\section{Neutral Particle Spectrometer}

The neutral-particle spectrometer offers unique scientific capabilities to study the transverse spatial and momentum 
structure of the nucleon in the Jefferson Lab experimental Hall C. Much progress in imaging nucleon structure can be 
made with electron-scattering reactions, yet experiments with high-energy photons play a unique complementary role. 
The small scattering probabilities of exclusive reactions demand high-intensity photon beams. Understanding strengthened 
by imaging longitudinally-polarized and transversely-polarized nucleons. Five experiments have been currently approved 
using the NPS. The experiments will use a high-intensity beam of electrons, with a typical lumonisity 
of $\sim 10^{38}\;{\rm cm^{-2} s^{-1}}$ as well as a secondary beam of photons incident on a liquid hydrogen target.


Similar to the FCAL insert in experimental Hall D, the NPS will be built from the same type of crystals, and instrumented 
with the same PMTs and readout electronics. The NPS consists of 1080 $PbWO_{\rm 4}$ crystals, which form an array of 
30x36 modules. The detector is positioned on a movable platforms in a temperature controlled frame designed by INPN Orsay. 
In different experiments, the detector will be placed at different angles between 5.5 and 60 degrees with respect to the 
Hall C beamline. A sweep magnet will be installed in front of the spectrometer in order to reduce background of charged 
particle tracks originating from the target. The integrated field of the magnet varies between 0.3 Tm and 0.6 Tm depending 
on the distance of the NPS detector from the beamline. The maximum fringe field originating from the magnet in the detector 
region is 200 Gauss.


$PbWO_{\rm 4}$ crystals produced by CRYTUR are expected to have slightly better radiation hardness compared to the SICCAS 
crystals~\cite{pwo_crystals}. The CRYTUR crystals will be installed in the region close to the beamline, where the radiation 
level is the highest, whereas the SICCAS crystals will be placed at the edges of the detector further away from the beam. Each 
crystal will be wrapped with the reflective ESR foil. The crystal's support structure consists of horizontal and vertical layers 
of 0.5 mm thick carbon plates. Each crystal is positioned inside a cell surrounded by the carbon plates. Hamamatsu R4125 PMT 
is situated inside a round $\mu$-metal housing with an out diameter of 19.6 mm  and is attached to the back side of each module.
In order to  provide additional magnetic field shielding, 0.5 mm thick $\mu$-metal plates are used around the PMT housing. 
The initial magnetic field is reduced by surrounding the NPS frame, except the face and the back of the detector, by a 10 mm 
thick soft iron shield box.

The design of the PMT base will be adjusted according to run conditions of experiments with the NPS. The gain of the amplifier 
will be determined according to rates expected in the detector, which are currently sumulated using Geant detector simulation. 
Signal pulses from the PMT are digitized  using flash ADCs hosted in VXS crates. High voltage for PMTs is supplied by a 36-channel 
CAEN 7030N module installed in a  SY4527 crate. Energy deposition  in the calorimeter, will be used in the trigger system of 
the experiments. Integration of the detector to the trigger will be performed by means of the trigger electronics modules designed 
at Jefferson Lab. A blue LED is distributed to each calorimeter module through a quartz optical fiber, glued to the crystal from 
the PMT side. The LED system will be used for calibration and allow to cure crystals whose performance is degraded due to radiation.  
The cluster reconstruction software of the NPS is currently under development. It will adopt the already existing clustering 
algorithm of the DVCS experiment in experimental Hall A [reference needed].  
