The New Uppsala Neutron Beam Facility - Extras Springer

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Abstract. A new quasi-monoenergetic neutron beam facility has been constructed at the The Svedberg ... of nuclear waste [1], fast-neutron cancer therapy [2-5],.
The New Uppsala Neutron Beam Facility S. Pomp1, A. V. Prokofiev2, J. Blomgren1, O. Byström2, C. Ekström2, N. Haag2, A. Hildebrand1, C. Johansson1, O. Jonsson2, P. Mermod1, L. Nilsson1,2, D. Reistad2, N. Olsson1,3, P.-U. Renberg2, M. Österlund1, U. Tippawan1,4, D. Wessman2, and V. Ziemann2 1

Department of Neutron Research, Uppsala University, Sweden 2 The Svedberg Laboratory, Uppsala University, Sweden 3 Swedish Defence Research Agency (FOI), Stockholm, Sweden 4 Fast Neutron Research Facility, Chiang Mai University, Thailand Abstract. A new quasi-monoenergetic neutron beam facility has been constructed at the The Svedberg Laboratory (TSL) in Uppsala, Sweden. Key features include an energy range of 20 to 175 MeV, high fluxes, and the possibility of large-area fields. Besides cross-section measurements, the new facility has been designed specifically to provide optimal conditions for testing of single-event effects in electronics and for dosimetry development. First results of the beam characterization measurements performed in early 2004 are reported.

INTRODUCTION Over the past years, a growing interest in nuclear applications, such as accelerator-driven transmutation of nuclear waste [1], fast-neutron cancer therapy [2-5], measurement of dose delivery to personnel in aviation [2,6], as well as electronic failures due to neutrons produced by cosmic rays [7,8] has put new emphasis on the need for a detailed understanding of nuclear interactions involving neutrons at intermediate energies (20-200 MeV). To satisfy these needs, a new quasi-monoenergetic neutron-beam facility has been constructed at the The Svedberg Laboratory (TSL), Uppsala. Emphasis has been put on a high neutron beam intensity in combination with a high flexibility in energy and shape.

TECHNICAL SPECIFICATION The facility uses the 7Li(p,n)7Be reaction (Q= 1.64 MeV) to produce a quasi-monoenergetic neutron beam. The proton beam is provided by the Gustaf Werner cyclotron with an energy variable in the 20-180 MeV range. A drawing of the neutron-beam facility is shown in Fig. 1. The proton beam is incident

FIGURE 1. Drawing of the new neutron beam facility. The neutron beam is produced in the lithium target and continues along the D-line. The lithium target, the deflecting magnet, and the collimator are indicated. The drawing shows also the position for two permanent but movable experimental setups, Medley and SCANDAL.

CP769, International Conference on Nuclear Data for Science and Technology, edited by R. C. Haight, M. B. Chadwick, T. Kawano, and P. Talou © 2005 American Institute of Physics 0-7354-0254-X/05/$22.50

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on a target of lithium, enriched to 99.99% in 7Li. The available targets are 2, 4, 8, 16, and 24 mm thick. The targets are rectangular in shape, 20x32 mm2, and are mounted in a remotely controlled water-cooled copper ring. An additional target position contains a fluorescent screen viewed by a TV camera, which is used for beam alignment and focusing. Downstream from the target, the proton beam is deflected by a magnet into a 10-m-long dumping line, where it is guided onto a heavily shielded water-cooled graphite beam dump.

TABLE 1. Parasitic irradiation positions. Position Distance from Angle to the Li Target the n-Beam (m) Direction (°) PARTY 1.9 1.6 TUNIS 1.1 7.5 1)

Gain in the Neutron Peak Flux 2.5 1.7 – 2.21)

dependent on the peak neutron energy.

CHARACTERIZATION OF THE FACILITY

The neutron beam is formed geometrically by a cylindrically shaped iron collimator block, 50 cm in diameter and 100 cm long, with a cylindrical hole of variable diameter. The collimator is surrounded by concrete to form the end wall of the production line towards the experimental area. Thereby, efficient shielding from the production target region is achieved. A modular construction of the collimator allows the user to adjust the diameter of the neutron beam to the needs of a specific experiment. The available collimator openings are 2, 3, 5.4, 10, 15, 20, and 30 cm. Other collimator diameters in the 0-30 cm range, as well as other shapes than circular can be provided upon request. Beam diameters of up to 1 m are obtainable at a larger distance from the production target, which may be used for testing a larger number of devices simultaneously, or larger devices like entire electronic boards. The facility is capable to deliver neutrons in the 20-175 MeV range. This makes TSL the only laboratory in the world offering full monoenergetic neutron testing according to the JEDEC standard [8].

The first neutron beam at the new facility was delivered in January 2004. Since then, extensive commissioning runs of the facility have been performed, including optimization of beam transport, diagnostics, vacuum and background conditions, as well as measurements of neutron flux, spectra, and profile. First results are reported below. The measured contamination of the neutron beam at the experimental area due to interactions of the primary protons with beam transport elements such as the target frame did not exceed 0.2%. Such interactions only lead to a minor surplus of neutrons in the experimental area because charged particles produced near the Lithium target and upstream are removed by the deflection magnet. The relative contamination of protons with energies above 15 MeV in the neutron beam is about 10−5. These measurements have been performed for a proton beam energy of 98 MeV. The energy and angular distribution of neutrons delivered to the experimental area is mainly defined by the double-differential cross section of the 7Li(p,n) reaction at forward angles. The reaction energy spectrum is dominated by a peak situated a few MeV below the energy of the primary protons and comprising about 40% of the total number of neutrons. Neutron spectra have been obtained by measuring elastic np-scattering with the Medley setup [11]. The scattered protons are registered at an angle of 20° relative to the neutron beam. Besides the energy of the scattered proton, the time-of-flight (TOF) relative to the RF signal from the cyclotron for each event is recorded. As an example, the measured proton energy vs. neutron TOF is shown in Fig. 2. All proton events for a peak neutron energy of 74.8 MeV are contained. The horizontal and vertical straight lines indicate the position of the proton peak in time and energy for elastic scattering events caused by peak neutrons. The bent line shows the calculated position of elastic scattering events for different neutron energies. The neutron spectrum is deduced by application of a cut

Neutrons reach the experimental area at a distance of about 3 m from the production target. Reduction of this distance has led to an increase of the neutron flux by about one order of magnitude in comparison with the old TSL neutron facility [9,10], using the same target thickness, proton energy, and current. Beam currents of up to 10 µA can be achieved for energies below 100 MeV. Above 100 MeV, when the cyclotron operates in FM mode, the achievable beam current is about a factor of 12 lower. The resulting lower neutron fluence can be partly compensated for by the use of thicker lithium targets. Two additional irradiation positions, which can be used parasitically with other experiments, are provided (see Table 1). The increase of the neutron flux at these positions is reached at the expense of limited accessibility, limited size of irradiated objects, and more intense γ-ray background.

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induced fission of 238U. Finally, a Faraday cup, installed in the proton beam dump, integrates the beam current and offers relative monitoring of the beam intensity.

around this bent line, proper background subtraction, and calculation of the corresponding incoming neutron energy on an event-by-event basis. The measured neutron spectra for five incident proton energies between 24.7 and 147.4 MeV are shown in Fig. 3. The measurements are compared with the systematics by Prokofiev et al. [12] for the four higher energies (Fig. 3 b-e). The systematics is not applicable at the lowest beam energy (Fig. 3 a). Instead, an evaluation of Mashnik et al. [13] was employed for the description of the neutron spectrum. The differential cross section for high-energy peak neutron production at 0° was obtained by multiplication of the total cross section of the 7Li(p,n)7Be reaction [13] to the “index of forwardness” from the systematics of Uwamino et al. [14]. The narrow peaks in the upper continuum region correspond to excitation of higher states in residual 7 Be nuclei. This process was included in the model calculation of Mashnik et al. [13]. However, the energy resolution in the experiment does not allow us to observe these peaks.

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FIGURE 3. The neutron spectra at 0º for different peak neutron energies (see Table 2 for incident proton energies and 7Li target thicknesses). Symbols connected by a solid line represent experimental data obtained in the present work. Predictions are shown as dashed lines (see text). Note that no carbon background subtraction could be applied to the 77 MeV data (c) (see text).

FIGURE 2. Measured proton energy vs. time-of-flight (TOF) for a peak neutron energy of 74.8 MeV registered at a scattering angle of 20° (see text).

The experimental data agree with the calculations except for the region below 10 MeV in the 24.7-MeV spectrum where the model overpredicts the experimental results by up to a factor of 2, and the region just below the peak in the 77.4-MeV spectrum. The later is due to the fact that no carbon background subtraction could be applied to these data. This would reduce the difference between the experiment and the calculation in the 30- to 70-MeV region where elastic events caused by tail neutrons and protons from 12 C(n,xp) reactions caused by peak neutrons overlap (cp. Fig. 2). Table 2 summarizes the main features of the measured spectra and the achieved neutron fluence. The later has been measured with the thinfilm breakdown counter (TFBC) [15]. Another monitoring option is provided by an ionizationchamber monitor (ICM). Both monitors, usually installed after the Medley chamber, utilize neutron-

Figure 4 shows a horizontal beam profile for 143-MeV neutrons, measured at a distance of 4.77 m from the production target. The measurement was performed by counting neutron-induced single-event upsets in a set of electronic chips positioned across the beam [16]. Another measurement of the beam profile performed at 94.7 MeV is currently under analysis.

SUMMARY AND OUTLOOK A new neutron beam facility has been constructed at TSL and is now available for regular operation. It is capable to deliver neutrons in the 20-175 MeV range, which makes TSL the only laboratory in the world

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TABLE 2. First results for the neutron spectra and beam intensities. The fluences have been measured with the TFBC and refer to the entrance of the experimental hall. Higher fluences can be achieved by using thicker Li targets. Beam Li Target Resulting Average Fraction of Neutrons in the Proton Beam Peak Neutron Fluence Current Mono-Energetic Peak (%) Thickness Energy of Peak Energy (MeV) (105 neutrons/(cm2 s)) (mm) Neutrons (MeV) (µA) Measured Calculated 24.68 ± 0.04 2 10 21.8 ~50 -1.3 49.5 ± 0.2 4 9.5 46.5 39 36 2.8 40 2.4 77.4 ± 0.2 4 9.9 74.8 34 1) 97.9 ± 0.3 8 3.4 94.7 41 39 3.1 147.4 ± 0.6 24 0.26 142.7 55 2) 40 0.9 1)

lower limit since no carbon background runs are available for this energy; 2) upper limit due to poor energy resolution.

REFERENCES 1. Koning, A., et al., High and Intermediate energy Nuclear Data for Accelerator-driven Systems (HINDAS), J. Nucl. Sci. Technol. 2, 1161 (2002). 2. Blomgren, J., and Olsson, N., Radiat. Prot. Dosim., 103, 293 (2003). 3. Orecchia, R., et al., Eur. J. Cancer 34, 459 (1998). 4. Schwartz, D. L., et al., Int. J. Radiat. Oncol. Biol. Phys. 50, 449 (2001). 5. Laramore, G. E., and Griffin, T. W., Int. J. Radiat. Oncol. Biol. Phys. 32, 879 (1995). 6. ICRP, 1990 Recommendations of the International Commission on Radiological Protection: Publication 60, Annals of the ICRP, Vol. 21, No. 1-3, Oxford: Pergamon Press, 1991. 7. Single-Event Upsets in Microelectronics, topical issue, edited by H. H. K. Tang and N. Olsson [Mater. Res. Soc. Bull. 28 (2003)]. 8. JEDEC Standard. Measurements and Reporting of Alpha Particles and Terrestrial Cosmic Ray-Induced Soft Errors in Semiconductor Devices. JESD89, August 2001. 9. Condé, H., et al., Nucl. Instrum. Methods Phys. Res. A 292, 121 (1990). 10. Klug, J., et al., Nucl. Instrum. Methods Phys. Res. A 489, 282 (2002). 11. Dangtip, S., et al., Nucl. Instrum. Methods Phys. Res. A 452, 484 (2000). 12. Prokofiev, A. V., et al., J. Nucl. Sci. Techn., Suppl. 2, 112 (2002). 13. Mashnik, S. G., et al., LANL Report LA-UR-00-1067 (2000). 14. Uwamino, Y., et al., Nucl. Instrum. Methods Phys. Res. A 389, 463 (1997). 15. Smirnov, A. N., et al., Radiat. Meas. 25, 151 (1995). 16. M. Olmos, private communication.

FIGURE 4. The horizontal beam profile for 142.7-MeV neutrons, measured at the distance of 4.77 m from the production target. Vertical dashed lines represent boundaries of the beam expected from the geometry of the collimator.

offering full monoenergetic neutron testing according to the JEDEC standard [8]. First beams for commercial electronics testing, as well as for nuclear physics research, have been delivered. For spring 2005, it is planned to measure the flux of thermal neutrons in the experimental hall. As a response to the needs of SEE users, the possibility of delivering peak neutrons with lower energies (