Radiation field inside the tunnel of the Linear Collider TESLA

Transcription

1 Laboratory Note DESY D3 113 April 2000 Radiation field inside the tunnel of the Linear Collider TESLA Dark current, first attempt A. Leuschner, S. Simrock Deutsches Elektronen-Synchrotron DESY Abstract Highly integrated electronic circuits are planned to be installed inside the tunnel of the TESLA collider. Life time und performance of complex control systems have to be investigated in a pulsed radiation field. First measurements around the cryo-modules of the TESLA Test Facility are presented indicating energy dose rate levels in the order of 1 Gy/h. Assuming a dynamic heat load of 0.1 W/m a dose rate of 10 mgy/h is predicted by a Monte-Carlo calculation. The comparison of the measured effective photon energy with the calculations shows that the radiation field mainly originates from accelerated dark current (electron energies above 20 MeV) rather than from onecell dark current (electron energies below 5 MeV).

2 1 Introduction Design studies of the linear accelerators for the TESLA collider raised the issue of the radiation level inside the tunnel. The questions are how those lots of computer, processors and highly integrated circuits perform in a pulsed radiation field and what their life times are. In principle, there are two different radiation sources: beam losses and dark current from the cavities. We concentrate on the latter. In general, it is considered black magic to pin down the source strengths. At a first attempt they are coupled to a heat load in the cold mass of 0.1 W/m. As this concept fixes the dose rate in the cold mass the radial distribution inside the tunnel has to be calculated only. Here, the Monte-Carlo code FLUKA99 [1] was used. Neutrons are not expected to play a significantrole in this context. But nevertheless, a rough estimation of the neutron flux is also given. The calculations are compared to experimental data from the TESLA Test Facility. The electromagnetic component of the radiation field around the two cryo-modules was measured by means of LiF thermoluminescence chips and ionisation chambers. 2 Geometry The cryo-module consists of various materials with a complex geometry. For the estimation of an average dose in a long tunnel section it is sufficient to use a simplified geometry. All structures are approximated by a pure cylindrical geometry by keeping the densities and masses per unit length in longitudinal direction unchanged. The result is compiled in tab.1. Region 8 is listed twice representing 2 different options: First, it is filled with air in the normal case, and second, it is filled with ordinary concrete in the shielding case. A concrete shell inside the tunnel might be somewhat unrealistic Λ, but it gives a rough impression about the dose attenuation in lateral direction. 3 Beam losses The main linac is expected to be operated at the highest possible gradient limited by the cooling capacities. They are designed to accept an additional dynamic heat load of 0.1 W/m. In fact only the total heat load (integral) of a 2.5 km long section is the limiting factor. With this constraint, the losses of the high energy beam and the dark current are estimated as an average over long linac sections and time. Λ One could imagine a horizontal or vertical wall but not a whole shell. 1

3 no region materaial inner radius outer radius density mass/length [cm] [cm] [g/cm 3 ] [kg/m] 3 soil sand wall concrete tunnel air tunnel air tunnel air tunnel air shielding concrete tunnel air tunnel air module tank iron outer coolant liquid He vacuum magnetic shield iron cavity tank titanium coolant liquid He cavity niobium beam vacuum Table 1: Region description of the pure cylindrical tunnel. Region 8 is filled with air in one option, and filled with concrete in another. Dark current: Given the absorbed power per length one can immediately deduce an average energy dose rate in the cold mass (see tab.1, regions 15-17). The following variables determine the dose rate in a particular region: _D ::: energy dose rate [Gy/h] E abs ::: absorbed energy [J] t ::: time [s],[h] m ::: mass [kg] l ::: length [m] The energy dose rate in a region is defined by Taking the length into account one gets _D = E abs t m : (1) _D = E abs 1 : (2) t l m=l The first factor is the average heat load assumed to be 0.1 W/m. The cold mass of the module (niobium, liquid helium, titanium) is heated up by the intercepted electrons. Its mass per length (second factor) is about 38 kg/m. With these 2 values one yields a dose rate _D =0:1 W/m 38 kg/m 1 ß 10 Gy/h : (3) High energy beam: Summing up the total loss power for one main linac and assuming that only 1/2 is absorbed by the cold mass one yields 0.1 W/m 2 15km=3kW.Compared to the averaged beam power of 12 MW/2 the relative loss is Rather uniformly distributed losses of that kind can hardly be detected by monitors. 2

4 Time structure: The cavities are operated in pulsed mode with an rf puls duration of about 1 ms. During that time, dark current is produced and a beam can be accelerated. The repetition rate will be up tp 10 Hz, giving a duty factor of 100. So, for example, an average dose rate of 1 mgy/h consists of 1 ms long bursts of 100 mgy/h. All dose rates given in the other sections are averaged over long times as they are derived from or compared with a heat load per unit accelerator length. 4 Electron-photon dose 4.1 Dose measurements Integral electron-photon doses D fl were measured by means of thermoluminescence dosemeters TLD-105 (LiF chips). The accelerating module ACC1 had been operated at a gradient of 19.7 MV/m and a repetition rate of 1 Hz for 7 hours. All other components of the linac (injector, ACC2, :::) were turned off. The dosemeters were positioned under each cavity on the tank surface. The measured dose distribution along the modul is displayed in fig.1. The cavities 1 and 2 (counted downstream) radiate most. But the other cavities also contribute to the dose. The average dose rate on the surface of the cryo-module is in the order of 1 mgy/h and at a distance of 2 m from the module 0.1 mgy/h y. Simultaneously the attenuation in lead was measured. A set of dosemeters was covered by lead cylinders of different wall thicknesses from 0.2 cm to 1 cm. It was positioned under cavity 4 in a distance of 35 cm from the tank. The attenuation curve is displayed in fig.2. The fitted half-valuethickness is 0.82 cm corresponding to an effective photon energy of 0.7 MeV and a tenth-valuethickness of 27 cm for a concrete shielding. The above mentioned measurement was repeated with cavity 2 on resonance only and the others detuned. The longitudinal dose distribution is also displayed in fig.1. The comparison of both curves shows that there are more than 1 radiating cavity in the module. 4.2 Dose rate measurements For the dose rate measurements an ionization chamber of 1000 cm 3 volume was mounted on a carrier movable on rails along the cryo-module. A dose rate distribution along the cryo-module no.1 at a distance of2misshown in fig.5. The module was operated with a gradient of 17 MV/m, a flat top of 800 μs and a repetition rate of 1 Hz. The dose rate distribution is much less characteristic than the dose distribution (upper curve of fig.1). The reasons may be that the distribution is flattened out at larger distances and at lower gradients. At a fixed position (9 m downstream of cryo-module no.1, 4 m upstream of cryo-module no.2), the dose rate as a function of gradient was measured. The data are plotted as boxes in figs.3 and 4. In fig.3 fits of the Fowler-Nordheim current to the cryo-module no.1 (dashed line, c 1 = y A1/r scaling of a line source is used. 3

5 0:033 Gy/h, fi = 210) and cryo-module no.2 (solid line, c 1 = 0:013 Gy/h, fi = 180) are shown. To be more precise, the Fowler-Nordheim current for the rf-case is multiplied by the field to get a value proportional to the energy of the dark current and thus to the dose [2]: _D = c 1 E 7=2 e c 2=(fiE) with c 2 = MV/m; (4) where c 1 and fi are parameters for the fit. The other solid line curves were obtained by changing the parameter fi only. fi was introduced ad hoc for the enlargement of the electric field on the cavity surface. It characterizes the emitters of dark current. Fig.4 shows the same data together with the scaled dark currents taken from [3]. The steeper slope of the calculation is due to the parameter fi =80being much lower than that of the data. 4.3 Calculations The radial distribution of the electron-photon dose D fl was calculated by means of the Monte- Carlo code FLUKA [1]. In this context, dose means that the deposition of electro-magnetic energy was scored on a region basis. No hadrons were generated. The geometry is described in section 2. The doses are scaled to a dose rate such that the average dose rate in the cold mass (see tab.1, regions 15 to 17) is 10 Gy/h (see equ.(3)). The distributions are calculated for 5 different energies: 2 MeV and 4 MeV with a isotropic angular distribution to model the dark current in one cell, and 20 MeV, 100 MeV, 500 MeV with a hollow cylindrical beam on beam axis to model the accelerated dark current. The distributions are displayed in figs.6, 7, 8, 9, and 10. For each energy, both options are shown: the region 8 (radius 150 cm to 200 cm) filled with air and filled with ordinary concrete. The concrete layer of 50 cm thickness attenuates the radiation field by a factor of 100 rather independant of the energy in the range from 20 MeV to 500 MeV. This is a tenth-valuethickness of 25 cm corresponding to a effective photon energy of 0.5 MeV. For the 2 MeV and 4 MeV case the attenuation is much stronger, factor 1000, 500, respectively. The correspondent tenth-value-thicknesses are 16.7 cm and 18.5 cm with effective energies of about 0.25 MeV. The measured effective energy of 0.7 MeV (see section 4.1, fig.2) indicates that at higher gradients the radiation field outside the cryo-module mainly originates from accelerated dark current rather than from one-cell dark current. 5 Neutron fluxes The neutron flux Φ around a thick target absorbing an electron beam can easily be estimated by a simple formula: Φ= 1 4r Y P ; (5) where r = 200 cm is the distance to the target, Y = 0:3/1GeV is the neutron yield per 1 GeV electron energy, P = 0:1 W/m is the standard power line source used. These values result in a neutron flux 1 Φ = 2300 (ψ cm 2 1 msv/h) : (6) s This calculation (5) is an overestimation for different reasons: Neutron selfabsorption in the target is not included. The neutron yield goes to zero as the electron energy decreases down to the threshold (ß 10 MeV). Roughly speaking, only electrons with energies well above 30 MeV contribute. 4

6 6 Summary An average heat load of 0.1 W/m due to dark current and beam losses leads to a dose rate level of about 10 mgy/h inside the tunnel. Most probably, the dose rate distribution due to dark current along the linac is strongly position dependent. Maxima are expected near the quadrupols and the worst cavities. The comparison of the measured effective photon energy with the calculations indicates that at higher gradients the radiation field outside the cryo-module mainly originates from accelerated dark current (energy above 20 MeV) rather than from one-cell dark current. So, it is desirable to adapt the design of the cryo-module such that accelerated dark current is suppessed. A conservative estimation of the neutron flux gives 2300 neutrons per cm 2 and s. Translating this flux into a dose equivalent one yields 1 msv/h. In terms of dose equivalent, it is a factor of 10 less than the photons, and in terms of energy dose a factor of 100. Measurements are still missing, but they are expected to give much smaller values. Measurements have been planned to study the impact of the radiation on computer performance. There are 2 aspects: The total dose limits the life time of the computer. And single event effects are caused by high peak dose rates. 5

7 References [1] A.Fasso, A.Ferrari, and P.R.Sala. Designing electron accelerator shielding with FLUKA. In 8th Intern. Conf. Radiation Shielding, Arlington, [2] R.J.Noer. Electron field emission from broad-area electrodes. Appl.Phys., 28:1 24, [3] Ch. Stolzenburg. Untersuchung zur Entstehung von Dunkelstrom in supraleitenden Beschleunigungsstrukturen. PhD thesis, University of Hamburg,

8 List of Figures 1 Measured dose distribution along the cyomodule no.1 under the tank (exposure 7 h, gradient 19.7 MV/m, flat top 800 μs, repetition rate 1 Hz). The dosemeters were positioned in parallel to the cavity midpoint on the tank surface. Upper curve: all cavities except no.3 on resonance. Lower curve: cavity no.2 on resonance, rest detuned. The cavities are counted in beam direction Measured dose attenuation in a set of lead cylinders. The curve is a fit of an exponential function on the data with it s parameter half-value-thickness d 1= Measured dose rates as a function of gradient (repetition rate 10 Hz). The set of curves shows the Fowler-Nordheim current for different parameters fi Measured dose rates as a function of gradient (repetition rate 10 Hz). The curves show nomalized current simulations from [3] Measured dose rate distribution along the cyomodule no.1 at a distance of 2 m (gradient 17 MV/m, flat top 800 μs, repetition rate 1 Hz) Radial dose rate distribution inside the tunnel due to an isotropic electron source of 2 MeV energy Radial dose rate distribution inside the tunnel due to an isotropic electron source of 4 MeV energy Radial dose rate distribution inside the tunnel due to an electron source in beam direction of 20 MeV energy Radial dose rate distribution inside the tunnel due to an electron source in beam direction of 100 MeV energy Radial dose rate distribution inside the tunnel due to an electron source in beam direction of 500 MeV energy

9 Figures beam direction 10 cryomodule #1 8 dose [mgy] distance [m] Figure 1: Measured dose distribution along the cyomodule no.1 under the tank (exposure 7 h, gradient 19.7 MV/m, flat top 800 μs, repetition rate 1 Hz). The dosemeters were positioned in parallel to the cavity midpoint on the tank surface. Upper curve: all cavities except no.3 on resonance. Lower curve: cavity no.2 on resonance, rest detuned. The cavities are counted in beam direction. 8

10 3 2.5 dose [mgy] d 1/2 = 8.2 mm E eff = 0.7 MeV wall thickness of the lead cylinders [mm] Figure 2: Measured dose attenuation in a set of lead cylinders. The curve is a fit of an exponential function on the data with it s parameter half-value-thickness d 1=2. dose rate 10 Gy/h 1 Gy/h 100 mgy/h 10 mgy/h 1 mgy/h 100 ugy/h limit for 0.1 W/m cryo-module no. 1 cryo-module no. 2 β = 210 β = 210 β = 180 β = 150 β = 120 β = 90 β = ugy/h gradient [MV/m] Figure 3: Measured dose rates as a function of gradient (repetition rate 10 Hz). The set of curves shows the Fowler-Nordheim current for different parameters fi. 9

11 10 Gy/h 1 Gy/h dose rate 100 mgy/h 10 mgy/h 1 mgy/h 100 ugy/h cryo-module no. 1 cryo-module no. 2 limit for 0.1 W/m 10 ugy/h gradient [MV/m] Figure 4: Measured dose rates as a function of gradient (repetition rate 10 Hz). The curves show nomalized current simulations from [3]. beam direction 1.4 cryomodule #1 dose rate [mgy/h] distance [m] Figure 5: Measured dose rate distribution along the cyomodule no.1 at a distance of 2 m (gradient 17 MV/m, flat top 800 μs, repetition rate 1 Hz). 10

12 100 Gy/h 10 Gy/h 1 Gy/h 2 MeV 100 mgy/h air dose rate 10 mgy/h 1 mgy/h 100 µgy/h 10 µgy/h concrete 1 µgy/h radius [cm] Figure 6: Radial dose rate distribution inside the tunnel due to an isotropic electron source of 2 MeV energy. 100 Gy/h dose rate 10 Gy/h 1 Gy/h 100 mgy/h 10 mgy/h 1 mgy/h 100 µgy/h 10 µgy/h air concrete 4 MeV 1 µgy/h radius [cm] Figure 7: Radial dose rate distribution inside the tunnel due to an isotropic electron source of 4 MeV energy. 11

13 dose rate 100 Gy/h 10 Gy/h 1 Gy/h 100 mgy/h 10 mgy/h 1 mgy/h 100 µgy/h 10 µgy/h air concrete 20 MeV 1 µgy/h radius [cm] Figure 8: Radial dose rate distribution inside the tunnel due to an electron source in beam direction of 20 MeV energy. 100 Gy/h 10 Gy/h 1 Gy/h 100 mgy/h air 100 MeV dose rate 10 mgy/h 1 mgy/h 100 µgy/h concrete 10 µgy/h 1 µgy/h radius [cm] Figure 9: Radial dose rate distribution inside the tunnel due to an electron source in beam direction of 100 MeV energy. 12

14 dose rate 100 Gy/h 10 Gy/h 1 Gy/h 100 mgy/h 10 mgy/h 1 mgy/h 100 µgy/h 10 µgy/h air concrete 500 MeV 1 µgy/h radius [cm] Figure 10: Radial dose rate distribution inside the tunnel due to an electron source in beam direction of 500 MeV energy. 13