The Low Energy Neutron Source at Indiana University first produced cold neutrons in April of 2005. Ten years after first reaching this milestone, the facility has three instruments in operation on its cold target station, and a second target station is devoted to thermal and fast neutron physics offers capabilities in radiation effects research (single-event effects in electronics) and radiography. Key elements in our success over these last ten years have been the diversity of activities we have been able maintain (which often involves using each of our instruments for multiple different activities), the close relationship we have developed with a number of major sources, and the focus we have had on innovation in neutron

instrumentation. In this presentation, we will introduce some of the highlights from our most recent activities, provide an update on some of our technical challenges, and describe some of our ideas for the future

Baxter, D. V.

Rinckel, T.

English

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DOI 10.1393/ncc/i2015-15183-1Colloquia: UCANS-VIL NUOVO CIMENTO 38 C (2015) 183Status report on the Low Energy Neutron Source for 2015D. V. Baxter and T. RinckelCEEM, Department of Physics, Indiana University - Bloomington, IN, 47405, USAreceived 22 February 2016Summary. — The Low Energy Neutron Source at Indiana University first producedcold neutrons in April of 2005. Ten years after first reaching this milestone, thefacility has three instruments in operation on its cold target station, and a secondtarget station is devoted to thermal and fast neutron physics offers capabilities inradiation effects research (single-event effects in electronics) and radiography. Keyelements in our success over these last ten years have been the diversity of activitieswe have been able maintain (which often involves using each of our instrumentsfor multiple different activities), the close relationship we have developed with anumber of major sources, and the focus we have had on innovation in neutroninstrumentation. In this presentation, we will introduce some of the highlights fromour most recent activities, provide an update on some of our technical challenges,and describe some of our ideas for the future.PACS 28.20.Gd – Neutron transport: Diffusion and moderation.PACS 61.05.fg – Neutron Scattering.1. – OverviewSince it first produced thermal neutrons late in 2004, LENS has provided a flagshipexample of a proton-beam-based CANS facility with capabilities in materials research,neutron education, and innovation in instrumentation [1-4]. In previous UCANS andICANS meetings we have outlined a number of the operational issues associated witha source of this design (in particular issues with RF systems, target cooling and targetblistering [3, 5]). We have now found solultions to a number of these issues, although insome cases the work around has not been completely satisfactory. In this paper we give anupdate on some of the issues of greatest general interest and highlight some of our moreimportant recent accomplishments. We would like to emphasize that a crucial element inthe success of LENS over the years has been the close ties that we have had to the majorfacilities within the USA (NCNR, ORNL and LANL). This has involved joint researchprojects, educational programs, and joint efforts on instrumentation development.Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 D. V. BAXTER AND T. RINCKEL2. – Operational issuesTarget lifetime has remained a vexing problem for us over the last two years. Afterconsiderable success with our thinner target design [3], where our targets were regularlylasting longer than the o-rings we used to provide the vacum seal around them (i.e.75–100 kWdays of integrated beam power), we have experienced cracking of targets aftershort periods of time (as short as several hours at 3 kW beam power). We have cometo the conclusion that this must represent some sort of transient issue with the beamoptics which leads to an anomalously intense portion of the beam spot on the target.We have not, however, been able to identify the origin of the anomaly. This experiencesuggests that it would be helpful to have more extensive diagnostics available for studyingthe proton beam near the target than was afforded by the LENS construction budget.Therefore, this should be considered to be an important lesson-learned for facilities thatare currently under construction: adequate diagnostics of the accelerated beam are veryuseful for long term health of the facility.In our most recent run cycle, we chose to operate at a beam power of only 600W (sincemost studies for this cycle involved moderator characterizations or neutron transmissionexperiments, for which this power is adequate) and we ran for almost 6 weeks without aproblem with the target. During this most recent run, we noticed that one of the klystronswas running with very little available overhead power, and have come to wonder if thismay have been a contributing factor, as it could lead to occasional transients where thebeam energy is lower than expected. This could, in turn, result in the beam passingat the wrong position through the octupoles which are designed to flatten the beamprofile, but which can produce hot spots if the beam is improperly tuned. Retuning ofthe klystron electromagnets has significantly increased the gain of that tube, so this mayprovide a solution to our target problem, but we will need more experience with this newklystron tune. During our next run cycle, we intend to go back to more standard powerlevels (2.5 to 4 kW) with this retuned klystron to confirm these suspiscions. We are alsoconsidering the use of an anodized aluminum target in the sample position to providea scintillation screen with which we can directly view the proton beam profile, albeit atthe cost of a significant reduction of neutron flux during the tests.3. – InstrumentationThe instrumentation suite at LENS has changed relatively little since we last providedan overview of our activities in 2013. We have added some additional monitoring to theproton beam and in-pile neutron field on the second target station. This includes a secondsystem for integrating proton charge in each pulse of the accelerator (similar to the one wehave previously been using for monitoring proton current during moderator neutronicsexperiments) and a low-efficiency N2 detector mounted in the shielding surrounding thetarget assemby. With these systems we can monitor the dose delivered on a pulse by pulsebasis and in the future we hope to combine this with in situ monitoring of tranisistorperformance as a means for studying neutron radiation effects in silicon on time scalesfrom several ms to several minutes. The physics relevant to radiation effects in electronicstakes place over time scales from several picoseconds all the way up to several monthsor more, and proton-accelerator based CANS facilities may be in a unique position toprovide insight in this intermediate range of time scales due to the intense neutron fieldsnear the target, their pulsed time structure and the relatively weak gamma fields nearthe target. We hope to capitalize on these features of our source over the next few years.STATUS REPORT ON THE LOW ENERGY NEUTRON SOURCE FOR 2015 30.0000010.000010.00010.0010.010.110.1 1 10 100I (C/(frame.bin))t (ms)Empty cellD2OCd-blockedFig. 1. – Count rates on the LENS SANS instrument in “Total cross section” mode with a3He detector. The background count rate in this mode, operating at 10Hz, is approximately3× 10−7 counts/frame per 100μs bin.Over the last couple of years we have also continued to demonstrate the flexibility ofour instrumentation suite. Our SANS instrument has been used in studies of complexfluids [6] and block-copolymers [7] as well as in support of our development of spinmanipulation devices (checking the transmission and scattering properties of materialsfor windows and Meissner screens) and components for various fundamental nuclearphysics experiments. Preliminary measurements on our instrument have also played akey role in securing beam time at major facilities to study nanotube composites and avariety of polymer composite systems. As we have reported previously, this instrumentcan also be reconfigured as a time-focussed spectrometer for measuring emission timedistributions as well as measurements of neutron total cross sections. In the formercase, we have recently demonstrated a roughly 3 order of magnitude dynamic range inmeasuring peak shapes [8]. In this mode, we run the accelerator with a 10μs pulse widthand 40Hz repetition rate, but we have been able to distinguish changes in moderatorimpulse responses as small as 2 to 3μs at neutron energies near 25 to 70meV. Wealso note that we have demonstrated the abilty to demount and remount the detectorand crystal for this mode of operation without having to adjust the detector position orcrystal orientation to optimize the time focused condition.As described in more detail in another paper in these proceedings [9], for the mea-surement of total cross sections, we have demonstrated the capability to measure σtotto energies below 0.1meV at variable temperature. In this mode of operation, we re-duce the accelerator frequency to 10Hz with a 150μs pulse width. This combinationof parameters extends the bandwidth to below 0.1meV while simultaneously allowingreasonable measurements above 1 eV for checking the normalization of the data againstknown free-gas results. In cases where greater precision is needed at either the high orlow energy extremes of this range, we could combine results with either longer pulsewidths or a higher frequency (to provide greater count rates at low energies or highenergies respectively). We perform these measurements with a 2.5 cm diameter 50 kPapresure 3He detector viewing the beam through a roughly 1.5 cm2 hole in a BN shield,a secondary flight path length of 57 cm, and an efficiency of roughly 9% at λ = 0.1 nm.This combination subtends a solid angle of only 0.46msR, limits dead time correctionsto less than a few percent near the peak in the spectrum, and yet provides an essentially4 D. V. BAXTER AND T. RINCKELblack detector at the longest wavelengths accessible to the apparatus (roughly 4 nm) [9].Background count rates with the facility running in this mode are roughly 0.2 c/minin this detector, and typical count rates, along with the Cd-blocked beam spectrum areshown in fig. 1. In this figure, the background rate lies roughly a factor of 3 below thelower limit of the vertical axis. Even at λ = 3.5 nm, the count rate in the direct beamis some 30 times this dark rate. This suggests to us that CANS facilities such as LENSare ideal places for conducting total cross-section measurements as part of a coherentprogram is evaluating materials effects in nuclear data over neutron energies from below0.1meV to 1 eV or higher.Perhaps the most significant activity in instrumentation at LENS over the last severalyears, however, has been our program developing in neutron spin-manipulation technol-ogy using superconducting components to produce and shape the magnetic fields per-forming the manipulation [10]. The devices we have demonstrated include a cryoflipperfor performing spin flips on large area (up to 18 cm2 or more) white beams [11], a de-vice for performing spherical neutron polarimetry in transition mode (such as SANSand radiography [10]), and Wollaston prisms [12], which can be used to encode neutrontrajectories or position into the neutron spin. These devices can be used to introducenovel contrast mechanisms into radiography and perform phonon focusing to increasethe resolution available in measuring the lifetime broadening of elementary excitationsin materials [13] among other applications. Over the next several years we will continueto develop these technologies and expand their applications.∗ ∗ ∗LENS was constructed with funding from the NSF (grants DMR-02200560 andDMR-0320627), the department of Defense, Indiana University, and the State ofIndiana. Its operations are supported by Indiana University. The authors also acknowl-edge partial support from the Department of Commerce under cooperative agreement70NANB10H255.REFERENCES[1] Baxter D. V. et al., Nucl. Instrum. Methods B, 241 (2005) 209.[2] Lavelle C. M. et al., Nucl. Instrum. Methods A, 587 (2008) 324.[3] Rinckel T. et al., Phys. Proc., 26 (2005) 161.[4] Baxter D. V. et al., Phys. Proc., 60 (2014) 175.[5] Rinckel T. et al., Phys. Proc., 26 (2012) 168.[6] Das N. et al., Langmuir, 28 (2012) 11962.[7] Yan C. et al., J. Polym. Sci. B, 51 (2012) 1705.[8] Baxter D. V. et al., Phys. Proc., 26 (2012) 153.[9] Ma´rquez Damian J. I. et al., these Proceedings, article 178.[10] Wang T. et al., J. Phys. Conf. Ser., 528 (2014) 012024.[11] Parnell S. R. et al., Nucl. Instrum. Methods A, 722 (2014) 20.[12] Li F. et al., Rev. Sci. Instrum., 85 (2014) 053303.[13] Li F. and Pynn R., J. Appl. Cryst., 47 (2014) 1849.

Status report on the Low Energy Neutron Source for 2015

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Status report on the Low Energy Neutron Source for 2015

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