This note briefly discusses the radiation hazard which may be posed by the neutrino radiation generated in the decay of an intense 50GeV muon beam circulating in the storage ring of a future Neutrino Factory. Of the various options which are being considered for the decay ring, for the present estimate the triangle-shaped one, with three arms of equal length, was taken into account. The neutrinos emerging from the ground along a direction which is the prolongation of the arm pointing towards the surface may represent a local radiation hazard. Assuming 10 21 muons per year, the annual dose equivalent will exceed 15m mSv

Silari, Marco

Vincke, H H

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EDMS ID: 3337601EUROPEAN ORGANISATION FOR NUCLEAR RESEARCHEUROPEAN LABORATORY FOR PARTICLE PHYSICSTECHNICAL NOTETIS-RP/TN/2002-01CERN NUFACT Note 105Neutrino radiation hazard at the planned CERN neutrino factoryMarco Silari and Helmut VinckeAbstractThis note briefly discusses the radiation hazard which may be posed by the neutrinoradiation generated in the decay of an intense 50 GeV muon beam circulating in thestorage ring of a future Neutrino Factory. Of the various options which are beingconsidered for the decay ring, for the present estimate the triangle-shaped one, withthree arms of equal length, was taken into account. The neutrinos emerging from theground along a direction which is the prolongation of the arm pointing towards thesurface may represent a local radiation hazard. Assuming 1021 muons per year, theannual dose equivalent will exceed 15 mSv.CERN, Geneva, Switzerland9 January 2002CERN-OPEN-2002-04609/01/2002EDMS ID: 33376021. IntroductionThe present design of a future Neutrino Factory to be possibly built at CERN after theLHC era includes a proton driver producing a 4 MW beam of 2.2 GeV protons, a pionproduction target, a system to capture and focus the pions, a muon focussing andcooling system, two muon accelerators to raise the muon energy to 50 GeV and a muondecay ring (see, for example, ref. [1]). In the latter the muons circulate at constantenergy and their decay originates an intense neutrino fluence. These neutrinos areaimed at one or more far-located detectors, at typical distances of a few hundreds to afew thousands of kilometres.The radiation protection issues of such a high-intensity proton accelerator complex are(at least) comparable to those posed by state-of-the-art spallation neutron sources.Before such a facility can be built a throughout study will have to be conducted todefine shielding requirements, assess induced radioactivity in accelerators, targets andauxiliary equipment, evaluate the risk of ground water activation and the environmentalimpacts due to prompt radiation streaming through the access and ventilation shafts aswell as to releases of radioactive air and fluids. Also, the amount of radioactive wasteto be handled at the time of the facility’s decommissioning will have to be assessed. AtCERN these aspects have been addressed until now only in a very preliminary way andlimited to the proton driver [2-4].In addition to the above “conventional” radiation issues a more exotic problem may beposed by the neutrino beam itself. This aspect was first investigated for high-energymuon-muon colliders [5-7], where it was shown that above about 1.5 TeV per beam theneutrino-induced secondary radiation emerging from the ground even at very largedistances (tens of kilometres) from the machine would pose a serious hazard (bothradiological and political). In spite of the fact that in a Neutrino Factory the muonenergy is much lower than in a muon-muon collider (50 GeV in the CERN referencescheme), a potential radiation hazard from the neutrino beam may still be presentbecause of the intensity of the circulating muon beam. The present CERN design of aneutrino factory foresees the production of 1021 neutrinos per year. Of the variousoptions which are being evaluated for the decay ring, for the purpose of the presentestimate we shall here consider the triangle-shaped one, with three arms of equallength. The machine will be located underground and tilted in such a way that two armsof the triangle (at slopes of 22% and 5.6%, respectively) point towards two far-locatedneutrino detectors, whilst the third arm, needed to close the storage ring, points towardsthe surface at a slope of 23.3%. The minimum depth at which the ring will be located isabout 20 m (at the starting point of the first “detector arm”, which coincides with theend point of the “return arm”). These construction parameters provide a minimumshielding of earth of 100 m for muons lost in the storage ring. Under this circumstanceall muons are stopped before reaching the surface. Since the return arm has the samelength as the two detector arms, the same number of neutrinos (approximately one thirdof the total) are produced in this section of the ring. These neutrinos, emerging from theground along a direction which is the prolongation of the arm, may represent a localradiation hazard, as discussed in this note.EDMS ID: 33376032. Neutrino induced radiationThe attenuation length (λ) of a neutrino of energy Eν in a material of density ρ isapproximately given by [6]:λ= 0.5·106 km (1 TeV/Eν)·(3 g cm–3/ρ)  (1)Neutrinos have a very small interaction cross-section. Interactions of muon neutrinos(νµ) can be reduced to two categories: Charged Current, when they weakly interactthrough the exchange of a W-boson to form muons, and Neutral Current, when theyproduce uncharged particles through the weak exchange of Z-particles. A hadronicshower accompanies both reaction types. The νµ induced radiation hazard thus comesfrom two sources: secondary muons and secondary hadronic shower products. Thelatter have a much shorter range than the muons produced in the neutrino interactions,but on the other hand the number of particles produced in the cascade is very large.We shall follow here the same simple model used for assessing the radiation hazard ofthe neutrino radiation from a high-energy muon-muon collider [6], using the fluence todose equivalent conversion coefficients of Cossairt et al. [8]. These data show that theradiological hazard is much larger (up to three orders of magnitude at TeV energies,two orders of magnitude at 50 GeV and still almost an order of magnitude at 1 GeV) ifthe neutrino beam is “shielded” rather than left “unshielded”. This is because of thesecondary radiation produced in the shielding material (in practice, earth, if theaccelerator is installed underground). As stated above, the secondaries with the longestrange are the muons, the maximum energy of which cannot obviously exceed theenergy of the muons circulating in the decay ring. The machine must be shielded andthe shield must be thick enough to absorb the muon beam circulating in the ring in caseof a full beam loss. It follows that the shield must be thicker than the maximum rangeof all secondaries, i.e. the neutrino radiation emerging from the shield willautomatically be in equilibrium with its secondary radiation. It has been shown that,down to a few GeV, the data of Cossairt et al. do not differ substantially from those ofMokhov calculated with the MARS Monte Carlo code [5].The increasing energy and increasing number of the secondaries is the main reason fora rising fluence to dose equivalent conversion coefficient with increasing neutrinoenergy for shielded neutrino radiation. Another reason is the energy dependence of theneutrino attenuation length, which decreases with increasing energy of the primaryneutrinos (see expression 1 above).3. Assessment of the produced neutrino radiationIn the decay ring of the Neutrino Factory the circulating muons will produce 1021neutrinos per year. The energy distribution (or differential fluence) of these neutrinos inthe laboratory system, averaged over all production angles, is given by [6]:Φ⋅−⋅= )1(2)(00 EEEEdENd ννν  (2)EDMS ID: 3337604where N(Eν) is the number of neutrinos per cm2, Eν is the neutrino energy, E0 is theenergy of the primary muons and Φ is the integral neutrino fluence (Nν cm-2, where Nνis the total number of neutrinos).Here we shall just consider the case in which the neutrinos have traversed a shieldsufficiently thick to have reached secondary particle equilibrium, i.e. the “shielded”case. In the neutrino energy range from 0.5 GeV to 10 TeV, the fluence to doseequivalent conversion coefficients as given by Cossairt et al. [8] can be fitted by theexpression [6]:C(Eν) = 10-15 Eν2  (3)in which C(Eν) is the fluence to dose equivalent conversion coefficient (µSv·cm2) andEν is the energy of the neutrinos (GeV).To calculate the dose equivalent (H) induced by an energy dependent neutrino radiationspectrum, one has to sum up the radiation effects over the relevant energy range:                            Φ⋅⋅⋅=⋅= −∫ 201501061)()(0EEdECEdENdHEνννν (4)where H is expressed in µSv, E0 in GeV and Φ in Nν cm-2.The neutrino fluence (Φ) is the number of neutrinos crossing a given surface behind theshielding. This surface is determined by the divergence of the neutrino beam and by thedistance r from the neutrino source. The divergence of the neutrino beam (the openinghalf-angle) induced by the decay (expressed in radians) is the inverse of the relativisticfactor:θ = 1/γ = 1/(10·E0) (5)in which θ is the opening half-angle in radiant of the decay cone, γ is the relativisticfactor and E0 is the energy of the primary muon in GeV.Hence, the neutrino fluence at a given distance r from the muon decay point is given bythe expression:Φ = Nν/((θ·r)²·π) (6)With expression 4, 5 and 6 the neutrino-induced dose equivalent as a function of theprimary muon energy and of the distance to the muon decay point can be estimated as:H = 5.3·10-15 · E04· Nν /r² (7)in which H is the dose equivalent in µSv, E0 is the primary muon energy in GeV and Nνis the number of produced neutrinos.EDMS ID: 3337605Expression 7 provides the dose equivalent if all muons decay at the same point. Sincethe muons actually decay all around the storage ring with the same probability, the doseequivalent has to be calculated as an integral over the length of the return arm of thedecay ring:))(11(103.51103.5 401524015 lddlNErdrlNEHldd +−⋅⋅⋅⋅=⋅⋅⋅=−+− ∫ νν (8)in which H is the dose equivalent in µSv, E0 is the primary muon energy in GeV, Nν isthe number of produced neutrinos, l is the length of the return arm of the storage ring(cm) and d is the distance (cm) from the end of the return arm to the surface, along thedirection of the arm.The radius of the produced neutrino beam at surface level is 140 cm, thus generating awhole body exposure. In order to calculate the expected annual dose equivalent at thepoint where the radiation emerges from the ground, the parameters listed in Table 1have been chosen:Table 1. Basic parameters assumed for the estimate of the annual dose equivalent fromneutrino radiation.Parameter ValueEnergy of the muons in the storage ring, E0 50 GeVLength of the return arm pointing towards the surface, l 60000 cmThickness of material traversed by the neutrino beam between the end ofthe return arm and the surface, d (approximate minimum thickness of earthneeded to absorb the 50 GeV circulating muons in case of total beam loss)10000 cmNumber of neutrinos produced in the return arm of the decay ring, Nν 1021/3 per yearUsing the above figures, equation 8 gives a dose equivalent of 16 mSv per year. Thishas to be compared with an annual CERN limit of 15 mSv for occupational exposedworkers and of 1 mSv for workers who are not under individual dosimetric control. Thevalue of ambient dose equivalent caused by ionising radiation emitted by CERNbeyond the boundaries of its site must not exceed 1.5 mSv per year [8]. Theradiological impact on the environment of a Neutrino Factory built at or nearby CERNwill have to comply with these limits.4. ConclusionsThis note has shortly addressed the radiological hazard which may be posed by theneutrino radiation from a future CERN Neutrino Factory. The magnitude of such anhazard is strongly dependent on the energy and intensity of the muon beam circulatingin the decay ring, on various machine parameters as well as on the shape and siting ofthe accelerator. Here only the energy and intensity of the muon beam was taken intoconsideration, along with a preliminary information on the shape and tilting of the ringand on the depth at which it will be installed. The value of annual dose equivalent of16 mSv here calculated for an integral number of 1021 muons per year decaying in theEDMS ID: 3337606storage ring will scale linearly with the muon intensity. Although limited to a relativelysmall area, the radiation hazard is such that, if the decay ring is not sited on CERNterritory, the area where the radiation emerges from the ground must be within theCERN perimeter.References[1] R. Garoby. Current activities for a neutrino factory at CERN. HEACC’2001, 26-30 March 2001, Tsukuba (Japan). Report CERN/PS 2001-007 (RF) andNUFACT Note 074 (2001).[2] S. Agosteo and M. Silari. Preliminary shielding calculations for a 2 GeV super-conducting proton linac. Technical Memorandum TIS-RP/TM/2001-028 andCERN NUFACT Note 088 (2001).[3] L. Bruno, M. Silari and H. Vincke. A preliminary design of a dump for 2.2 GeV,4 MW proton beam, to be published.[4] M. Silari and H. Vincke. Preliminary shielding estimate for the SPL Front End, tobe published.[5] N.V. Mokhov and J.D. Cossairt. Radiation studies at Fermilab. Proceedings ofthe Fourth Workshop on Simulating Accelerator Radiation Environments(SARE4), Knoxville (Tennessee, USA), 14-16 September 1998, Ed. T. Gabriel,Oak Ridge National Laboratory (1999), p. 291-302.[6] B. Autin, A. Blondel and J. Ellis, eds. Prospective study of muon storage rings atCERN. CERN 99-02 (1999), pp. 105-113 and C. Johnson, G. Rolandi and M.Silari. Radiological hazard due to neutrinos from a muon collider. Internal ReportTIS-RP/IR/98-34 (1998).[7] N.V. Mokhov, S. Striganov and A. Van Ginneken. Muons and neutrinos at high-energy accelerators. Proceedings of Monte Carlo 2000 – Advanced Monte Carlofor Radiation Physics, Particle Transport Simulation and Applications, Lisbon(Portugal), 23-26 October 2000, Eds. A. Kling, F. Barão, M. Nakagawa, L.Távora and P. Vaz, Springer-Verlag (2001), p. 985-990.[8] J.D. Cossairt, N.L. Grossman and E.T. Marshall. Assessment of dose equivalentdue to neutrinos. Health Phys. 73 (1997), p. 894-898.[9] CERN Radiation Safety Manual, Safety Code F (1996).

Neutrino radiation hazard at the planned CERN neutrino factory

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