Proposal for Experiment S291: " Residual radioactivity induced by U ions - experimental investigation and longtime predictions" GSI, Darmstadt: G.Fehrenbacher, Radiation Protection & Safety Group, G.Moritz, Magnets and Beam Transport Group, E.Mustafin, High-Current Beam Physics Group, D.Schardt, Biophysics group, P.Spiller, Accelerator Development Group, K.Weyrich, Plasma Physics Group, ITEP, Moscow: A.Golubev, A.Fertman, STU Bratislava: M.Pavlovic.

The Proposal activity is based on the INTAS Project confirmed for funding in : GSI-INTAS call Project # with the following institutions involved: GSI Darmstadt, INR RAS Moscow, ITEP Moscow, STU Bratislava, VNIIEF Sarov.

Motivation From experience with the 1 GeV proton accelerators: to ensure „hands-on“ maintenance the beam loss must be kept below ~1 W/m. There is no recognized limit of allowed losses in the heavy ion accelerators. Proposed experiments must answer the question: is the „1 W/m – criteria“ applicable in the heavy ion case too, what kind of radioactive inventory must we expect in normal accelerator operation and what is the long time behavior of the residual activation.

SIS100 operation with the U beam The intensity of up to ions of U +28 with energy of E=1 GeV/u is foreseen in the SIS100 synchrotron (~40 kW total power). The „ 1 W/m – criteria“ for the U beam in SIS100 would result in the allowed loss level below ~3% distributed uniformly along the ring, which is very tough and requires elaborated system of collimators in the ring (to distribute the „hot spots“ safely) overloaded by various equipment. But if the activation scales with the neutron yield then one may expect the tolerable heavy ion losses to be higher than 1 W/m: U ion with E=1 GeV/u produces about 458 neutrons/projectile (SHIELD code calculations), which is 5 time less compared to the neutron yield from the equivalent 238 protons with E=1 GeV (8.4 neutrons/proton).

Present time radiation problems In May 2004 the power supply box for the extraction system was destroyed and operators discovered highly activated dust inside the device. The reason was beam particle losses on the collimator: it happened after the slow- extraction run with N ions of intensity about /spill. There are more such high-intensity runs foreseen for the next year. Special shielding is necessary to ensure safe operation.

Experimental setup of the measurements of induced radioactivity after irradiation with heavy ion beams

Air Al 100  m Secondary electron monitor Ionization chamber 600 mm Target “front“ surface (foil 0) Vacuum Ar Beam 50 mm Experimental setup already used for Ar ions irradiating copper targets Cu target set of disks mm: Energy of Ar ions and SRIM ranges Target thickness Total dose on the target (ions) 1,7, 1, 7, 1, 5, 1, 1, 1, 5, 1, 5, 1, 5, 1, 3, 1 500MeV/u 23.6 mm 47 mm4.8* , 7, 1, 7, 1, 7, 1, 7, 1, 7, 1, 5, 1, 1, 1, 7, 1, 7, 1, 7, 1, 5, 1, 5, 1, 5, 1, 5, 1 800MeV/u mm 97 mm1.2* , 3, 1, 3, 1, 1, 1, 1, 3, 1, 5, 1 300MeV/u mm 22 mm3*10 11 The one-millimeter foils were inserted after each thick disk, which allowed to obtained the spatial distribution of the activation rates in the copper target along the range. Besides that, the radionuclide spectra measurements were carried out for all irradiated foils and discs. Based on these experimental data the predictions were made of the induced radioactivity for continuous irradiation of the accelerator equipment during normal operation.

Spectra of the Cu sample irradiated by 12 C beam (E 0 = 200 MeV/u) up to the dose of 9.88*10 11 ions on the target. The target thickness was 20 mm and the ion range was 14.61mm The produced radioisotopes can be identified and quantified by the energies and by the area under the peak determined in the measurements of their characteristic  -rays.

Spatial distribution of activation rates of different nuclides in the copper target after 40 Ar E 0 =800MeV/u irradiation. 7 Be (53.29d), 46 Sc (83.79d), 51 Cr ( d), 54 Mn (312.11d), 56 Co(77.233d), 57 Co (271.79d), 58 Co (70.86d)

Limits for beam intensity in Cave A Beam-time requirements are determined by the limitations imposed to the neutron flux by the safety detector in Cave A. From experience: tolerable average intensity for 800 MeV/u Ar beam was about ≈2.2∙10 7 /spill The SHIELD code calculations: 800 MeV/u Ar produces neutrons in the forward 3 0 cone and 1 GeV/u U produces 1.22 neutrons in the same cone. Thus the tolerable maximum intensity in U beam pulse must roughly be one order of magnitude lower ≈ 3∙10 6 From the experience: in order to have prominent peaks in the gamma-spectra the samples must be irradiated with total doses about ≈ ions.

Measurement and SHIELD simulation: total number of the residual nuclei in the 97 mm length, 25 mm radius Copper target irradiated by Ar & U ions. proj resid Argon, 800 MeV/u SHIELD calculation Argon, 800 MeV/u experimental results Uranium, 1000 MeV/u SHIELD calculation 7 Be (1.7) 46 Sc (1.1) 51 Cr (4.0) 54 Mn (1.1) 52 Mn (11) 56 Co (10) 57 Co (3.0) 58 Co (1.8)

Beam-time requirement for irradiation of two samples (stainless steel and copper) at U beam energies of E = 100 (injection into SIS100), 500 (intermediate) and 1000 (extraction from SIS100) MeV/u. RunIonEnergy, MeV/u Intensity, particles per pulse Duration, shifts U100~ U500~ U1000~ 3∙ HHD