PEARL technical information
24 Oct 2022
- Christopher Ridley



The PEARL diffractometer is a medium resolution high-flux diffractometer


​Detector layout on PEARL diffractometer, showing two key scattering geometries used with the Paris-Edinburgh press. Further details in technical summary table (below).​​​​​


Technical Summary

​ISIS, TS1, be​amline S9

Poisoned​ liquid CH4 at 100K

Beam geometry

Evacuated ​primary flight path, L1= 12.75 m. Sample to detector distance, L2= 0.8 m. 

PEARL is fitted with a comprehensive set of primary beam-scrapers, tapering the beam down to a 20 mm diameter profile. Three sets of adjustable jaws are then used to shape the beam into a square or rectangular profile, minimum dimensions 0.2x0.2 mm, with maximum beam divergence (approximately 0.5o vertical and horizontal). PEARL is not fitted with any choppers, instead making full use of the available beam over the shorter wavelengths.

Scattering geometries


Illustrated in page image (above)​

‘Transverse’ mode

Incident beam directed through the back of the anvil for access to the main detector bank covering 81.2° < 2𝚯 < 98.8°

‘Longitudinal’ mode

Incident beam directed through the gap between the two anvils (through the gasket) for access to the low-angle (20° < 2𝚯 < 60°) and high-angle banks (100° < 2𝚯 < 160°).


PEARL is by default calibrated against NIST standards CeO2 (SRM 674a), and Si (SRM 640c). A 10 mm diameter V sphere is used to correct for detector efficienies, and the moderator output profile. Alternative standards are sometimes used to calibrate the low-angle detector modules.


Each detector module on PEARL consists of 78 ZnS scintillator elements, with 3mm width (viewed in 2𝚯). The scintillator elements are fibre-optically coupled to 15 photo-multiplier tubes using a modified 2CN15 coding scheme.

PEARL has a total of fourteen detector modules. Nine modules constitute the ‘transverse’ bank, with three modules forming the back-scattering, and two modules forming the forward-scattering arms of the `longitudinal' bank. 

Information on the d-spacing range and resolution for each of these banks is summarised below. The d-spacing range quoted assumes the standard 20 ms frame on TS1. When measured with a 40 ms frame instead, where counting is extended during the 1 out of 5 pulses that goes to TS2, the d-spacing range can be approximately doubled on each bank, with the same resolution, though with considerably longer data collection times. 

Module #
Nominal 2𝚯​
d-spacing range
​Resolution, 𝛅​​d/d
0.31 - 4.10 ​Å
0.39 %​
​0.27 - 3.60 Å
0.34 %​
​0.25 - 3.24 Å
0.31 %​
​0.23 - 3.03 Å
​0.28 %
0.95 - 12.10 ​Å
​3.86 %
0.55 - 7.37 ​Å
​1.50 %