Properties, Sources, and Detection of Radiation

6.4.4 Line and Area Detectors

A position sensitive detector (PSD) employs the principle of a gas proportional counter, with an added capability to detect the location of a photon absorption event. Hence, unlike the conventional gas proportional counter, the PSD is a line detector that can measure the intensity of the diffracted beam in multiple (usually thousands) points simultaneously. As a result, a powder diffraction experiment becomes much faster, while its quality generally remains nearly identical to that obtained using a standard gas proportional counter. 25

The basic principle of sensing the position of the photon absorption event by the PSD is based on the following property of the proportional counter. The elec- trons (born by the X-ray photon absorption and creation of Xe ion – electron pairs) accelerate along a minimum resistance (i.e., linear) path toward the wire anode, where they are discharged, thus producing the electrical current pulse in the anode circuit. In the point detector, the amplitude of this pulse is measured on one end of the wire. Given the high speed of modern electronics, it is possible to measure the same signal on both ends of the wire anode. Thus, the time difference between the two measurements of the same discharge pulse is used to determine the place where the discharge occurred, provided the length of the wire anode is known, as illustrated schematically in Fig. 6.11.

The spatial resolution of the PSDs is not as high as that attainable with the precise positioning of point detectors. Nevertheless, it remains satisfactory (approaching about 0 .01 ◦ ) to conduct good-quality experiments. Yet, a minor loss of the resolu- tion is a small price to pay for the ability to collect powder diffraction data in a wide range of Bragg angles, simultaneously, which obviously and substantially decreases

24 This particular X-ray tube was in service for more than a year and the total number of hours of operation was approaching 2000. The tube was regularly operated at ∼75% of rated power.

25 A significant deterioration of the quality of X-ray powder diffraction data may occur when the studied specimen is highly fluorescent because it is impractical to monochromatize the diffracted

beam when using line or area detectors. Also see Sect. 11.3.2.

6.4 Detection of X-Rays 129 Counting electronics

Counting electronics

Window t 2 Window t 1 Xe+CO 2 or CH 4 Xe+CO 2 or CH 4

Anode

Anode

Window Window

X-rays X-rays Fig. 6.11 The schematic comparing the conventional gas proportional detector, where the signal is

collected on one side of the wire anode (left) and the position sensitive detector, where the signals are measured on both sides of the wire anode (right). The position of the electron discharge (dark

dot ) is determined by the counting electronics from the difference between times t 1 and t 2 it takes for the two signals to be recorded.

the duration of the experiment. A typical improvement is from many hours when using a point detector to several minutes or less when using a position-sensitive detector.

Different models of the PSD’s may have different geometry, resolution and Bragg angle range: short linear PSD’s cover a few degrees range (from ∼5 to 10 ◦ ), while long curved PSD’s may cover as much as ∼120 ◦ –140 ◦ 2 θ. The biggest advantage of the long range PSD’s is the considerable experimental time reduction when com- pared to short or medium range position sensitive detectors. Their disadvantage arises from often substantial differences in the photon counting properties observed at different places along the detector, for example in the middle vs. the ends of its length. The large angular spread of long detectors also puts some restrictions on the quality of focusing of X-rays and usually results in the deterioration of the shape of Bragg peaks. Relevant discussion about the geometry of powder diffractometers equipped with PSDs is found in Sect. 11.3.2.

Comparable to PSDs, which fundamentally are gas proportional counters with an added functionality of detecting the location of photon absorption events, the real time multiple strip detector (RTMS) , employs the principle used in a standard solid state detector (see Sect. 6.4.3). Strip detectors are typically manufactured by using a photolithographic process, during which narrow p-doped strips are deposited on an n-type Si wafer. The simplified schematic of a strip detector is shown in Fig. 6.12. The electric potential difference applied to each strip across the wafer results in the photon-induced electric current, which is amplified and measured. Since both the holes and electrons created as a result of each photon absorption event travel along the path of least resistance, the current measured individually for each strip pro- vides information about the position at which the X-ray photon was absorbed by the detector. The current remains proportional to the number of the generated electron– hole pairs, thus providing information about photon flux at a specific location of the detector.

Area detectors record diffraction pattern in two dimensions simultaneously. Not

130 6 Properties, Sources, and Detection of Radiation

p-type strips

X-rays

+ hole

n-type Si

− electron

Fig. 6.12 The schematic of a strip detector.

advanced to a commercial status, and are being frequently used in modern X-ray powder diffraction analysis.

In a charge-coupled device detector, X-ray photons are converted by a phos- phor 26 into visible light, which is captured using a charge-coupled device (CCD). The latter is a chip similar to (or even the same as) those used in modern digital cam- eras. In order to reliably measure a large area, in some detector designs the phosphor may be made several times larger than the chip, and then the generated visible light is demagnified to the size of the chip by using fiber optics, while in other designs several chips (e.g., a 2 × 2 or 3 × 3 chip arrays) are glued together. Similar to solid- state detectors, CCD chips are cooled with thermoelectric cooling device to reduce random (thermal) noise.

In an image plate detector (IPD) X-ray photons are also captured by a phos- phor. 27 The excited phosphor pixels, however, are not converted into the signals immediately. Instead, the information is stored in the phosphor grains as a latent image, in a way, similar to the activation of silver halide particles in the photo- graphic film during exposure. When the data collection is completed, the image is scanned (or “developed”) by a laser, which deactivates pixels that emit the stored energy as a blue light. Visible light photons are then registered by a photomultiplier in a conventional manner, and the plate is reactivated by another laser. Image plates are integrating detectors with high counting rates and dynamic range but they have relatively long readout times.

26 A typical CCD phosphor is Tb 3 + doped Gd 2 O 2 S, which converts X-ray photons into visible light photons.

27 A typical image plate phosphor is Eu 2 + doped BaFBr. When exposed to X-rays, Eu 2 + oxidizes to Eu 3 + . Thus produced electrons may either recombine with Eu 3 + or they become trapped by

F-vacancies in the crystal lattice of BaFBr. The trapped electrons may exist in this metastable state for a long time. They are released when exposed to a visible light and emit blue photons

during recombination with Eu 3 + ions, e.g., see K. Takahashi, K. Khoda, J. Miyahara, Y. Kanemitsu, K. Amitani, and S. Shionoya, Mechanism of photostimulated luminescence in BaFX:Eu 2 + (X = Cl, Br) phosphors, J. Luminesc. 31–32, 266 (1984).

6.6 Problems 131 Table 6.4 Qualitative comparison of the most common area detectors.

MWD Active area size

Small Readout time

Small

Large

Short Counting rate

Medium

Long

Low Dynamic range

High

High

Medium Spatial resolution

Another type of area detector that finds more use in powder diffraction than other area detectors is a multi-wire detector (MWD) which uses the same principle as gas proportional counters. The multi-wire detector has two anodes made of multi- wire grids which allow detection of the X and Y positions at which the photons are absorbed in addition to the total number of the absorbed photons. Table 6.4 compares three types of area detectors discussed here.

6.5 Additional Reading

1. International Tables for Crystallography, vol. A, Fifth revised edition, Theo Hahn, Ed. (2002); vol. B, Third edition, U. Shmueli, Ed. (2008); vol. C, Third Edition, E. Prince, Ed. (2004). All volumes are published jointly with the International Union of Crystallography (IUCr) by Springer. Complete set of the International Tables for Crystallography, Vol. A-G, H. Fuess, T. Hahn, H. Wondratschek, U. M¨uller, U. Shmueli, E. Prince, A. Authier, V. Kopsk´y, D.B. Litvin, M.G. Rossmann, E. Arnold, S. Hall, and B. McMahon, Eds., is available online as eReference at http://www.springeronline.com.

2. R.B. Neder and Th. Proffen, Teaching diffraction with the aid of computer simulations, J. Appl. Cryst. 29, 727 (1996); also see Th. Proffen and R.B. Neder. Interactive tutorial about diffraction on the Web at http://www.lks.physik.uni-erlangen.de/diffraction/.

3. P.A. Heiney, High resolution X-ray diffraction. Physics department and laboratory for research on the structure of matter. University of Pennsylvania. http://dept.physics. upenn.edu/ ∼heiney/talks/hires/hires.html.

4. Electron diffraction techniques. Vol. 1, 2. J. Cowley, Ed., Oxford University Press. Oxford (1992). 5. R. Jenkins and R.L. Snyder, Introduction to X-ray powder diffractometry. Wiley, New York (1996). 6. J. Als-Nielsen and D. McMorrow, Elements of modern X-ray physics, Wiley, New York (2001).

6.6 Problems

1. A typical energy of electrons in a modern transition electron microscope is 300 keV. Calculate the corresponding wavelength of the electron beam assuming that the vacuum inside the microscope is ideal.

2. Calculate the energy (in keV) of the characteristic Cr K α 1 and Mo K α 1 radiation.

132 6 Properties, Sources, and Detection of Radiation

3. You are in charge of buying a new powder diffractometer for your company.

The company is in business of manufacturing alumina (Al 2 O 3 ) based ceramics. The powder diffractometer is to become a workhorse instrument in the quality con- trol department. Routine experiments will include collecting powder diffraction data from ceramic samples to analyze their structure and phase composition. High data collection speeds are critical because a typical daily number of samples to be ana- lyzed using the new equipment is 100+. The following options are available from different vendors:

Sealed Cu X-ray tube, scintillation detector; the lowest cost. Sealed Cu X-ray tube, solid state detector; $10,000 more than the first option. Sealed Cu X-ray tube, curved position sensitive detector; $25,000 more than the

first option. What recommendation will you make to you boss without a fear of being fired during the first month after the delivery of the instrument?