"Explore how FLUKA simulations are used to enhance the Frascati Neutron Generator (FNG) for neutron fusion research. Learn about upgrades, trigger systems, and cross-section challenges in neutron production."
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Table of Contents Introduction; The Frascati Neutron Generator (FNG); Use of FLUKA in mine Phd; Conversion and validation of the source code; Development of new features for the source; Simulation to study the possibility to install a trigger on FNG; Conclusion and future development.
Introduction I am performing the research activity of my Phd at the Frascati Neutron Generator (FNG) at the Enea in Frascati. FNG is a machine that produce fusion neutrons inducing D-T, principally, or D-D fusion reaction. To achieve this, a deuteron beam is accelerated to an energy of 260 keV and make collide with a titanium-tritium target. My Phd work concerns to study possible upgrades of the machine, especially the installation of a trigger system on it.
Use of FLUKA in my Phd The mains uses of FLUKA in my Phd work are: Create a user routine that simulates the production of neutrons and alphas at FNG in FLUKA; Validate the new source code; Study the possibility to install a trigger system at FNG running simulations on FLUKA; Study new possible configurations for the alphas detector to create a effective trigger system.
The Frascati Neutron Generator (FNG) FNG is a machine that starts its activity in 1992 to make measurement with fusion neutrons. Deuterium nuclei are accelerated up to an energy of 260 keV. The collision with a titanium-tritium target triggers the fusion reactions with neutrons production. It is possible to have even DD reactions. The neutrons are produced at FNG with the following reactions: ? + ? ??4 (3,5 ???) + ? (14,1 ???) ? + ? ??30,82 ??? + ? (3,2 ???) For the DT reaction we have a yield of ?~1011 n/s and for the DD one of ?~109 n/s with a current of 1 mA.
Simulate the FNG environment The montecarlo code used until now to simulate the FNG environment was MCNP. Now we decided to implement simulation also in FLUKA. Simulate the machine it is necessary to study the feasibility of new upgrades that we want to install on it. FLUKA has not the fusion reactions cross sections for deuterium and tritium. So, to generate the neutron fusion spectrum emitted at FNG, it is necessary to write a specified source. MCNP has the same problem. A source was already written for it by M. Pillon and A. Milocco. It uses part of the code of the TRIM routine that describe the scattering of the deuteron ion. After the scattering is evaluated, the program check if the collision happens with a titanium atom or a tritium one. If the collision is with the last one there is a probability check to see if the fusion reaction occurs. If not the deuteron continue to scatter until it exits the target or goes below a specific energy or a fusion reaction occurs. If the neutron is produce all the cinematics is calculated and then the kinetic energy, the direction and the starting position are passed to the main program. So, the first task for me was to convert this source subroutine from MCNP to FLUKA.
Porting of the code from MCNP to FLUKA Fortran 90 MCNP FLUKA Fortran 77 First Step: convert the programming language of the source to adapt it to the FLUKA one. Second Step: adapt the code to the logic of the new program. Third Step: substitute the MCNP intrinsic function with the FLUKA ones. Fourth Step: put the external subroutine of the original code in the same file of the main one. Fifth Step: Check if the porting of code was done correctly with equal simulations in FLUKA and MCNP
Geometries for the validation simulations For the check simulation two types of geometries have been built. The first consists of a vacuum sphere divided in two main regions. We have a little middle region, few m thick, where the point of origin of the source lays. The second one is formed by eight sphere of radius 15 cm at 50 cm from the point of origin of the source. These are placed every 45 . The firsts simulations with the spheres are run in vacuum. Then, we put three different materials inside them: Lead; Polyethylene; Water. The simulation in MCNP were done by A. Colangeli, researcher at ENEA, expert on the code.
Scoring for the validation simulations For the simulation with the hemispheres I put a USRBDX scoring between the central region and each of the two bigger ones. The quantity scored is the current with linear binning in energy and in solid angle. For the ones with the spheres, I put a USRTRACK scoring in each of the spheres with linear energy binning.
Comparison between FLUKA and MCNP V Spheres simulations integrals: Angle (deg) Integral Fluka Integral MCNP Reaction Material Difference Error % DT Vacuum 0 1.256E-05 1.256E-05 1.525E-09 0.0121 Vacuum DT 180 1.034E-05 1.033E-05 9,376E-09 0.0908 DT Lead 0 5.703E-06 5.736E-06 3.285E-08 0.573 DT Lead 45 1.112E-05 1.139E-05 2.712E-07 2.381 DD Vacuum 0 6.408E-06 6.416E-06 8.541E-09 0.133 DD Vacuum 180 3.496E-06 3.498E-06 2.279E-09 0.0652 DD Lead 0 4.504E-06 4.593E-06 8.969E-08 1.952 DD Lead 45 7.126E-06 7.397E-06 2.715E-07 3.670
Development of the source Original source code Add the option that allows to simulate the alpha/helium-3 particles Add option that allows to simulate both of fusion products at the same time Possibility to study the feasibility of a trigger system at FNG
Source Card details The SOURCE card given to the FLUKA run to simulate the FNG neutrons and alphas spectrum. There is still some work ongoing. So, some of the options could change in the near future. Working in this section, it will be changed in the near future Option obsolete, it will be eliminated soon
FNG modeling With the help of a graduating student (F. Chiarelli), the geometry of the FNG machine and bunker was represented in FLUKA. From that I extracted the part of geometry that I need and then I added the detectors for the alphas and the neutrons.
Correlation study To study the feasibility for the trigger installation at FNG it is necessary to check if the neutrons and the alphas maintain their correlation between them. If the D-T reaction happen in a rest frame the two particles would be emitted back- to-back. Since the deuteron has a no zero kinetic energy it will be a little discrepancy between them. Others elements that could decrease the correlation could be: The structure of the machine deviating the neutrons; The titanium deviating the alpha particle; Reflection of the alpha on the beam pipe internal walls.
Discrepancy between Neutrons and Alphas I studied the discrepancy between the angle of emission of the neutrons and the alphas and 180 deg. It comes out that this start to be significant from ~20 deg. The angle of emission of the alphas that hit the detector is 0.79 deg. So, the discrepancy is very little in our case. Our range of interest
Collimated Source vs Real Source To study the deflection due to the machine structure and the titanium, I wrote a focalized source (left). In void the efficiency and correlation are 100%. So, any decrease would come from these factors. Instead, firsts simulations with the real source showed that the alphas reflection by the beam pipe walls could lead to significant number of false positive signals.
Scoring for the Correlation Study At the beginning I chose to use the EVENTBIN scoring to record if in one of the detector there was an energy deposition. After the simulation the output files were analyzed to check if, when an event occurs in the alpha detector, it happens in the neutron one too. However this approach was disk space consuming. It filled 20 TB of space on the server!!!! So, I decided to change scoring to DETECT. I use two scoring, the first score the energy deposition in the alpha detector without any trigger, the second one scores only if there is a signal in the neutron detector. To have a 100% efficiency in the neutron detector it is set to BLACKHOLE.
Results with the Collimated Source The efficiency of the neutron detector it not affected by the thickness of titanium crossed. The alphas one, on the contrary, drops to zero over 7 m. The Correlation between the two particles remains constant independently to the efficiency of the alpha detector. Not significant point. It comes from a run with only one particle scored in the alphas detector
Conclusion The source code for FNG was successfully converted from MCNP to FLUKA; The new source code was validated and it showed a good agreement with the old one; New features that expand the source were added and they proved to work well; When the alphas are emitted in direction of the detector, as in the focalized source, the scattering induced by the titanium target on them is almost negligible as the discrepancy due to the kinematic; The scattering and absorption in the titanium become significant for the alphas when the length crossed rises, over 7 m the efficiency of the alphas detectors go to 0; The neutron are not significantly scattered by the machine structure; The studies with the real source, until now, revealed that the reflection of the alpha particles by the beam pipe walls, plays a large factors in the decreasing of the C correlation of the two particles.
Future developments Conclusion of the correlation measurement with the real source and understanding of the results; Continuing the study of a new geometry configuration for the alpha particles detector, like a circular ring around the beam pipe; We foreseen, also, the acquisition of neutron spectrum with neutron detector placed in the hall of FNG in coincidence with alpha detectors.
