Measurement of the n_tof beam profile in the second experimental area (EAR) using a silicon detector Fidan Suljik Supervisors: Dr. Massimo Barbagallo & Dr. Federica Mingrone September 8, 7 Abstract A new beam line and a second experimental area (EAR) have been recently built at the neutron Time- Of-Flight (n_tof) facility at CERN. The characterization of the neutron beam in terms of spatial profile is a prerequisite for high accuracy cross-sections measurements. A silicon strip detector equipped with a neutron converter has been used to determine the beam profile as a function of incident neutron energy, in particular neutron beam profile has been measured from thermal energy up to ev. Preliminary results have been compared with those collected with a MicroMegas detector also installed during the measurement. Introduction High accuracy measurements of cross sections for neutron-induced reactions are performed at n_tof, the neutron Time-Of-Flight facility at CERN [], producing important data for several fields such as nuclear astrophysics, nuclear technology, fundamental physics and medical physics. To this end, GeV protons are accelerated at the CERN s Proton Synchrotron (PS), with typically 7 protons per pulse, and impinge on a lead target. As a result of the spallation process taking place and the subsequent moderation, a white spectrum of neutrons is produced []. Two neutron beam lines lead the neutrons to the corresponding experimental areas, the first one (EAR) located in the horizontal direction at 8 m from the target and the newly built second experimental area (EAR) reached through a shorter vertical flight path of about 9 m. The aim of this work is to determine the beam profile in EAR using a silicon strip detector. In particular it is important to determine the profile as function of incident neutron energy. neutron beam profile the detector is coupled to a LiF converter layer, deposited onto a carbon fibre plate ( mm thick), which can be seen in Figure. The converting reaction proceeds as follows: Li + n H (.7 MeV) + α (. MeV) () As depicted in Figure b, after a neutron is captured an alpha and a triton are emitted and eventually detected in the silicon strips according to the efficiency of the system. (a) (b) SiMonD detector The neutron beam profile monitor SiMonD is a x + y strip double sided silicon detector having x cm total active area and µm thickness, while each strip is mm wide []. In order to measure Figure : View of the strips of the front layer ( - ) of SiMonD and scheme of the interaction of the incoming neutron with the converter layer and detection of the produced particles. For the measurement, the neutron beam is coming from the bottom with the front layer of the detector facing directly the beam.
Laboratory test In order to get hands on the detector and the corresponding electronics, a laboratory test was conducted and the energy resolution was investigated. Figure : Scheme of the setup for the laboratory test. An Am- alpha source of 9.9 kbq was placed in a vacuum chamber, facing the SiMonD detector (see scheme of the setup in Figure ). The signals from the detector are transported through two vacuumtight feedthrough connectors to the preamplifier and subsequently to the amplifier and the multichannel analyzer (MCA). The spectrum presenting the alpha peak for each strip was collected. As an example the spectrum recorded for strip is reported in Figure. counts [a.u.] 9 8 7 Strip 8 8 Figure : Alpha source spectrum for one the strips during the laboratory test. A preliminary energy resolution was extracted for each strip and the data acquired were later on compared with values extracted during the in-beam measurement. Measurement and data analysis The measurement for the beam profile was performed in two configurations and using two different detec- Figure : Setup of the measurement (Config. B): upper position for the SiMonD detector with the neutron beam coming from the bottom. tors, SiMonD and an ancillary XY-MicroMegas, in order to be able to compare independent results. Moreover, the beam profile was measured at two different positions, where normally samples are placed during physics run at n_tof. In the first configuration (Config. A), SiMonD is placed in the lower position with respect to the Micromegas detector, facing directly the neutron beam coming from the bottom. The positions of the detectors are inverted in the second configuration shown in Figure (Config. B). In the picture, it is possible to see the vacuum chamber and the preamplifiers used for the silicon detector, as well as its actuator to insert and to remove the detector from the beam axis. After the completion of the measurement, the data analysis was performed. First, the amplitude spectrum has been investigated for all the strips, as it is presented for strip in Figure. Both alpha and triton peaks are well separated from the signals induced by background and electronic noise, nevertheless in the data analysis only the triton peak was used as a signature of the occurrence of neutron capture in the LiF sample. The stability of the detector response was checked in
counts [a.u.] Strip Figure : Amplitude spectrum for strip : each run is drawn in a different color in order to check the stability of the response of the detector. Figure 7: Gaussian fit of the triton peak for one strip of the front layer for the determination of the energy resolution. 8 8 Strip : Mean of gaussian fit run number Figure : Stability of strip. As expected, the gain has a decreasing trend but the variation is less than %. order to investigate possible degradation of the detector performances during the measurement. Indeed, since the detector is directly placed in the beam, the radiation damage in the silicon crystal lattice could affect the accuracy of the results. Figure presents the evolution of the position of the triton peak with respect to the run number for a selected strip, showing only a % change. The position of the peak is defined as being the mean of its gaussian fit. The effect of such a small change is completely negligible in terms of results provided by the measurement. The energy resolution of the strips of the detector was also determined by fitting the triton peak with a gaussian. Figure 7 shows a selected strip of the detector. Resolution (%) 8 Front Back 8 strip number Figure 8: Energy resolution for the strips of the front and back layers, which is better for central strips. FWHM E, The energy resolution defined as R = where E is the center of the peak, is reported in Figure 8 for all the strips, and it is between % and % in general. It is possible to see that the energy resolution is better for the middle strips and the results are consistent with what expected. It is important to note that each strip may have different levels of radiation damage. In order to extract the beam profile, coincidence events between strips on the front side and in the back side were taken in consideration. Prior to that, a suitable threshold in amplitude must be applied to each strip to select the triton peak, allowing the reconstruction of the X-Y neutron distribution. To this purpose, a proper time-of-flight to energy calibration had to be applied. If L [m] is the effective path and t [µs] the flight time, the non-relativistic neutron en-
Cut for tritons (strip ) Results The profile for configuration A (SiMonD in lower position) and for different energy decades, obtained by selecting corresponding intervals of time of flight, can be seen in Figure. neutron energy [ev] Figure 9: Energy deposited in strip as a function of neutron energy. In red is reported the threshold condition used for the selection of the triton peak. Reproduction of the Li(n,α)t cross section Li(n,α)t cross section data (normalized to neutron flux) 7 neutron energy (ev) Figure : Cross section of the reaction Li(n, α)t and normalized data. The curves are superimposed in the energy range from to ev. ergy E [ev] is given by []: ( ) 7, 9 L E =, () t the relativistic expression (or relativistic corrections) should be used above a few kev. The energy deposited (amplitude) as a function of the neutron energy could then be determined and the proper cuts selected, as shown in Figure 9 for strip. In order to determine the neutron energy range where the response of the detector is reliable, it has been checked how the reference Li(n, α)t cross section matches the data collected. As can be seen in Figure, it resulted that the detector could provide accurate data up to ev. - - - ev - ev 8 - - ev - ev Figure : Profile for different energy windows for config. A. The detector is not reliable anymore after ev. One observes that the profile is similar for energy decades from to ev, but it is not well reconstructed after ev, the detector being not reliable anymore beyond this energy. The profile integrated over all the energy range obtained with SiMonD is shown in Figures a and b for configurations A and B respectively. Investigating the sigma of the beam and the mean position in both directions, one observes that from the lower to the upper position the sigma of the beam increases, as expected since the beam is less focused at that position. Moreover, one can see that the shape changes slightly to a more turned and sharp ellipse, visible for example in the bottom right part of the shape. Indeed, in both configurations, the sigma of the beam is different in the two directions (the Y direction is less gaussian than the X direction), meaning that it is more elliptical rather than circular. The comparison of these first results with the ones obtained with the XY-MicroMegas detector shows that the results are compatible, since the shape and 8
- Beam profile: < En < (ev) 8 (a) SiMonD : lower position (Config. A) 8 - Beam profile: < En < (ev) 8 (b) SiMonD : upper position (Config. B) 8 I would like to express my gratitude also to Dr. M. Diakaki for her analysis using the MicroMegas detector and all the resulting discussions about the comparison of the preliminary results. Finally, I wish to thank all n_tof members for their warm welcome and for making this project possible. References 7 (c) XYMGAS : lower position (d) XYMGAS : upper position Figure : Beam profile obtained with SiMonD and XYMGAS (Courtesy of M. Diakaki) for both configurations. the behavior of the sigma of the beam are similar, increasing comparably from lower to upper position. Figures c and d present the profile integrated over the energy range for the two configurations with the XY-MicroMegas. The beam profiles obtained by the detectors for a given configuration are in agreement. [] F. Gunsing et al., Nuclear data activities at the n_tof facility at CERN, The European Physical Journal Plus () : 7 [] M. Barbagallo et al., High-accuracy determination of the neutron flux at n_tof, The European Physical Journal A () 9: [] L. Consentino et al., Silicon detectors for monitoring neutron beams in n-tof beamlines, Review of Scientific Instruments 8, 79 () [] G. Lorusso et al., Time-energy relation of the n_tof neutron beam: energy standards revisited, Nuclear Instruments and Methods in Physics Research A () - Conclusions The characterization of the n_tof beam in EAR in terms of spatial profile was performed using the silicon strip detector SiMonD, in an energy range from to ev. The extracted profile was compared with results from another detector (XY-MicroMegas) used for the measurement during the same experiment. The first comparison shows consistency of the results obtained. 7 Acknowledgements I would like to sincerely thank Dr. M. Barbagallo and Dr. F. Mingrone for their availability, help and valuable suggestions throughout this work.