Induced radioactivity and dose rates in the vicinity of a collimator at the Linear Collider TESLA

Transcription

1 Laboratory note DESY D3-104 September 1999 Induced radioactivity and dose rates in the vicinity of a collimator at the Linear Collider TESLA H. Dinter and A. Leuschner

2 Abstract: The collimators positioned in the tunnel of the Linear Collider absorb a significant amount of energy during operation of the accelerator. They are important sources of secondary particles which cause induced radioactivity inside the tunnel and in the environment outside. Using a simple geometry that approaches a tunnel section housing a collimator, radioactivities were calculated by the Monte Carlo code FLUKA and dose rates were estimated. An irradiation time of 5000 h and a constant beam loss of 100 kw per collimator at 250 GeV were assumed as maximum operation conditions per year. Dose rates of a very high level produced by the activated objects inside the tunnel after shut-down of the accelerator were found. These dose rates prohibit any handling of objects in the collimator areas for a long decay time, and even a simple passage is related with a considerable dose charge. Measures have to be taken that these dose rates are lowered by at least 2 orders of magnitude. The radioactivity of the soil around a collimator is higher than in the case of a beam absorber (factor 50); the radioactivity of the ground water is comparable with that expected around a beam absorber. The release of radioactivity with the tunnel air leads to doses well below legal limits. /projekte/tesla/collim/lb104.tex 2

3 1 Introduction In TESLA two high energy electron/positron beams are brought to collision. After having passed the interaction point and the detector, the beams are separated and are either used to produce positrons or are dumped into specially designed absorbers. The quality of the spent beams is low with respect to emittance so that they have to be upgraded by use of a series of collimators positioned in the accelerator tunnel, before further processing. In these collimators a considerable amount of beam power is dissipated. During operation of the accelerators each collimator represents a strong source of secondary particles leading to a high level of radioactivity of itself and of all objects in its vicinity. As a consequence, radiation damage in the components is expected as well as restrictions in the accessibilty of the tunnel area and impacts on the environment outside the tunnel. In this paper calculations are reported concerning induced radioactivity and estimations of dose rates inside the tunnel after the end of accelerator operation. From these results conclusions can be drawn after which period of time the area nearby a collimator can be accessed, to what extend persons can work there and about the possibilities to repair or exchange components. In addition the radioactivity of nuclids with relevance to the environment are calculated. A preliminary design version of a total collimator section is shown in Fig.1. It consists of 5 spacial separated collimators positioned very close to or even within a dipole magnet. In the following we restrict ourselves to a prototype of one of these collimators. 2 Calculations The program code FLUKA in its version 98 has proved to be well suited to calculate residual nuclei produced in hadronic cascade processes [Fas97]. The main difficulty for the usage of the program is to keep the running time, needed to obtain statistically significant results, reasonably low (that means less than a few days). One measure to reach this goal is to simplify the geometry in a way that it is effective for a fast processing but does not lose the relation to the reality. 2.1 Geometry For the calculations it is not necessary to use a very detailed arrangement of the collimator and the components next to it because the essential features can be approximated without reduction of the applicability of the results. Therefore, a cylindrical arrangement was adopted with a simple and clear distribution of the materials in the vicinity of a collimator. As shown in Fig.2, a hollow beam hits the front face of an annular aluminium cylinder, simulating a beam halo scraping the inner surface of a collimator. This source of primary interactions is surrounded by additional annular cylinders of iron, air, concrete and wet sand, representing a dipol magnet, the tunnel wall, and the adjacent soil. The soil region was divided into an 3

4 inner and an outer layer with thicknesses of 50 cm each. The total length of the arrangement amounted to 30 m, the scoring length was 6 m, centered around the shower maximum. The components of the materials and their dimensions are listed in Tab.7 and 8 in the appendix. 2.2 Operational parameters The following parameters were assumed for the calculations: Beam energy 250 GeV Operation time 5000 h/year ower of beam loss 100 kw 2.3 Calculation of induced radioactivity The option RESUCLEi of FLUKA results in a table of nuclei (corresponding to a region of material) produced per primary electron, either by inelastic reactions or by low energy neutrons ( low means 20 MeV). In an additional evaluation program these nuclei are filtered in a way that only such with half-lifes between 10 minutes and 50 years are selected. Only nuclei of this type are regarded to be relevant for the present study. The multiplication of the numbers of produced nuclei per electron by the rate of the assumed beam loss (assumed: 250 GeV and 100 kw giving e/s) results in saturation activities. Saturation activity is the maximum achievable activity obtained by an infinitely long period of irradiation and the decay rate being equal to the production rate. 2.4 Calculation of dose rates The rate of the dose equivalent in the accelerator tunnel after the shut-down of operation was calculated (or better estimated ) for a distinct point of interest in the middle of the tunnel, approximately at 1 m sideward of the activated beam line section (see Fig.2). The rate of dose equivalent due to radioactive nuclides was calculated by the following relation: with being the conversion coefficient that converts the activity of a nuclid into rate of dose equivalent produced in soft tissue at a depth of 10 mm and at a distance of = 1 m from the source [et93]. The quantity takes into account the attenuation by self-absorption in the source and absorption by other objects between the source and the point of interest, and is the mean distance between the source and the point of interest. For the objects under consideration the following assumptions are met: (1) Collimator: An attenuation factor! "$# was applied because it is shielded by 20 cm of iron being equivalent to 2 tenth-thicknesses (% &' (#) cm at 1.5 MeV). 4

5 0 6 0 Dipol magnet: To facilitate the integration of the activity over the volume the following simplifications were made: The activity of the 6 m long magnet is assumed to be in a circular cross-section area in the same plane as the point of interest. A quadratic cross-section was formed with the same area in which the total activity is homogenously distributed. The integration results in an effective area *,+.-/- that contributes to the dose rate, and the rest of the area was neglected with respect to the dose rate because of selfabsorption. Then the factor is just the ratio of the effective area to the total area * ; 0 and 021 being the outer and inner radius of the magnet ring, respectively % &8 #= 5:9<; *4+.-/-5* >@? BA (2) 0 1C This relation results in 3D!E#. Tunnel wall: The self-absorption of the concrete wall can easily taken into account by the ratio of an effective depth and the wall thickness: 3 % &' #) 5:9E; A (3) 0/1 With % &8 FG cm for ordinary concrete one gets 3D$ "H. The distance between the wall and the point of interest is IH m. Soil: The soil around the tunnel wall is not regarded as a radiation source with respect to the point of interest because of its low activity compared with the main sources and its shielding by the tunnel wall. Air: The activation of the air inside the tunnel is also not taken into consideration as a source of dose rate inside the tunnel. 3 Results 3.1 Activations and dose rates inside the tunnel The lists of produced nuclei inside the tunnel and the resulting dose rates after shutdown of the accelerator and with respect to the point of interest (see Fig.2) are shown in details in Tab. 9 to 16. The following table (Tab.1) is a summary of these tables and shows the dose rate contributions of the different materials for a set of decay times after shut-down. The found dose rates are extremely high. Even after a decay period of 1 year an access to the area would not be possible without restrictions. According to the German Regulations for Radiation rotection ( Strahlenschutzverordnung ) a rohibited Area ( Sperrbereich ) has to be established when dose rates higher than 3 msv/h may occur. Although the geometry of the tunnel section is highly simplified the dose rates in case of the realistic arrangement of Fig.1 (and together with the assumptions for accelerator operation and beam loss) are supposed to be in the same order of magnitude. As expected, the object with the highest saturation activity concentration is the collimator with appr. 110 Bq/(g W), see Tab.2. evertheless, its contribution to the dose 5

6 Decay time after shut-down Object none 1 hour 1 day 1 month 1 year Collimator Magnet Wall Total Table 1: Dose rates in msv/h in a distance of appr. 1 m from the collimator (point of interest), after one year of operation. Contribution of the regarded sources, calculated for the geometry shown in Fig.2. Object Sat.activity concentration per beam loss (Bq/(W g)) Collimator Dipol magnet Tunnel wall J Soil J Table 2: Saturation activity concentrations per beam power of objects in the tunnel. For soil and wall it was assumed that 90% of the total activity is located in a length of 6 m, centered around the shower maximum. rate at the point of interest is lower than that resulting from the magnet. The manganese isotopes produced in the iron of the magnet dominate with half-lifes between 2.6 h (KML Mn) and 312 days (KM Mn). The magnet acts as a source and a shield of the collimator as well. Therefore, a geometry with an isolated collimator (the magnet being installed in a certain distance from the collimator) would not improve the situation. In contrary, the collimator would contribute 100 times more dose rate when unshielded. In case of the collimator being the main source, the isotopes of sodium govern the dose rate with half-lifes of 15 h ( a) and 2.6 years ( O a). Other materials like the concrete of the tunnel wall are of minor importance compared to the main sources collimator and magnet. 6

7 3.2 Environmental impacts Air activation The air in the tunnel is continuously transported with a velocity of 0.6 m/s. It was assumed that the collimator is located at the center of the collider so that the activated air has to travel 15 km before being released into the environment. As the collimator arrangement used for the calculations has a length of 6 m (see Fig.2) the effective activation time amounts to 10 s and is repeated 5000 h/10 s times per year (= L ). The decay time is approximately 7 hours. The results are displayed in detail in Tab.15 and 16. The activities reaching the end of the tunnel can be compared with figures reported in the earlier paper [Tes98]. But differences in the methods as well as in the scope of both calculations have to be taken into account. While in the present paper the main view is directed to the embedding of the collimator in a more or less realistic geometry, in paper [Tes98] the worst case situation was studied. There, an isolated target was placed in the tunnel, optimized to result in a maximum number of secondary neutrons. Then the tracklengths were determinated and together with the cross-sections of relevant nuclear reactions the number of nuclei of interest were calculated. This method leads to an upper limit of radioactivity whereas the method used in the present paper is expected to give lower activities. To be able to compare the results of both methods, an additional calculation was performed were the dipol magnet was omitted. In Tab.16 the summary of the results of both methods are compared after being normalized to the same loss of beam power of 100 kw. The values calculated by the present paper are lower by a factor of 35 on the average. The released activities produced by the actual geometry (they are summarized in Tab.3) are 10 to 200 times lower than those of [Tes98] and consequently the doses are lower by the same factor. They are well below the legal limits Soil and ground water For the calculations the tunnel was surrounded by a layer of wet sand, representing a mixture of dry soil and ground water (27% water, 73% dry soil, see Tab.8). In the previous paper [Tes97] the activities in soil and groundwater were calculated for the environment of the beam absorbers. We use the same composition of the dry soil as being typical for the region under consideration. As mentioned above, the methods of the calculations are different in a way that we make use of the option RESUCLEi and the data library provided by FLUKA. The relevant nuclei found by this method are listed in details in Tab.17. All these figures are related to the inner layer of the soil (see Fig.2). The nuclei giving the main activities agree with those found in [Tes97]. They are shown in Tab.4 together with their saturation activities. The total activity of the soil for a power dissipation of 100 kw amounts to 210 Bq/g (see Tab.4, col. 4). This is considerably higher than in the case of the well shielded beam absorber (see [Din98] option 1 of absorber shielding: 4.0 Bq/g). The natural activation concentration of sand amounts to 0.3 to 1 Bq/g. 7

8 Q Q Released uclid Half-life activity (MBq) H 12 a 23. R Be 53 d 210. C 20 min 2.7 RS C 5730 a 4.0 RT Cl 37 min 22. Q Cl 56 min 160. Ar 35 d 37. Ar 1.8 h Table 3: Radioactivity released per year with the air at the end of the tunnel. Sat. activity Sat. activity uclid Half-life per beam power concentration (kbq/w) per beam power (Bq/(g kw)) H 12 a O Be 53 d a 2.6 a KU Mn 312 d KRK Fe 2.7 a atural activity of sand Bq/g Table 4: Saturation activities in wet sand surrounding the accelerator tunnel (inner layer, see Fig.2). 8

9 D Activity Activity Critical uclid Half-life concentr. concentr. activity inner ly. outer ly. concentration (Bq/g) (Bq/g) (Bq/g) R H 12 a a 2.6 a atural activity 0.4 Bq/g Table 5: Radioactivity produced during 400 days in the inner and outer layer of the soil (see Fig.2) and removed by the ground water flow. The Critical Activity Concentration produces a dose to a person of 0.3 msv when 800 ltr of water (yearly consumption of a person) are ingested. Concerning radioactivity in the ground water, H and R a are assumed to be the only nuclei necessary to be regarded. All other nuclei produced in soil can be neglected because of their low solubility (see [Tes97]). H is produced in soil and water and 100% will be dissolved in the water, wheras the solubility of R a amounts to only 15%. From the expert s report [Koe97] we know that an average residence time of the ground water of 1150 days can be assumed in the area around Ellerhoop. This period of time is related to a cylinder with a length of 17 m and a diameter of 11 m. As the center part of our geometry amounts to 6 m it was concluded that after an irradiation time of 400 days (17 m/6 m D 3; 1150 d/3 D 400 d) the irradiated water is exchanged and together with the activities of H an R a produced during 400 days it is removed, dilluted and mixed with natural ground water. The activity concentrations of both nuclei are listed in Tab.5. A comparison of the activity concentrations of the inner and outer layer of the soil surrounding the tunnel shows a decrease of 1/3. To estimate an upper level of radioactivity tolerable with respect to the German Regulations for Radiation rotection a Critical Activity Concentration was defined (see [Tes97]) that leads to a dose of 0.3 msv when activity in water is ingested (find more information to this concept in section 4.2). The critical activity concentrations determined in [Tes97] are listed in Tab.5 for comparison. In the case of R a this limit is exceeded by a factor of 3 in the outer soil layer. The activity concentration of both nuclei in the outer layer is higher than the natural activity by a factor of 5. The activity of R a is in the same order of magnitude than that around a beam absorber (option 1 of absorber shielding: 0.22 Bq/g; see [Din98]). 9

10 Ḣ = 70 msv/h (see Tab.1) AWV assage: 5 s 100 X Sv AWV Short stay: 50 s 1000 X Sv Weekly dose limit for persons ) 400 X Sv Table 6: Dose charge to a person when entering a collimator area 1 hour after shutdown. ) = Dose limit of 20 msv per year equally distributed over 50 weeks. 4 Conclusions 4.1 Inside the tunnel The dose rates listed in Tab.1 reveal that the configuration of a collimator as simulated in Fig.2 prohibits practical operations in this area of the accelerator tunnel after shutdown. Even a decay time of 1 year would request a rohibited Area ( Sperrbereich ) and a simple passage could only be allowed under the control of a Radiation Officer. As an example for an immediate access to this area in case of emergency the dose equivalent received by a person is shown in Tab.6. The official regulations being in progress dictate an upper limit of 20 msv per year to a person in case of an occupational exposure. If equally distributed this means a dose of 400 X Sv per week (1 year equals 50 weeks) or a stay of roughly half a minute in this area. ormal maintenance work (even with time restriction) could only be performed if the dose rates are lowered by at least 2 orders of magnitude. The occupation of persons not in a contract with DESY (Fremdfirmen, Gastinstitute) is only possible when owing a special permission of the Supervisory Authority. One measure to prevent prohibitive dose rates are to avoid irradiation of objects around the collimator and to shield radioactive sources after shut-down of the accelerator by unirradiated material. The attenuation of Y -radiation by 2 orders of magnitude is achieved by 2 tenth-thicknesses of shielding material (20 cm iron or 12 cm lead). Another possibility could be to fill the room (if there is any) between collimator and dipole magnet with a low-z material (like carbon or water) in which the electromagnetic casade can fully develop and in which only a few radionuclides ( Q Be, H) are produced. 4.2 Environment General remarks In the German Regulations for Radiation rotection ( Strahlenschutzverordnung ) no exemption values for the activation of soil and ground water are foreseen. 10

11 In the case of soil near the accelerator tunnel an activation appears to be rather uncritical as long as the nuclei are not moved and are not soluble in water. The only scale to measure the degree of activation is the natural activity concentration. In contrast to soil the ground water transports the radioactive nuclei produced in the water itself or dissolved from soil, and can contribute to human exposure following different pathways. In order to get a relation between the activity concentration and a dose value laid down in the regulations the Critical Activity Concentration (see section 3.2.2) was used in [Tes97]. This concentration is calculated in a way that it leads to a dose of 0.3 msv when dissolved in water and ingested. The annual dose of 0.3 msv is the upper limit of radiation exposure to persons in public areas, caused by radiation producing installations. The demand not to exceed these concentrations immediately at the outer surface of the tunnel would be extremely conservative because it is absolutely unrealistic to assume that a person supplies its consumption of drinking water exclusively from that point. The concept to compare calculated activity concentrations with Critical Activity Concentrations is presently the only possibility to relate to legally fixed dose limits. It will be a matter of the Supervisory Authorities to determine at what distance from the tunnel surface the equality has to be achieved. But, DESY has not only to prove that the project is conform with respect to the regulations, in particular it has also to prove that Z 28 (rinciples of radiation protection 1 ) is fulfilled. In this sense it is advisable in the planning phase of the project to aim at activity concentrations near to the tunnel in the order of magnitude of the Critical Activation Concentrations Soil and ground water The activation of soil and groundwater occurs during operation of the accelerator. Especially to the nuclei being soluble in water ( H and O a ) one has to pay attention. As shown in Tab.5 the activity concentration of R a in the layer up to 50 cm from the tunnel surface (inner layer) exceeds the critical values by a factor of 12. In the second layer from 50 to 100 cm the mean activity concentration has decreased by a factor of 3, and in another layer of comparable thickness (not calculated) the values of the Critical Activity Concentration can be reached. The isotope H is of minor interest in this context. If the various collimators are not installed too concentrated, so that a superposition of activities has to be expected, no additional shielding around the tunnel wall would 1 ) Wer eine Tätigkeit nach [ 1 dieser Verordnung ausübt oder plant, ist verpflichtet, 1. jede unnötige Strahlenexposition oder Kontamination von ersonen, Sachgütern oder der Umwelt zu vermeiden, 2. jede Strahlenexposition oder Kontamination von ersonen, Sachgütern oder der Umwelt unter Beachtung des Standes der Wissenschaft und Technik und unter Berücksichtigung aller Umstände des Einzelfalles auch unterhalb der in dieser Verordnung festgesetzten Grenzwerte so gering wie möglich zu halten. 11

12 be necessary. A comparison with the situation around the beam absorbers shows that in case of a collimator soil activity is 50 times higher (option 1 of absorber shielding 2 ), Air water activity is comparable. The air activity calculated in [Tes98] give values at the release shaft leading to doses below legal limits. The activities at the shaft calculated in this report are lower by a factor of 35 and therefore do not represent difficulties. References [et93]. etoussi, M. Zankl, G. Fehrenbacher, G. Drexler, Dose distributions in the ICRU sphere for monoenergetic photons and electrons and for ca. 800 radionuclides, Institut für Strahlenschutz, GFS-Bericht 7/93 [Fas97] A. Fassò, A. Ferrari, J. Ranft, R.. Sala, ew Developments in FLUKA Modelling Hadronic and EM Interactions, roceedings of the Third Workshop on Simulating Accelerator Radiation Environments (SARE3), KEK, Tsukuba, Japan, 1997 [Koe97] E. Doerks, A. Kölling, Hydrologisches Übersichtsgutachten Ellerhoop, Fa. lanum, Dec [Tes97] K. Tesch, roduction of radioactive nuclides in soil and groundwater near the beam dump of a Linear Collider., Internal Report, DESY D3-86, 1997 [Din98] H. Dinter, Radiologische Auswirkungen auf die Umwelt beim Betrieb des Linear Colliders, Laborbericht DESY D3-97/1, 1998 [Tes98] K. Tesch, H. Dinter, roduction of radioactive nuclides in air inside the collider tunnel and associated doses in the environment., Internal Report, DESY D3-88, ) Absorber shielding: 3 m concrete and 80 cm concrete-equivalent, presented by building and selfabsorption, see [Din98] 12

13 Figure 1: Designed positions for collimators in the collider tunnel 13

14 m qo m qo ]]\]^_ defg `abc ch g cm Z R Soil tt tuv rs llo Air mop nkl ij kl mj Iron Beam Aluminium Beam Iron mop nkl ij kl mj tt tuv rs llo Soil Figure 2: Geometrical approach of a collimator area applied for the calculations 14

15 1 5 Appendix Object Material (cm) (cm) Mass (10 kg) Collimator Aluminium Dipol magnet Iron Tunnel Air Tunnel wall Concrete Inner soil Wet sand Outer soil Wet sand Table 7: Radii of cylindrical regions of the simplified tunnel geometry. The quantities 1 and mean inner and outer radius of the cylinder, see Fig.2. 15

16 Object: Collimator Magnet Material: Aluminium Iron Element Weight % Element Weight % Al 98.2 Fe 100. Mg 0.6 Si 0.6 Fe 0.3 Mn 0.1 Cu 0.1 Zn 0.1 Object: Tunnel room Tunnel wall Material: Air Concrete Element Weight % Element Weight % 75.5 O 52.9 O 23.1 Si 33.7 Ar 1.28 Ca 4.4 C Al 3.4 a 1.6 Fe 1.4 K 1.3 H 1.0 Mg 0.2 C 0.1 Object: Soil Material: Water Sand Element Weight % Element Weight % O 24.0 O 39.0 H 3.0 Si 23.0 Al 4.0 Ca 3.0 Mg 2.0 Fe 2.0 Table 8: Material composition of the objects used in the geometry of Fig.2 16

17 umber per Saturation Specific Relative uclid Half-life primary activity per saturation statistic 250 GeV beam power activity error electron [MBq/W] [Bq/(g W)] [%] S O F 110. m a 2.60 a a 15.0 h KU Mn 310. d a n 15.0 h Table 9: Radioactivity of the collimator. n means: caused by low energy neutrons. Dose rate Dose rate Dose rate Dose rate Dose rate uclid after 1 hour after 1 day after 1 month after 1 year after shut down shut down shut down shut down shut down [msv/h] [msv/h] [msv/h] [msv/h] [msv/h].s R F a a a n KM Mn total Table 10: Dose rates at the point of interest, after an operation of 5000 hours with constant 100 kw beam loss; source: collimator; n means: caused by low energy neutrons. 17

18 S K T K S K T K umber per Saturation Specific Relative uclid Half-life primary activity per saturation statistic 250 GeV beam power activity error electron [MBq/W] [Bq/(g W)] [%] RL Sc 83.8 d V 16.0 d Mn 5.6 d KU Mn 312. d KUL Mn 2.58 h KML Co 78.8 d KU Mn n 312. h KUL Mn n 2.58 h Fe n 45.1 d Table 11: Radioactivity of the dipol magnet. n means: caused by low energy neutrons; Dose rate Dose rate Dose rate Dose rate Dose rate uclid after 1 hour after 1 day after 1 month after 1 year after shut down shut down shut down shut down shut down [msv/h] [msv/h] [msv/h] [msv/h] [msv/h] OL Sc V Mn KM Mn KM Mn n KML Mn KML Mn n Fe n KML Co total Table 12: Doses rate at the point of interest after an operation of 5000 hours with constant 100 kw beam loss; source: dipol magnet; n means: caused by low energy neutrons. 18

19 K K umber per Saturation Specific Relative uclid Half-life 10 primary activity per saturation statistic 250 GeV beam power activity error electrons [MBq/W] [Bq/(g kw)] [%] R R C 20.4 m a 2.60 a a 15.0 h Mn 5.6 d w a n 2.60 a Si n 2.62 h K n 12.4 h Table 13: Radioactivity of the tunnel wall. n means: caused by low energy neutrons; Dose rate Dose rate Dose rate Dose rate Dose rate uclid after 1 hour after 1 day after 1 month after 1 year after shut down shut down shut down shut down shut down [msv/h] [msv/h] [msv/h] [msv/h] [msv/h] R R C a a w a n Si n K n Mn total Table 14: Doses rate at the point of interest after an operation of 5000 hours with constant 100 kw beam loss; source: tunnel wall; n means: caused by low energy neutrons. 19

20 Q Q umber per Saturation Specific Relative uclid Half-life 10L primary activity per saturation statistic 250 GeV beam power activity error electrons [kbq/w] [Bq/(g kw)] [%] H 12.3 a R Be 53.3 d C 20.4 m RS C 5730 a RT Cl 37.2 m Q Cl 56. m Ar 35.0 d H n 12.3 d RT C n 5730 a Q Cl n 56. m Ar n 35.0 d Ar n 1.83 h Table 15: Radioactivity in the air. o air flow assumed. n means: caused by low energy neutrons; Q Q roduced Released Released Released uclid activity activity activity activity no magnet ref. [Tes98] [MBq] [MBq] [MBq] [MBq] H H n R Be C L C RS C n RT Cl RT Cl Cl n Ar Ar n Ar n K K Table 16: roduced and released air activity during an operation of 5000 hours with constant 100 kw beam loss. n means: caused by low energy neutrons; 20

21 Q T umber per Saturation Specific Relative uclic Half-life 10 primary activity per saturation statistic 250 GeV beam power activity error electrons [kbq/w] [Bq/(g kw)] [%] H 12.3 a R Be 53.3 d a 2.60 a OL Sc 83.8 d J V 330. d KM Mn 312. d KRK Fe 2.7 a KML Co 78.8 d J KM Mn n 312. d KRK Fe n 2.7 a Table 17: Radioactivity in the soil (inner layer, see Fig.2). n means: caused by low energy neutrons. 21