Aperture slits

A typical streak camera image tube is shown in Fig. 17(a) (Hadland Photonics Ltd., Imacon). Briefly, its method of operation is as follows. The light signal strikes the photocathode and is converted with the associated focusing electrode beam. Until the camera is triggered, this beam is deflected onto the aperture slit by a constant potential on the shutter plates. When triggered, the voltage on the shutter plates drops to... 

The design of the camera corresponds to a pinhole eamera with aperture slits after each optical element and a guard slit front of the detector. A further slit in front of the monochromator defines the beam. The resolution is discussed in 3.3. 

The small angle scattering resolution is determined by the closest approach to the primary beam (Fig. 27). This is hmited by the aperture slits which produce a diffuse halo. The guard slits are only used to limit the diffuse scattering without cutting into the primary beam. For a given size of the aperture slits — — the size of the... 

Fig. 3. Schematic representation of the SAXS/WAXS Polymer Beamline A2 at HASYLAB/Ham-burg SI, S2 slits, Mo crystal monochromator, Mi quartz mirror, S3 aperture slit, S4 guard slit and sample position, D detector...

In this formula Lt is the distance aperture slit — guard slit, S, the width of the aperture slit, L2 the distance guard slit — detection plane, and S2 is the width of the guard slit which can be calculated from... 

The simplest solution of the first problem is to place an ionization chamber somewhere between aperture slit and sample. Another possibility is to place there a thin Kapton foil, inclined 45° to the primary beam. The small portion of intensity, that is scattered by the foil, can then be registered by a scintillation counter. If a second foil is mounted close behind the sample, then the absorption of X-rays in the sample can be measured additionally by comparing the intensities registered at the two foils. This design has been realized at some SSRL beamlines in Stanford. At the polymer beamline at the HASYLAB it became evident, that fluctuations of the primary beam position can lead to fluctuations of the intensity at the sample, which cannot be registered by a monitor placed in front of the sample. This observation is obviously caused by restrictions of the beam directly at the sample holders. 

Figure 2 shows schematically the focussing geometry and the origin of diffuse scattering around the focus in the detector plane, due to scattering from the aperture slits (St) and the optical elements. 

The largest observable d-spacing of such a camera depends on the limits of diffuse scattering in the detector plane. This limit — L — can be approximately calculated from the size of the aperture slits (St), the distance of the detector plane from the guard slits (Lj) and the size of the focus (a) [4], In real space one can write ... 

The size of the aperture slits in the horizontal (S ) and vertical (S ) directions is calculated from the photon beam source point size — x, z (ct) —, divergence — x, z (a) and F1 which is the distance source point to aperture slits ... 

The extent of the diffuse scattering in the detector plane is therefore dominated by the size of the aperture slits (S ) and the length of the camera after the optical elements (F2). 

A decision between an undulator and a wiggler for SAXS experiments has to be based on the optimum flux at the sample for a given SAXS resolution and the necessity to tune the wavelength for anomalous dispersion studies. The beamprofile should furthermore be rather symmetric at the aperture slits in order to get a comparable SAXS-resolution both in the horizontal and vertical directions. 

Evidently these type of sources result in a quite symmetric beamprofile at the aperture slits. The high P-undulator is, however, preferable if one wants to have the smallest possible aperture slits. 

Abbr. U undulator M mirror Mo monochromator D-Mo double monochromator Mu multilayers A aperture slits G guard slits S sample D detector. The length scale is determined by the insertion of the camera into the experimental hall, The different length of the mirror/ monochromator camera is due to the horizontal deflection by 20 as 27° (X 1.5 A)... 

Figure 4-57 Suppression of scattered light due to the sUt aperture (slit effect).

---