ARCS instrument Components

• Overview
• Neutron Guide
• T₀ chopper
• Fermi Choppers
  • Slit Package Description
• Slits
• Attenuators
• Oscillating Radial Collimator
• Sample Environment
• Vacuum Chambers
• Detectors
• Beam Monitors

Overview

The ARCS instrument is a wide angle Fermi chopper spectrometer. As it selects the incident energy and uses time of flight to measure the final energy it is known as a direct geometry spectrometer, one of four at the SNS which are compared in Review of Scientific instruments 85, 045113 (2014) . An overview of the instrument is provided below. Followed by a table giving the main distances of the instrument. Then their is a brief discussion for the instrument components. A more complete description of the instrument is provided in Review of Scientific Instruments 83, 015114 (2012).

Neutron Guide

The neutron guide reflects lower energy neutrons providing higher flux, at the expense of beam divergence, at the sample position. It starts in the Shutter and is elliptically shaped to focus the beam to the sample position. The last section of guide is inside the sample chamber and is removable for when larger pieces of sample environment equipment are used.

T₀ chopper

The purpose of the T₀ chopper, which is located 8.77 m from the moderator, is to suppress the prompt pulse of fast neutrons produced when the proton beam strikes the target. This suppression is accomplished by having an ~0.20-m-thick piece of the alloy inconel in the beam when the proton pulse hits the target. This piece of inconel must be out of the beam in sufficient time for the 0.01 - 1 eV neutrons to pass. It only turns counter clockwise and can operate at rotational speeds between 30 Hz and 180 Hz in multiples of 30 Hz. The following table provides guidance on the choice of the T₀ chopper base on your choice of incident energy. Note for energies larger than 1eV, the T₀ chopper is not fully closed when the prompt pulse hits the target. Thus the background is somewhat elevated for these incident energies. Please consult with instrument staff about the appropriate T₀ chopper frequency for your experiment.

Fermi Choppers

The Fermi chopper is the primary energy selection device on ARCS. It consists of a series of closely spaced neutron-absorbing blades (slit package) held together by a rotor that spins about a vertical axis in the path of the beam. The slit package is curved to be optimized for a specific E_i at a specific frequency. All slit packages are 100 mm in length. Each Fermi Chopper can be spun from 0 to 600 Hz in increments of 60 Hz. Two Fermi complete choppers are mounted on a translation table so that either can be chosen independently. The are installed on ARCS 11.61 m from the moderator. The table below provides the parameters of the Fermi choppers. Only one Fermi chopper may be spinning at full speed at a time. When decreasing the rotation speed of the chopper, use increments of 60 Hz down to 360 Hz. Otherwise the chopper will fault and needs to be reset by the instrument staff or Hall coordinators. Your local contact can assist you in choosing the appropriate chopper and rotational speed for your experiment.

Slit Package Description

Slits

There are two sets of beam defining slits on ARCS. The most upstream is after the Fermi chopper but before the Monitor. The second set of slits is located in one of two positions. When smaller sample environment equipment is used, it is located 20 cm upstream of the sample position. If larger sample environment is used it is located just inside the wall of the sample vessel.

Each set of slits has four independently operating blades. Each blade will fully cover the beam at its full extent of travel. Each blade is made of 99% isotopically enriched hot pressed ¹⁰B₄C blades (74 – 77% B, 23 – 26% C). The slits are typically controlled by a set of virtual motors that provide a center and gap of the beam in the horizontal and vertical directions. Typically only the second set of slits is adjusted to limit the beam on the sample.

The default setting for slit 1 is for them to be centered at 0 mm in both directions. The vertical gap is 60 mm and the horizontal gap is 56 mm.

For slit 2 each blade has a full travel of 50 mm. 25mm is completely out of the beam and -25mm is completely in the beam.

Attenuators

Two attenuator plates, located between the Fermi chopper and the upstream slits, may be independently placed in the incident beam. Each plate is 1 mm thick and is 2% borated aluminum.

Oscillating Radial Collimator

Inside the sample chamber there is an oscillating radial collimator on a vertical translation table. For detectors with a scattering angle above ~30^o The radial collimator filters out scattering that is from components that are beyond ~20mm from the center of the sample The collimator blades are spaced at 1.6^o and there is a 12.8^o gap around the straight through beam. This gap reduces potential scattering from the blades in a region where the collimator is ineffectual. The oscillation is 1.6^o in a period of 21s. Details of the Collimator are provided in: (M. B. Stone et al. Rev. Sci. Instrum. 85, 085101 (2014) and M. B. Stone et al. EPJ Conf. 83, 03014) For sample environments that contain their own vacuum and/or several heat shields, this can significantly reduce the background. However for experiments using very low background sample environment equipment, this is not necessary and should be lowered out of the beam. For the largest Sample environment the Radial Collimator can not be used.

The radial collimator is most efficient near 90^o scattering and does nothing at the lowest scattering angles. Therefore it works better for studies that do not analyze in the low Q region. Discuss with the instrument staff if the radial collimator is appropriate for your specific experiment.

Sample Environment

Details of sample environment mounting are provided on the sample environment website

More instrument specific sample environment information is provided here

Vacuum Chambers

The sample environment is positioned in a vacuum chamber that is contiguous with a detector vacuum chamber for the purpose of minimizing extraneous material between the sample and detectors.The chambers are operated below 1x10^-5 Torr. Both chambers are lined with B₄C neutron absorber such the the detectors only see the sample space or the absorber. The chambers have additional shielding on the outside to minimize the time independent background. A gate valve isolates the two chambers during sample changes to minimize the pumping time during such a change.

Detectors

The detector array on ARCS is an assembly of 1.0 m long by 25 mm diameter Linear Position Sensitive Detectors (LPSDs). The array has 920 detectors grouped into packs of 8 and located on a vertical cylinder with a radius of ~ 3.0 m (which is the distance between the sample position and the detectors in the horizontal plane). The detectors cover an angular range of the cylinder from -28^o to 135^o in the horizontal.

To avoid cross talk between detectors, absorbing baffles are placed around the tank. Nevertheless since ARCS has such a large angular coverage, all tube to tube scattering can not be eliminated. One should be wary of features that appear near 75% of E_i this can be examined by looking at the instrument view in Mantid to see if extra counts are observed in this energy transfer range in the lowest and highest angle detectors.

The LPSDs are filled with ³He at a pressure of 10 Atmospheres (1.0 MPa). To reduce background, there are no windows between the sample and the detectors; the detectors are located inside a detector chamber that is evacuated to a high vacuum (below 10^-6 atm) and is contiguous with the sample vessel. An incoming neutron is converted through the nuclear reaction n⁰ + ³He → ³H + ¹H + 0.764 MeV into charged particles tritium (T or ³H) and protium (p or ¹H) which then are detected by creating a charge cloud in the stopping surrounding gas. The electrons from the ionized gas are collected at an anode wire running down the center of the detector tube. This wire is at 1870 V above ground and has a high resistance so the proportion of charge seen at each end allows one to determine the position of the neutron detection event. The length of the detectors is divided into 128 pixels of ~1. mm length by the electronics. Each pixel subtends an angle of ~0.485° perpendicular to the length of the tube and ~0.125° along the length of the tube. The pixels are often binned together during reduction. Discuss the current binning with your Local Contact. Each pixel has a timing resolution of 1 µs and saturates at no less than 70,000 n/s. After saturation a tube is ready for measurement within 10 µs.

There are two labeling conventions for the detector banks the following table maps the bank number and the bank name.

Beam Monitors

There are two neutron beam monitors located along the direct beam, one located just downstream of the Fermi chopper and variable aperture and a second located farther downstream of the sample near the beamstop. These two monitors are primarily used to determine the speed of the incident neutrons. To detect the neutrons, the beam monitors use a low pressure of ³He