The INPOL (Indiana Neutron Polarimeter) time-of-flight detector is housed in a small garage-like building at a flight path length of meters. The polarimeter consists of four planes of scintillator oriented perpendicular to the incident neutron flux. A schematic view of the detector is shown in figure .
Figure: A schematic view of the INPOL neutron time-of-flight polarimeter. The charged particle veto paddles are not shown.
The first three planes (NA0, NA1, and NC0) are constructed out of thin-walled stainless steel tanks filled with a BC-517s organic liquid scintillator. Each of the tanks is subdivided into ten optically isolated cells with the dimensions . The fourth plane (NA1) consists of 10 bars of BC-408 solid scintillator, each with the same dimensions as the cells of the first three planes, and stacked to create a fourth plane similar to the first three. The first two planes, as a group, are the polarimeter's `analyzer' and the second two are the `catcher'. The liquid scintillator in the first three planes was used because it has a high hydrogen to carbon ratio (H:C = 1.7), and the elastic scattering of the neutrons off the hydrogen, , in the scintillator is the most efficient analyzing reaction in the scintillator.
There are also two planes of thin scintillator paddles one located in front of NA0 (CA0) and the other in front of NC0 (CC0). They are shown in figure .
Figure: A side view of the INPOL Neutron Polarimeter.
These paddles serve to identify, and veto, charged particles. The first charged particle plane, taken from the upstream side, (CA0) will veto any incoming particle which is non-neutral. This keeps charged particles created at the target or in the flight path from contaminating the data. The second charged particle plane (CC0) tags those events which create a charged particle in the analyzing reaction. As mentioned above neutron elastic scattering, , is the preferred reaction. The charge-exchange analog, , has a very small analyzing power at forward scattering angles for the energies used in this experiment. This means that that reaction does a very poor job of determining the polarization of the incident neutron, and, therefore, CC0 is used to veto those events from the data.
Each of the cells in the neutron detection planes is viewed at each end with an Amperex XP2262 photomultiplier tube coupled to the cell through a lucite light guide. The timing of a ``hit'' (meaning an interaction of the neutron with the scintillator) is determined by a mean timing between the signals at each end of the cell. The neutron energy is determined by the time of flight to the first two planes with respect to a corrected RF stop signal which is derived from the cyclotron RF signal. Discrete transitions of known Q value are used to give an absolute scale. With a meter flight path the typical energy resolution at MeV is keV.
The position of a `hit' in a cell is determined by the timing difference between the signals at each end. Using this technique it is possible to get a position resolution of cm along the long axis of a cell. Therefore, overall position of a hit in the detector is determined by knowing which cell was hit and the timing of the signals from the ends of that cell. Once an elastically scattered neutron from the `analyzer' plane is detected in the `catcher' plane the position information from both planes can be used to determine the polar scattering angle, , and the azimuthal scattering angle, (see figure ). The angular distribution can then be used to determine the polarization of the incident neutrons. This procedure will be discussed in more detail in Chapter .
Aside from the neutron-neutron events, which the detector is meant to measure, cosmic ray events are also collected. The cosmic events are generally high-energy muons created by the interaction of cosmic rays with the upper atmosphere. These events are used to calibrate the detector, and are saved in the data stream if they fall into one of two categories: COSMIC or PP. The COSMIC events are those which traverse and are detected in all 10 detector cells in one of the four planes as shown in figure .
Figure: Illustration of a cosmic event in the NA0 plane (third plane). A hit in all 10 cells is required to define a COSMIC event.
The track created by the ten hits is fit to a straight line and used to calibrate the position offsets for those cells. The PP event, so-called because it fires both charged particle veto paddles, is one which has one hit in each of the six different planes: CA0, NA0, NA1, CC0, NC0, and NC1. These are generally also caused by cosmic rays and, given the assumption that the cosmic rays are moving at approximately the speed of light, are used to calibrate the distance between the different planes. More details on these calibration techniques will be given in Chapter .
For more information on the general design considerations of this type of neutron time-of-flight polarimeter see [Tad85]. For information on this detector's design and performance while in use at LAMPF see [Cis89], [Pro91], [Mer93], and [Che93]. For information on its implementation at IUCF see [Pal95].