Fermi chopper

Fig. 3.28 A Fermi chopper. The rotor with the slit package is on the left.

The resolution, AE/ i, of a chopper instrument largely depends on the width of the pulse from the moderator, Atnu and the opening time of the Fermi chopper, Atchop- The moderator term is fixed for any given moderator but the second term depends on the rotation speed of the chopper and the slit width. The resolution is given by... 

To obtain S(Q,a>) from the measured data it is necessary to know the instrumental flight paths, the energy of the incident neutrons and the angles of the detectors. The incident energy is found from the beam monitors. The first monitor is normally placed before the Fermi chopper to monitor the incident flux for the purposes of normalisation. The... 

The original Fermi chopper consisted of a cylinder 1.5 in in diameter with a maximum rotational speed of 15,000 r/min. The open channels consisted of aluminum sheets inch thick. 

On HET, a high resolution (in neutron terms) of 1-2% is achieved by using a fast 600 Hz Fermi chopper located 10 m from the source. Scattered neutrons are detected in two azimuthal arrays of He detectors, the first at 4 m from the sample covering 3° to T and the second at 2.5 m covering 9° to 29°. A third detector array at 136° has proved to be invaluable in determining (multi) phonon and multiple scattering contributions to the signal. 

Fermi chopper thermal neutron time-of-flight experiments using a thermal column. 

Three low-efficiency (because of the high neutron flux) detectors are placed in the main beam. The first is placed before the background choppers to monitor the incident flux for the purposes of normalization. The second and third are placed just after the Fermi chopper and behind the sample respectively. These are used to accurately determine the incident energy of the neutrons. 

Figure 30.2 shows a diagram of the Fermi chopper. The shutter is a hollow steel cylinder, inside of which is a "sandwich" of alternate cadmium and aluminum sheets. The aluminum sheets, due to their small neutron cross section, are the "slits" that allow the neutrons to pass through when the slits are within l- -° of the horizontal position (parallel to the neutron beam). Due to the angular speed (cd) of the shutter, the slits are "open" for the time it takes the shutter to rotate 3 . 

The initial values of the Fermi chopper parameters (o), ]3, and L) will be given by the instructor. The normal values of these parameters which will be used are ... 

The bursts for neutrons of different velocities produced by a rotating shutter of the Fermi chopper have a great variation of time dura-... 

Now, in the following considerations, with only a few additional approximations, the above Eqs. (6") will be shown to define the extreme velocity in a channel of the Fermi chopper which uses a rotating shutter. 

---