Lead Iodide Single Crystals Research Articles

Improved performance lead iodide nuclear radiation detectors

Melt-grown lead iodide single crystals with an improved transport of positive charge carriers (holes) have been developed. It is shown that an energy resolution of about 3% can be obtained for gamma-radiation at 60 keV ( 241Am isotope source). Improvement of the spectrometric performance of lead iodide detectors at lower gamma-ray energies has been achieved as well due to better purification of the starting material and optimization of the etching procedure. Correlation between the detector performance and the structure of lead iodide crystals has been studied using X-ray diffraction analysis.

Growth and Polytype Transformation of AgI-Doped and Undoped PbI2 Crystals

Both undoped and AgI-doped single crystals of lead iodide have been grown by the vapour technique and characterized by X-ray diffraction (XRD) to study their polytype formation. All undoped crystals are exclusively found to contain the stable high-temperature modification, polytype 12R, while nearly 2/3 of the doped crystals are found to consist additionally of the polytype 4H and the room-temperature modification 2H. The AgI doping is held to be responsible for the formation of 2Hat the prevalent high temperature of crystal growth. The crystals were subsequently stored for nearly seven months and re-examined through XRD to study possible polytype transformation in them. The undoped crystals did not transform at all but all the doped crystals transformed into the stable room-temperature polytype 2H. It is concluded that only those crystals that initially have an appreciable number of 2H nuclei present eventually transform into the stable 2H structure after prolonged storage.

Polytypism of vapor grown PbI2 crystals

Abstract Pure single crystals of lead iodide have been grown from vapor and characterised by X-ray diffraction for their polytypism and related phenomena. The theoretical estimation of the Jackson's α-factor and the surface micrographs of the crystals, coupled with the results of X-ray diffraction, indicate crystal growth by two-dimensional nucleation. Comparison of the experimental results with the earlier ones on gel-grown and melt-grown crystals reveals an obvious influence of the impurities on polytype formation, besides confirming the high purity of the vapor-grown crystals.

Characterization of PbI/sub 2/ radiation detectors using the response of /spl alpha/-rays

An improved method is proposed for the production of single crystals of lead iodide (PbI/sub 2/). For the work reported here a modified Bridgman method is used. Bulk-type nuclear radiation detectors have been fabricated by use of crystals grown by this method. The characteristics of the detectors, particularly the response for 5.48 MeV /spl alpha/-particles from /sup 241/Am source, have been measured and compared with characteristics of other detectors fabricated with crystals grown by the ordinary Bridgman method. An /spl alpha/-peak has been clearly observed in the pulse-height spectrum. On the other hand, no peak has been observed in the detectors made from crystals grown by the ordinary Bridgman method. From the relations of peak position versus applied bias voltage, the average energy to produce electron-hole pairs and the /spl mu//spl tau/ product of holes are estimated to be about 30 eV and 10/sup -7/ cm/sup 2//V, respectively.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An improvement in growing large, oriented lead iodide single crystals for detector applications

Abstract Recent improvements have been made in growing large, oriented lead iodide (PbI 2 ) single crystals for room temperature X-ray detector applications. The effect of ampoule design on the growth of crystals by the Bridgman-Stockbarger technique was studied. Crystals, optimal for detector labrication, were obtained using a necked ampoule with a 60° bend. This ampoule produced single crystals with the c -axis parallel to the axis of the ampoule.

Ferroelectric Curie points in pure and silver-doped lead iodide single crystals

Ferroelectric Curie points in pure and silver-doped lead iodide single crystals

Phase transformations in PbI2 crystals with temperature

Abstract A systematic study of phase transformation with temperature has been undertaken in single crystals of lead iodide grown in gel. The crystals of the polytype 2H, which is known to be the common modification of PbI2 at room temperature, have been finally found to transform into polytype 12R after heating at 150°C. During the intermediate period of heating the crystals show features of disorder, viz. streaking or arcing or both, on their X-ray diffraction photographs. The higher polytypes 12H and 16H do not show any change even after prolonged heating at 150°C. If silver iodide is added in a minute quantity during crystallization, the process of phase transformation is appreciably altered. The results have been discussed.

Purification, growth and polytypism of single crystals of lead iodide

Single crystals of lead iodide have been purified and grown using the method of zone refining and their polytypism has been studied by X-ray diffraction. Oscillation photographs obtained from several different regions of the crystals have revealed that the polytype 12R has its maximum occurrence in melt-grown crystals, followed by an occasional occurrence of the polytype 4H. An examination of crystals with different degrees of purification has revealed that impurities present in the starting material are responsible for an occasional transformation of 12R into 4H.

Chemical reactions and transport processes in lead iodide single crystals

— An anisotropic ionic motion and properties of potassium ion and iodine ion vacancy dipole ( K + V | ) have been studied by measuring d.c. ionic conductivity, dielectric losses (DL) and ionic thermocurrent (ITC). The migration energy of V,by the application of the electric field parallel (case I) or perpendicular (case II) to c-axis of crystals were found to be the same value of 0.26 eV in K-doped crystals. On the other hand, the migration energy of Pb + + ion vacancy in Bi + -doped crystals for cases I and II were 0.58 and 0.35 eV, respectively. The DL peak near at 1.3 x 10 Hz was found at 402 K. From ITC measurements two current maxima at 167 and 185 K have been observed. The results from DL and ITC measurements were examined with respect to the preexponential factors and the activation energies for jump of V, of dipoles. It is found that the relaxation process of dipole is composed of two stages.

Preparation of lead bromide and lead iodide single crystals from solutions: K Petkov, Izv Otdel Kim Nauk Bolg Akad Nauk, 4 (1), 1971, 51–55 ( in Bulgarian)

Preparation of lead bromide and lead iodide single crystals from solutions: K Petkov, Izv Otdel Kim Nauk Bolg Akad Nauk, 4 (1), 1971, 51–55 ( in Bulgarian)

Crystal structures of three polytypes of lead iodide: correlation between phenomena of arcing and polytypism

An X-ray study of single crystals of lead iodide grown by the gel method has revealed that the arcing phenomenon occurs rarely in this compound. The polytypism is also not observed frequently. The arcing phenomenon originates from arrangements of partial or unit edge-dislocations created during crystal growth. On the basis of Jagodzinski's disorder theory the polytypism can also be explained in terms of partial edge dislocations. Thus, the two phenomena appear to be interrelated. The complete crystal structures of three PbI2 polytypes, 10H, 14H and 20H, have been computed. These are found to be (11)322, (11)522 and (11)72112, respectively, in Zhdanov symbols, with space group P3m1. Their formation can be explained in terms of partial edge dislocations created at regular intervals, providing further support to the possibility of the polytypism and arcing phenomena being mutually related.

Excitonic Effects in the Electroreflectance of Lead Iodide

The reflectance and electroreflectance spectra of single crystals of lead iodide (Pb${\mathrm{I}}_{2}$) were measured between 2.2 and 5.0 eV at 4.5 and 77\ifmmode^\circ\else\textdegree\fi{}K, respectively. A Kramers-Kronig dispersion analysis of the data was made, and the real and imaginary parts of the dielectric constant were calculated together with their field-induced modulation. The first three lines of the hydrogenic exciton series associated with the absorption edge are well resolved in reflectivity. In the same energy range, the electoreflectance spectrum is interpreted as the Stark shift of the first exciton line from which the binding energy of the exciton is deduced. In the region of fundamental absorption, the existence of several metastable exciton states gives rise to a new and large contribution to the electroreflectance spectra.

Dielectric properties and index of refraction of lead iodide single crystals

The paper describes measurements of the dielectric constant ( E∥ c-axis) and loss as a function of temperature (110–450°K) and frequency ( 10 2 c s to 10 5 c s ), and the refractive index ( E⊥ c) from 0.5 μ to 15 μ on PbI 2 single crystals grown by a gel technique. Above 10 4 c s , K' = 6.21. The index of refraction was measured by an interferometric method for E⊥ c-axis from 0.5 μ to 15 μ where in the long wavelength region n E⊥ c = 2.50 ± 0.10. At 2.3μ Δn E⊥c ΔT = −2.0 × 10 −4 °K . The long wavelength value of the index of refraction for E∥ c-axis is shown by optical transmission measurements to be close to that for E⊥ c. The results show that the material is much less strongly ionic than hitherto supposed. Values of the dielectric constant previously reported vary within an order of magnitude. The origin of these discrepancies is traced and discussed. It is linked with the observation of photocapacitive effects.

Fission fragment damage to crystal lattices: lead iodide I. Electron microscopy

Earlier papers (Bowden &amp; Chadderton 1962; Chadderton &amp; Montagu-Pollock 1963 ; Chadderton 1964) described the damage caused to various types of crystal by energetic fission fragments, and interpreted the electron microscope observations of damage on the basis of current theories of energy loss of charged particles, and of image contrast in the microscope. In this paper, electron microscope observations of fission fragment irradiated single crystals of lead iodide are described. This compound is heat-sensitive, and might therefore be expected to show damage through lattice excitation following the passage of a heavily ionizing energetic particle. The nature of the damage is sensitively dependent on the thickness of the crystal specimen, with tracks visible in thin crystals and a random array of dislocation loops in thicker ones. The variation in track visibility and appearance with thickness is described, and the possible mechanisms of energy loss which might account for these variations are discussed. The proximity of the crystal surfaces to the path of a fission fragment is a critical factor in determining the visible effects. Observations of damage in cadmium iodide are also discussed. The random distribution of dislocation loops in thick crystals, where surface effects are no longer influential, indicates a high mobility of the point defects created in the wake of a fission fragment by direct collision and thermal processes. Some possible mechanisms for the motion of energetic knock-ons in the lattice are suggested. These will be investigated more fully in part II.

The formation of photographic images in single crystals of lead iodide

Abstract A recent investigation of the electrical conductivity of small crystals of lead iodide has led to the discovery of an anomalous behaviour in photo-conductivity. The conductivity undergoes a sudden extinction at temperatures between 170°c and 190°c. This appears to be related to the occurrence of photo-decomposition which commences at the same temperature. The micrographs presented in this paper to illustrate photo-decomposition also demonstrate the remarkable localization of the effect. They suggest the possibility of using photo-decomposition in lead iodide crystals to ‘print out’ photographic images, providing a recording process which may have advantages over conventional techniques.

Nucleation of Ice on Lead Iodide

Abstract A cold box was constructed to study ice crystal growth on single crystals of lead iodide. When the temperature of the PbI2 substrate was slowly lowered past the temperature of the ice surface, crystal growth was observed first at the edges of the crystal and then at certain preferred sites on the PbI2 surface. If water drops appeared on the crystals, there was a reduction in the number of active, nucleating sites. This was especially noticed at the edges of the crystal. It was noted that the threshold temperature for ice crystal growth, as determined by visual observation, was −6C. From the temperature-vapor pressure curve, it appears that PbI2 acts as a freezing nucleus from −6C to about −12C to −14C and a sublimation nucleus at temperatures lower-than −12C to −14C.