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Research activities in fission chamber modeling in support of the nuclear energy industry

June 7-10, 2009. . . OBJECTIVE AND OUTLINE. Fission chamber modeling: improvement of the performance of those neutron detectors in terms of lifetime, calibration and online diagnosis. . . Principle of fission chamber Computational tool setSelection of the deposit compositionInfluence of the bias

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Research activities in fission chamber modeling in support of the nuclear energy industry

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    1. June 7-10, 2009 C. Jammes1, P. Filliatre1, B. Geslot1, L. Oriol1, F. Berhouet1, J-F. Villard1, L. Vermeeren2 1CEA-Cadarache 2SCKCEN Research activities in fission chamber modeling in support of the nuclear energy industry

    2. June 7-10, 2009 OBJECTIVE AND OUTLINE Fission chamber modeling: improvement of the performance of those neutron detectors in terms of lifetime, calibration and online diagnosis.

    3. June 7-10, 2009 FISSION CHAMBERS (1/2) A fission chamber has in general two coaxial electrodes. The cathod can have a diameter as small as 3 mm for miniature chambers, or even 1.5 mm for sub-miniature chambers. The inter-electrode space is filled with pressurized gas. A thin layer of fissile material (from a few ?g to a few g) is deposited in most case on the anode. When a neutron reaches the fissile deposit, it is likely to induce a fission that generates two heavily charged ions, the fission products emitted in two nearly opposite directions. The one emitted out of the deposit ionizes the filling gas on its trajectory. Given that a DC voltage of a few hundred volts is applied between the electrodes, the electrons and positive ions are separated and drift across the gas, generating a current signal that can be amplified and processed. That DC voltage must be high enough to collect all the charges, and low enough to prevent the production of secondary ionization pairs (a phenomenon occurring in proportional counters). If both conditions are fulfilled, the fission chamber works in the saturation regime, namely for which the neutron-induced current signal is proportional to the fission rate. The ? photons that directly ionize the filling gas also generate a signal.A fission chamber has in general two coaxial electrodes. The cathod can have a diameter as small as 3 mm for miniature chambers, or even 1.5 mm for sub-miniature chambers. The inter-electrode space is filled with pressurized gas. A thin layer of fissile material (from a few ?g to a few g) is deposited in most case on the anode. When a neutron reaches the fissile deposit, it is likely to induce a fission that generates two heavily charged ions, the fission products emitted in two nearly opposite directions. The one emitted out of the deposit ionizes the filling gas on its trajectory. Given that a DC voltage of a few hundred volts is applied between the electrodes, the electrons and positive ions are separated and drift across the gas, generating a current signal that can be amplified and processed. That DC voltage must be high enough to collect all the charges, and low enough to prevent the production of secondary ionization pairs (a phenomenon occurring in proportional counters). If both conditions are fulfilled, the fission chamber works in the saturation regime, namely for which the neutron-induced current signal is proportional to the fission rate. The ? photons that directly ionize the filling gas also generate a signal.

    4. June 7-10, 2009 FISSION CHAMBERS (2/2)

    5. June 7-10, 2009 We selected a few computational tools available either at CEA or in the worldwide scientific community. The question of the fissile deposit isotopic evolution can be addressed with the use of the DARWIN code developed by CEA for simulating the evolution of nuclear fuel and structure materials. Most of the other aspects will be able to be studied by means of the GARFIELD code suite. The electric field can be computed by GARFIELD for simple geometries. The trajectory of the fission products is simulated by Monte Carlo method using its energy loss and the directional stragglings (longitudinal and transversal) obtained by the SRIM code. The electron transport is then carried out by the MAGBOLTZ code that is fully embedded in GARFIELD. The filtering of the fission chamber signal through the pre-amplifier will be modeled using the SPICE code that is a general-purpose analog electronic circuit simulator. If those tools cover several needs for modeling a fission chamber, we will have either to enhance some of them or to develop our own computational tools for simulating the fission chamber output signal in fluctuation mode for instance. As an example, we will have to implement the electric field screening effect in GARFIELD and develop our own tool to simulate the Campbelling mode from the pulse shape obtained from that one computed by GARFIELD and convoluted with the impulse response of the amplifier. We selected a few computational tools available either at CEA or in the worldwide scientific community. The question of the fissile deposit isotopic evolution can be addressed with the use of the DARWIN code developed by CEA for simulating the evolution of nuclear fuel and structure materials. Most of the other aspects will be able to be studied by means of the GARFIELD code suite. The electric field can be computed by GARFIELD for simple geometries. The trajectory of the fission products is simulated by Monte Carlo method using its energy loss and the directional stragglings (longitudinal and transversal) obtained by the SRIM code. The electron transport is then carried out by the MAGBOLTZ code that is fully embedded in GARFIELD. The filtering of the fission chamber signal through the pre-amplifier will be modeled using the SPICE code that is a general-purpose analog electronic circuit simulator. If those tools cover several needs for modeling a fission chamber, we will have either to enhance some of them or to develop our own computational tools for simulating the fission chamber output signal in fluctuation mode for instance. As an example, we will have to implement the electric field screening effect in GARFIELD and develop our own tool to simulate the Campbelling mode from the pulse shape obtained from that one computed by GARFIELD and convoluted with the impulse response of the amplifier.

    6. June 7-10, 2009 SELECTION OF THE DESPOSIT The DARWIN code allowed us to select the Plutonium 242 isotope as the best candidate to monitor the fast component of the neutron flux for a fluence between 1020and 1021n.cm-2 in a MTR spectrum including both thermal and fast components. This study was a key contribution to our FNDS project. In order to achieve that goal, we conducted an exhaustive study of the depletion of many various isotopes. The main conclusion of that study is that Pu242 is the best choice for monitoring the fast neutron flux during long irradiation durations in MTR given that its sensitivity to fast neutrons is excellent at the beginning of irradiation, and slowly decreases with fluence.The DARWIN code allowed us to select the Plutonium 242 isotope as the best candidate to monitor the fast component of the neutron flux for a fluence between 1020and 1021n.cm-2 in a MTR spectrum including both thermal and fast components. This study was a key contribution to our FNDS project. In order to achieve that goal, we conducted an exhaustive study of the depletion of many various isotopes. The main conclusion of that study is that Pu242 is the best choice for monitoring the fast neutron flux during long irradiation durations in MTR given that its sensitivity to fast neutrons is excellent at the beginning of irradiation, and slowly decreases with fluence.

    7. June 7-10, 2009 MOBILITY AND DRIFT VELOCITY We have studied study the influence of the high-voltage bias, gas pressure and composition on the velocities of the ion-pairs, namely the electrons and positive ions that are collected by the anode and cathode, respectively, and take part to the electric signal produced by the detector. It is important to note that the velocity of each type of the created charges is an ensemble velocity, referred as to the drift velocity, which is parallel to the electric field (in the absence of any magnetic field as it is the case here). Actually, it is more relevant to study the drift velocity v as function of the reduced electric field that is the electric field E divided by either the density N of the filling gas or its pressure p given that fission chambers are sealed and thus have the same density at any temperature. That new entity accounts for the fact the charge transport depends on both the electric field and density of the filling gas, the atoms and molecules of which lessen the drift velocity by collisions. In case when the electric field E is weak enough that its resulting force can be regarded as a perturbation with respect to the Langevin force that accounts for the Brownian motion, one shows the drift velocity v is given by the product of the electric field E itself and a factor m called the mobility. Actually, that assumption is valid for the drift velocity of positive ions only. Unlike the electrons, their small kinetic energy makes them more affected by the Brownian motion in the gas. One has to note the mobility is assessed for the transport of a given ion in a given neutral gas. The ion mobility is thus estimated from data available in the literature for different reduced electric field and using the gas mixture rule. For the studied fission chambers (see Table I), all the mobilities, which are displayed in Table II, can be viewed as constant for the corresponding E/p range. Table II also provides the maximum collection time T+, which is the traveling time of the ion from the anode to the cathode. Unlike the transport of ions, that of electrons cannot be cannot be described by the mobility concept as aforementioned. And, the drift velocity of electrons has to be computed using the MAGBOLTZ code embedded in GARFIELD. We have studied study the influence of the high-voltage bias, gas pressure and composition on the velocities of the ion-pairs, namely the electrons and positive ions that are collected by the anode and cathode, respectively, and take part to the electric signal produced by the detector. It is important to note that the velocity of each type of the created charges is an ensemble velocity, referred as to the drift velocity, which is parallel to the electric field (in the absence of any magnetic field as it is the case here). Actually, it is more relevant to study the drift velocity v as function of the reduced electric field that is the electric field E divided by either the density N of the filling gas or its pressure p given that fission chambers are sealed and thus have the same density at any temperature. That new entity accounts for the fact the charge transport depends on both the electric field and density of the filling gas, the atoms and molecules of which lessen the drift velocity by collisions. In case when the electric field E is weak enough that its resulting force can be regarded as a perturbation with respect to the Langevin force that accounts for the Brownian motion, one shows the drift velocity v is given by the product of the electric field E itself and a factor m called the mobility. Actually, that assumption is valid for the drift velocity of positive ions only. Unlike the electrons, their small kinetic energy makes them more affected by the Brownian motion in the gas. One has to note the mobility is assessed for the transport of a given ion in a given neutral gas. The ion mobility is thus estimated from data available in the literature for different reduced electric field and using the gas mixture rule. For the studied fission chambers (see Table I), all the mobilities, which are displayed in Table II, can be viewed as constant for the corresponding E/p range. Table II also provides the maximum collection time T+, which is the traveling time of the ion from the anode to the cathode. Unlike the transport of ions, that of electrons cannot be cannot be described by the mobility concept as aforementioned. And, the drift velocity of electrons has to be computed using the MAGBOLTZ code embedded in GARFIELD.

    8. June 7-10, 2009 FILLING GAS INFLUENCE The figure shows the computed drift velocity ve of electrons as function of the reduced electric field E/p for various types of gas filling. That figure also shows the E/p ranges corresponding to the studied fission chambers. One can note, for the same fission chamber, how favorable it is to add some molecular nitrogen to the argon filling gas in order to speed up the electrons. In addition, it is noticeable the chosen couple (V,p) for the CFPR fission chamber corresponds to a E/p-range where not only the drift velocity ve reaches a maximum, but also is near constant.The figure shows the computed drift velocity ve of electrons as function of the reduced electric field E/p for various types of gas filling. That figure also shows the E/p ranges corresponding to the studied fission chambers. One can note, for the same fission chamber, how favorable it is to add some molecular nitrogen to the argon filling gas in order to speed up the electrons. In addition, it is noticeable the chosen couple (V,p) for the CFPR fission chamber corresponds to a E/p-range where not only the drift velocity ve reaches a maximum, but also is near constant.

    9. June 7-10, 2009 IMPACT OF GAS EVOLUTION We also studied how much the oxygen atoms released by the oxide of the deposit material under irradiation could also increase the electron velocity for two different gas mixtures. The velocity change is about 25% for the PHI4 and PHI8 fission chambers and only 1% for the CFPR one.We also studied how much the oxygen atoms released by the oxide of the deposit material under irradiation could also increase the electron velocity for two different gas mixtures. The velocity change is about 25% for the PHI4 and PHI8 fission chambers and only 1% for the CFPR one.

    10. June 7-10, 2009 INDIVIDUAL CURRENT SIGNALS The simulation of the electric pulse delivered by a fission chamber prior to the pre-amplifier with the help of the GARFIELD code suite was performed as follows: - Random selection of a fission product, denoted FP hereafter (weighted by independent fission yield only) and its kinetic energy, - Random initial direction of the FP that comes out of the deposit (isotropic distribution, assumed thin fissile layer with no auto-absorption taking place), - Simulation of the trajectory of each FP with the use of the SRIM data, - Generation of N electron-ion pairs for each interaction of the FP with a cluster of gas molecules at subsequent locations along its trajectory, - Transport of the electrons using the drift velocity and diffusion coefficients computed by MAGBOLTZ and that of ions using the available mobility data, Sampling of the electron and ion currents at the anode and cathode, respectively (carried out by GARFIELD). It is noteworthy that the number N of pairs is obtained from the W-value which is the average energy needed to create an ion pair. The figure shows the simulated current signals due to the electrons created along the trajectory of a fission product in a CFUR. First, the noticeable sharp rising time is accounted for by the non-instantaneous time-of-flight of the fission product between two interaction points in the gas. Second, the longer trajectory of a fission product is, the higher signal amplitude is. Third, the current signal starts to decrease as soon as all the electrons are created and keep being collected at the anode. The simulation of the electric pulse delivered by a fission chamber prior to the pre-amplifier with the help of the GARFIELD code suite was performed as follows: - Random selection of a fission product, denoted FP hereafter (weighted by independent fission yield only) and its kinetic energy, - Random initial direction of the FP that comes out of the deposit (isotropic distribution, assumed thin fissile layer with no auto-absorption taking place), - Simulation of the trajectory of each FP with the use of the SRIM data, - Generation of N electron-ion pairs for each interaction of the FP with a cluster of gas molecules at subsequent locations along its trajectory, - Transport of the electrons using the drift velocity and diffusion coefficients computed by MAGBOLTZ and that of ions using the available mobility data, Sampling of the electron and ion currents at the anode and cathode, respectively (carried out by GARFIELD). It is noteworthy that the number N of pairs is obtained from the W-value which is the average energy needed to create an ion pair. The figure shows the simulated current signals due to the electrons created along the trajectory of a fission product in a CFUR. First, the noticeable sharp rising time is accounted for by the non-instantaneous time-of-flight of the fission product between two interaction points in the gas. Second, the longer trajectory of a fission product is, the higher signal amplitude is. Third, the current signal starts to decrease as soon as all the electrons are created and keep being collected at the anode.

    11. June 7-10, 2009 AVERAGE ION AND ELECTRON COMPONENTS The average signals due to the collected electrons and ions are shown that figure. One can notice the two signals have two characteristic time scales: the pulse width of the electron component is of a few nanoseconds whereas that of the ion component is of a few microseconds. When using a fast amplifier of an impulse response width less than 1ms, the ion component shape is not imposed by the amplification conditioning. As a result, it makes the overall signal more affected by a variation of that ion component due to a change in bias voltage V or pressure p. That should be reason why it is better to filter out the ion component when operating a fission chamber in Campbelling mode.The average signals due to the collected electrons and ions are shown that figure. One can notice the two signals have two characteristic time scales: the pulse width of the electron component is of a few nanoseconds whereas that of the ion component is of a few microseconds. When using a fast amplifier of an impulse response width less than 1ms, the ion component shape is not imposed by the amplification conditioning. As a result, it makes the overall signal more affected by a variation of that ion component due to a change in bias voltage V or pressure p. That should be reason why it is better to filter out the ion component when operating a fission chamber in Campbelling mode.

    12. June 7-10, 2009 IMPACT OF THE W-VALUE The impact of the W-value is exhibited in that figure. At first glance, one can see the smaller the W-value is, the higher signal amplitude is. However, one can verify the amplitude is not proportional to the W-value. An 11% decrease in W-value from 26.9eV down to 24eV leads to an amplitude gain of 21%, whereas an 11% increase in W-value from 26.9eV to 30eV leads to an amplitude loss of 7%. That comes from the fact the smaller W-value is, the greater energy loss DE for each subsequent interactions between the fission product and the gas molecules. A good knowledge of the W-value is not needed when operating a fission chamber in pulse mode because of the use of a triggering threshold. On the contrary, the knowledge of the W-value directly impacts on the simulation of signals in current and Campbelling modes since the total collected charge Q, which is the integral of the pulse, is function of the W-value in the same way as it is for the amplitude. For the sake of understanding, one recalls the current signal is proportional to Q whereas the Campbelling one is proportional to Q2.The impact of the W-value is exhibited in that figure. At first glance, one can see the smaller the W-value is, the higher signal amplitude is. However, one can verify the amplitude is not proportional to the W-value. An 11% decrease in W-value from 26.9eV down to 24eV leads to an amplitude gain of 21%, whereas an 11% increase in W-value from 26.9eV to 30eV leads to an amplitude loss of 7%. That comes from the fact the smaller W-value is, the greater energy loss DE for each subsequent interactions between the fission product and the gas molecules. A good knowledge of the W-value is not needed when operating a fission chamber in pulse mode because of the use of a triggering threshold. On the contrary, the knowledge of the W-value directly impacts on the simulation of signals in current and Campbelling modes since the total collected charge Q, which is the integral of the pulse, is function of the W-value in the same way as it is for the amplitude. For the sake of understanding, one recalls the current signal is proportional to Q whereas the Campbelling one is proportional to Q2.

    13. June 7-10, 2009 CONCLUSION Selection of Pu242 for the FNDS project. Our simulation tool showed a given bias voltage and gas mixture can significantly enhance the charge collection time (design fission chambers). Signal simulation: Important to have a satisfactory knowledge of W-value (current and Campbelling modes). The different studies carried out in this paper allowed us to show how promising our endeavor to model the signal delivered by a fission chamber is. First, the simulation of the deposit evolution permitted to select the most appropriate fissile material for our FNDS project. Second, we showed how the choice of a given bias voltage and filling gas mixture can significantly enhance the charge collection time. As a consequence, our tool set relying mostly on the GARFIELD code suite appeared to help us successfully design fission chambers. Finally, the simulation of a pulse signal prior to amplification sensitivity showed how it is important to have a satisfactory knowledge of the energy for creating ion pairs to accurately simulate the signal amplitude in current and Campbelling modes. Our next step will be to improve the simulation of a fission chamber pulse by taking into accounts more physical self-shielding, auto-absorption, electric field screening effect, etc.). Then, we will develop our own tool to simulate the fission chamber signal in Campbelling mode. For that purpose, we will also have to get more accurate W-values if one aims at accurate assessment of the signal.The different studies carried out in this paper allowed us to show how promising our endeavor to model the signal delivered by a fission chamber is. First, the simulation of the deposit evolution permitted to select the most appropriate fissile material for our FNDS project. Second, we showed how the choice of a given bias voltage and filling gas mixture can significantly enhance the charge collection time. As a consequence, our tool set relying mostly on the GARFIELD code suite appeared to help us successfully design fission chambers. Finally, the simulation of a pulse signal prior to amplification sensitivity showed how it is important to have a satisfactory knowledge of the energy for creating ion pairs to accurately simulate the signal amplitude in current and Campbelling modes. Our next step will be to improve the simulation of a fission chamber pulse by taking into accounts more physical self-shielding, auto-absorption, electric field screening effect, etc.). Then, we will develop our own tool to simulate the fission chamber signal in Campbelling mode. For that purpose, we will also have to get more accurate W-values if one aims at accurate assessment of the signal.

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