Nucleon Polarimeter and PID3

This section details the new Phase II Nucleon Polarimeter that is in use during the August 2016 beamtime. There are two major new components :

  • Nucleon Polarimeter
    A schematic overview of the components inside the crystal ball

In addition to this there have been major changes to the hydrogen target, a short description of the changes to the target cover are included here. The Geant simulation of the polarimeter and new PID design can be found on GitHub. This version of the simulation includes additional PID options and also various polarimeter options. It will be updated frequently. Also added in is the ability to displace various components (e.g. target/MWPC/PID/Polarimeter) along the Z axis. This version will soon be updated with the ability to handle polarised events.

Motivation: Neutron Polarimetry

The Phase-II polarimeter will allow the simultaneous measurement of both the proton and neutron polarisation. This will allow for the measurement of polarisation observables in channels such as

  1. D(γ,p)n
  2. n(γ,n)π0
  3. p(γ,p)π0
  4. n(γ,n)η
  5. 2π production channels


To fit in as much analysing material as possible the new PID has been reduced in size. Still has 24 elements and readout is via the same Hamamatsu H3164-10 PMTs with the same

The new PID III

EJ-204 scintillator as PID-II, precision cut with angled edges to form a barrel. The scintillation light from the strips is read by heat moulded perspex lightguides to a photomultiplier tubes, held in a bespoke 3D printed support crown. Each PID element is individually wrapped in aluminised mylar to reduce light loss along the length of the element.

Neutron Polarimeter

Process used in determining the Polarization of the neutron, the new GRaphite polarimeter. The neutron polarisation can be measured with the Phase-II polarimeter via charge exchange (n,p) interactions with the carbon analysing material

Measuring the spin polarization of the final state of the nucleus following photodisintegration uses a secondary scattering reaction on a carbon nucleus. The nucleon-nucleon interation leads to a modulation of the angular distribution of the scattering process, according to the spin polarization of the initial nucleons. To measure this modulation a lrage sample of scatter events with close to full angular acceptance is necessary. This is achieved with the Edinburgh nucloen spin polarimeter which we constructed and installed inside the Crystal Ball detector at MAMI. The main components are a graphite scattering chamber, particle identification detector and a charged particle tracking system. The scattering material for the nucleons is a thick cylinder of high purity graphite. Inside the cylinder is the new PID III. The graphite scatterer and the PID are placed inside the MWPC cylinder tracking chamber. Charged particles traversing the chamber ionise the gas mixture in the detector volume and produce ionisation.

The new detector setup was successfully installed (more pictures on the installation can be found here ) in July 2016 and took data in August-September 2016.