-energy neutron detector as described above. One common fast neutron detector is a Bonner sphere. In this detector, a scintillator is placed in the center of a polyethylene sphere. Radiation transport calculations are used to produce efficiency curves that depend on the energy of the incident neutron. Another common fast neutron detector is a long counter. This detector uses a slow neutron detector (originally a BF3 chamber) at the center of a cylindrical moderator designed so that the detector is only sensitive to neutrons incident from one side. 

On the other hand, some of these species are stable enough to be isolated as salts. Nitronium cation N02 exists in equilibrium with nitric acid at ambient temperature however, more than 15 crystalline nitronium salts with a variety of counter ions have been isolated and characterized [ 1 ]. The most widely used salt N02+BF4- is made by the reaction of HN03 and BF3 in anhydrous hydrogen fluoride [41] ... 

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