H4IRRAD generic simulation results

Transcription

1 1. Introduction H4IRRAD generic simulation results The radiation field present in LHC critical areas can cause radiation damage on non specifically designed electronic equipment due to Single Event Effects (SEE, caused by a single, energetic particle), Total Ionizing Dose Effects (TID, cumulative long-term ionizing damage) and Non Ionising Energy Loss (NIEL, caused by accumulation of displacement damage in the lattice). In order to estimate the risk for the machine operation and safety, exposed devices needs to undergo radiation tests. Until now these have been carried out at CNRAD and at external facilities. However electronic tests of equipment requiring special services are not possible at CNRAD and in addition access to the test area is only possible during technical stops, thus strongly limiting either configuration changes or required reset/repair interventions during the tests. H4IRRAD is a planned irradiation area for testing LHC electronic equipment, especially high power converters. The aim is to reproduce a radiation field as similar as possible to LHC tunnel and shielded areas, taking into account the high energy hadron fluence, particle spectra shape (respective risk factors for thermal neutrons), dose maps, etc. The Monte Carlo FLUKA code was used for all calculations. 2. Simulation setup specification The current work is aimed at establishing the radiation field (high energy hadron fluence and dose rate) for a simplified geometry that is representative to the envisaged irradiation facility in the H4 beam line in the North Area at CERN (H4IRRAD). The simplified geometry (see Fig. 1) consists of a target, for which various target materials and sizes were studied, surrounded by a first and second layer of shielding. In the present proposal equipment would be installed within the first layer to simulate the tunnel locations of the LHC machine and between the first and second shielding in order to simulate LHC shielded areas. In order to establish the radiation field and to evaluate if we can reproduce a mixed field as similar as possible to the one that we have around the LHC, several regions in the geometry were set-up to evaluate the particle spectra and fluences. Simulations have been performed using a copper target of cylindrical shape, 8 cm diameter and 100 cm long. The inner shielding has 40 cm thick concrete walls except the front side (SH1, SH2) whose thickness changes from 0 to 50 cm and various combinations of concrete, iron and polyethylene are used for it. Outer concrete shielding is 80 cm thick. The whole space inside the outer shielding is filled with air. The inner test boxes (ITB) positions should reproduce LHC tunnel-like conditions while the outer test blocks (OTB) positions should reproduce LHC shielded-like conditions. These detectors are only testing volumes filled with air and without any material boundaries so not modify the radiation field. A proton beam of 320 GeV energy has been considered, with a Gaussian profile of 2 cm FWHM.

2 In order to evaluate the average dose rate and hadron fluence over a certain time interval, an SPS supercycle of 44 s was assumed, with 10 9 protons per spill, which give us average intensity of protons/s. All the results that will be presented are evaluated for this beam intensity and supercycle structure. Fig. 1. Simplified geometry. Fig. 2. Target without and with the iron box and inner detectors (ITB). Fig. 3. Outer detectors (OTB) behind the inner shielding.

3 3. Inner shielding influence simulations In order to optimise the radiation field in the external test location, the influence of the inner shielding material composition and thickness has been studied, aimed at the reproduction of condition as similar as possible to LHC shielded locations. 3.1 First simulation setup series In the first series the target was contained in an iron box, which was initially foreseen so to be able to manipulate it after irradiation. In Tab. 1 the various performed simulation configuration are reported with the different material for the two 20cm layers of inner shielding. In simulation case #8, 10 cm of polyethylene is added to 40 cm of concrete. Material Sim No. Sh 1 Sh 2 1 Concrete Concrete 2 Iron Concrete 3 Concrete Iron 4 Iron Iron 5 Concrete / 6 Iron / 7 / / 8 Concrete Concrete+polyethylene Tab. 1. Simulation setups 1 st series. By maximizing the high energy hadron fluence and the spectrum shape, the most promising setups were found to be #2, #5, and #7 (See the high energy hadron fluence in Tab. 2). Setup Fe+Con Concrete Location HEH fluence (/week/cm 2 ) 3.9E E E E+09 No Sh 3.8E E+09 Tab. 2. High energy hadron fluence evaluation. 3.2 Second simulation setup series After some iteration, the target container was removed from the simulation, since it was clarified that the target manipulation will be performed remotely. The geometry in Fig. 1 was used without target box. The most promising shielding setups from the first series and some additional setups were then studied (see Tab. 3).

4 Material Sim No. Sh 1 Sh cm Concrete 20cm Concrete 2 20cm Concrete / 3 / / 4 / 10cm PE 5 / 20cm PE 6 20cm Concrete 10cm PE 7 20cm Fe 20cm Concrete Tab. 3. Simulation setups 2 nd series. Setup #1 was used only for comparison. Setups #5 and #6 did not bring any significant improvement, while #4 could be reasonable solution. Therefore simulation results will be presented only for setups #2, #3, #4 and #7. In Tab. 4 the high energy hadron (HEH) fluence per week for the considered supercycle and beam intensity, as mentioned in chapter 1, are reported. Setup Concrete 10cm PE No Sh Location HEH fluence (/week/cm 2 ) 5.0E E E E E E+09 Fe+Con 5.1E E+08 Tab. 4. High energy hadron fluence evaluation. The goal of around HEH/cm 2 /week in shielded area could be reached by increasing the beam intensity, reducing the supercycle or a proper combination of the previous possibilities, in accordance with Radioprotection authorities and beam operation, respectively. In order to find the most suitable setup for the present purpose, in addition to maximize the hadron fluence, it is necessary to compare the simulated particle energy spectra with the coresponding spectra of critical LHC locations. With the purpose of producing a quantitatively comparison, the contribution of different neutron energy regions with respect to the neutron high energy hadron fluence as well as the contribution of the neutrons to the total high energy hadron fluence was evaluated ( nth thermal neutron fluence, HEn high energy neutron fluence, 5-20MeV 5-20MeV neutron fluence, HEall all high energy hadrons fluence). The H4IRRAD evaluated ratios are listed in Tab. 5 together with some examples of LHC tunnel and shielded areas.

5 Fluence fractions Setup Location nth / HEn 5-20MeV / HEn HEn / HEall Concrete 10cm PE No Sh Fe+Con LHC IR1 Q6 tunnel UJ14/ IR1 RR13/ UJ Tab. 5. Fluence fractions of energy distribution. Whilst values for the internal position don t differ markedly (only in the case of the setup without inner shielding the thermal neutron rate is lower), the ratios between the inner and outer shielding differ significantly. The largest differences between the spectra reachable at H4IRRAD and those in LHC area is for the thermal over HEH ratio, as expected due to the heavy shielding present in areas such as the UJ14/16. Nevertheless the setup combining an iron and concrete lining seems to be the most favourable in light of Tab. 5. However, the high energy hadron fluence is a factor of 5 lower than for pure PE shielding setup. In order to provide a comparison of the obtainable particle spectra, fig. 4 shows the neutron spectra for the H4IRRAD 10cm PE setup (ITB10 detector) and the LHC IR1 Q6 tunnel area, while Fig. 5 shows the neutron spectra comparison between the H4IRRAD 10cm PE setup (OTB11 detector), the 20cm concrete setup (OTB11 detector) and the LHC UJ14/16 area. Spectra are normalized to fit together in the thermal neutron energy region. Fig. 4. Neutron spectra comparison between the H4IRRAD inner region and LHC tunnel area.

6 Fig. 5. Neutron spectra comparison between the H4IRRAD outer region and LHC shielded areas. 3.3 High energy hadrons detailed evaluation From Fig. 4, it is evident that the high energy tail, whose effect is very important in producing SEE in certain equipment, reaches higher energy values for LHC spectra with respect to H4IRRAD (mainly due to the fact that in the LHC the hadronic cascade is generated by a proton beam of 7 TeV instead of 320 GeV). These energetic particles can cause spallation reactions with high corresponding energy densities on heavy materials (such as W) present in some equipment. These might significantly contribute to destructive SEE (SEL, SEB, SEGR) due to the localized energy deposited by secondary particles. Detailed evaluation of the HEH fluence for the setup with 10cm polyethylene shielding is presented in Tab. 6. Results are listed for three inner detectors in different position. We should expect a higher contribution from testing position ITB19 since it is located downstream of the target, at the level of the floor. Neutrons (/h/cm 2 ) >20 MeV >100MeV >200MeV >500MeV >1GeV >5GeV ITB10 2.2E E E E E E+00 ITB12 1.0E E E E E E+03 ITB19 1.2E E E E E E+04 HEH (/h/cm 2 ) >20 MeV >100MeV >200MeV >500MeV >1GeV >5GeV ITB10 3.2E E E E E E+00 ITB12 2.0E E E E E E+04 ITB19 2.7E E E E E E+05 Tab. 6. HEH and neutron fluence in various locations and for different low energy cuts.

7 3.4 High energy hadron fluence gradient The high energy hadron gradient should not be very high in correspondence of the location where the equipment is expected to be tested. This allows to have a well characterised fluence within a single power converter rack. In Fig. 6 and Fig. 8 the high energy hadron fluence x-y and z-x projection cuts are shown. The fluence gradient is also visible in the 2-D plots in Fig. 7 and Fig 9. Although outer detectors occupy the whole testing area height, the area of interest is only in position y: -100 cm 100 cm where the racks will be placed. In this range the fluence differences are smaller than 20 %. For the horizontal (z) direction the most important racks are 3, 4, and 5 where the fluence differences are lower than 12 % per rack. Fig. 6. x-y projection of the HEH fluence gradient in correspondence of the external racks.

8 Fig. 7. Trace of the fluence along the vertical direction for the outer detectors. Fig. 8. z-x projection of the HEH fluence gradient in correspondence of the external racks.

9 Fig. 9. Trace of the fluence along the horizontal direction for the outer detectors. 3.5 Detailed radiation field studies In addition to the high energy hadron fluence also dose maps have been performed by enabling in FLUKA the proper treatment of the electromagnetic cascade. These simulations (more CPU intensive) were performed only for the four best inner shielding setups (20cm Fe + 20cm concrete, 20cm concrete, 10cm PE, no shielding). A more realistic geometry was also used (see Fig. 10). The OTB and the outer shielding were moved closer to the inner shielding and some unused parts of the area between the inner and outer shielding was filled with concrete.

10 Fig. 10. Upgraded simplified geometry. In Tab. 7 and Tab. 8 the high energy hadron fluence and fluence ratios of energy distribution over the spectra (newly recalculated values from Tab. 4 and Tab. 5) are reported for this updated geometry. For some results, there is a noticeable difference (in comparison with Tab. 4 and Tab. 5) due to moving the OTB closer to the inner shielding. In Tab. 8 the fractions of the high energy hadron fluence over the dose (in Gy) and the Si 1MeV-neutron equivalent flux over the high energy hadron fluence have been added. Setup Fe+Con Con 10cm PE Location HEH fluence (/week/cm 2 ) 5.1E E E E E E+09 No Sh 5.0E E+09 Tab. 7. High energy hadron fluence evaluation.

11 Fluence fractions Setup Location nth / HEn 5-20MeV / HEn HEn / HEall HEall /Dose[Gy] Si1MeV / HEall Fe+Con Con 10cm PE No Sh LHC E E E E E E E E IR1 Q6 tunnel UJ14/ IR1 RR13/ UJ Tab. 8. Fluence fractions of energy distribution. When we compare the results reported in Tab. 4 and Tab. 7 (high energy hadrons), the values for the outer locations are increased by 17.5 % for the Fe + concrete setup, 29.8 % for the concrete one, 35.7 % for the polyethylene one and 37.8 % in the case of no shielding. Considering these new results we would need factor of 2.6 for PE shielding setup (and not the 3.5 as from Tab. 4) to reach the value of HEH/cm 2 /week. In the case of a single PE shielding it would be sufficient for example to reduce the SPS supercycle length to 16 s. For the setup with 20cm concrete shielding we report in the following pictures the projection of the dose, fluence of HEH, and the Si 1MeV-neutron equivalent flux (Fig. 11 Fig. 16). Fig. 11. x-y dose projection in Gy/week.

12 Fig. 12. z-x dose projection in Gy/week. Fig. 13. x-y projection of high-energy hadron fluence (per week). Fig. 14. z-x projection of high-energy hadron fluence (per week).

13 Fig. 15. x-y projection of Si 1 MeV-neutron equivalent flux (per week). Fig. 16. z-x projection of Si 1 MeV-neutron equivalent flux (per week). Fig. 17 shows the neutron fluence in the case of the 20cm concrete shielding setup for various locations inside and outside the inner shielding. We note that the high energy tail is shifting to higher energies for downstream positions with respect to the target (in accordance with Tab. 6). Fig. 18 and Fig. 19 show comparisons of the neutron fluence and of the high energy hadron fluence between four selected shielding setups, which confirm the results from Tab. 7 and Tab. 8.

14 Fig. 17. Neutron fluence in various inside and outside detectors (per week). Fig. 18. Neutron fluence comparison in outside OTB11 detector (fluence per week).

15 Fig. 19. High energy hadron fluence comparison in outside OTB11 detector (fluence per week). 4. Summary The generic calculations for planned H4IRRAD irradiation facility in the H4 beam line in the North Area at CERN were done using the Monte Carlo FLUKA code. For establishing the radiation field (high energy hadron fluence and dose rate) in H4IRRAD and for optimizing the inner shielding, the simplified geometry (See Fig. 1 and Fig. 10) was used. As the best compromise between high energy hadron fluence, particle spectra shape (the contribution of different neutron energy regions with respect to the neutron high energy hadron fluence as well as the contribution of the neutrons to the total high energy hadron fluence) and the HEH gradient in the equipment testing locations, the 20cm concrete inner shielding was identified. For the setup with the 20cm concrete inner shielding, we obtain HEH/cm 2 /week (average value over the second OTB row) for the considered SPS supercycle (44 s, 10 9 protons per spill). The HEH gradient is lower than 12 % for the horizontal direction and 20 % for the vertical direction per rack, which is sufficient. As a next step, the real implementation layout has to be designed and all FLUKA calculations have to be rerun for the real geometry.