LAX in brief
Investigations of the interaction between radiation and matter as well as the development of modern X-ray detectors for astrophysics and medical researches require the availability of an X-ray beam with wide ranges of variable energy, flux, transversal size and angular divergence.
Such an X-ray beam facility was installed at LAX (Laboratorio per sperimentazioni con radiazione X), jointly operated in Palermo by the DiFTeR and DEAF (Dipartimento di Fisica e Tecnologie Relative e Dipartimento di Energetica ed Applicazioni della Fisica) of the University of Palermo and IASF (Istituto di Astrofisica Spaziale e Fisica cosmica) di Palermo of INAF (National Institute of Astrophysics).
The X-ray beam has an operational energy range of 0.1 – 25 keV (extensible up to 60 keV) with a flux of 1010 – 1012 photons/sr . s. The beam, collimated on a length of 10.5 m, has a diameter at full aperture of 200 mm with a divergence of 1°. A view of the LAX facility is reported below.
Some interesting applications of the LAX facility are:
- Development, investigation and calibration of X-ray detectors: energy resolution, spatial resolution, quantum efficiency…
- Surface physics: material microstructure, reflectivity and transmission of materials, photoemission from materials…
- Absorption and transmission of matter: irradiation, radiation damage…
For the intended applications the LAX X-ray beam facility with good angular collimation fullfills the requirements and is much more convenient, for example, of the Synchrotron radiation sources.
As shown in the general layout figure below the X-ray beam line consists of:
- the X-ray Generators;
- the Beam Collimation System;
- the Sample Chamber;
- the Detection Chamber.
The X-ray Generator Chamber operates at 10-9 mbar, whereas the other parts operate at 10-6 mbar.
The beam line was developed and manufactured by RIAL Vacuum S.p.A. (Parma, Italy) following the specifications of the DEAF and IASF. The X-ray Generators The X-radiation is produced by electrons accelerated in a vacuum tube against an target as anode. X-ray lines can be obtained directly from the primary target (then requiring a specific target material for every line chosen) or by using the continuum derived from the primary target to excite the fluorescence from secondary targets. The high voltage can reach 60 kV. Three X-ray Generator systems are available, covering different energy ranges and allowing a flexible choice of the beam intensity. The generators are easily interchangeable following the requirements by the beam operation:
- A “Manson” model Ultrasoft Source, covering the energy range from 0.109 keV (Be-Ka) to 1.254 keV (Mg-Ka).
- A “Seyfert” Air-insulated diffraction X-ray tube type SN60 (max tube voltage 60 kV) covering the energy range from 1.0 keV up to 60 keV (W- Ka)
- An “in house source” electron gun induced variable X-ray emission lines from several targets covering the energy range from 0.109 keV (Be-Ka) to 25.2 keV (Sn- Ka).
Flux value ranges from 1010 to 1012 photons/sr . s that means at 10 meters a flux of the order of 105 photons/cm2 . s. A list of available spectral lines is reported below.
Table 1 : Targets and Energies of characteristic fluorescence lines available at LAX
|Target material||Fluorescence line||Energy (keV)|
|Pb||La, Lb||10.5 – 12.5|
To minimize the Bremsstrahlung continuum came from the primary target and to select the lines originate from the primary or secondary targets, absorption filters are also used. A list of available absorption filters is given in the following table.
Table 2 : Thickness and purity of the absorption filters available at LAX
|Filter material||Thickness (mm)||Purity (%)|
The Beam Collimation System
The maximum acceptable angular divergence is imposed by the specific problem under consideration: minimum and maximum values range from the arcsec to the degree of arc.
From the practical point of view, the angular divergence depends upon the distance L between the X-ray generator and the Sample Chamber and from the typical dimension D of the area illuminated, which in turn determines the required power of the X-ray generator.
In the LAX beam, L=10.5 m and values of the order of 1° obtainable for beam aperture of 200 mm diameter and of 1’ for a beam aperture of 2mm diameter.
The Sample Chamber
The Sample Chamber contains a table capable of translation along the X, Y and Z axes and rotation around Y and Z. It hosts the sample under analysis and/or monochromator/polarizes.
The Detection Chamber
The Detection Chamber hosts the X-ray detectors; positioning is achieved by a micrometer table capable of translation movements along the X and Y axes and rotation around Y.
The block layout of the X-ray beam line is shown in the figure below (click for larger image).
X-ray Generator Chamber
The cylindrical chamber (Pos. A – Level 100) is made of stainless steel for UHV, with 4 CF 35 viewports, three CF 16, one CF 100 and one CF 63, Copper seals are used.
The CF 100 closing flange has a CF 35 door.
The two windows for X-ray transmission are made of beryllium and mounted on CF 16.
The chamber is pumped by a 160 l/s diode ion pump provided with a CF 63 all-metal valve. The power supply can be used to measure the vacuum. The operative vacuum is better than 10-10 mbar.
- Motion system are used for the electron gun and the targets:
- A 30 mm translation system mounted on 2 CF 63 flanges. It is used for the translation of the electron gun, equipped with a bipolar feedthrough CF 63, with inside connections for filaments and outside connection for the HV connector (60 kV) compatible with the “Seyfert” power supply cable.
- A 360° rotation – 50 mm translation system is installed on a CF 100 flange to support the various targets on a double helical octagonal pyramid. The support is water cooled.
First Lateral Beam Interceptor
The first lateral interceptor (Pos. A – Level 473) of the X-ray beam is mounted after the CF 35 slide valve, with two linear translators with a stroke of 30 mm moved by a stepping motor where some slits are mounted to limit beam divergence.
X-ray Absorption Filter System
A disc filter system with 12 positions (Pos. A – Level 690) is mounted on a T chamber. Interchange of the filters is performed by a routable manipulator controlled by a stepping motor.
Beam line: Pipes
Three stainless steel tubes of 200 mm of diameter (DN 200) are installed to connect the X-ray source to the sample holder chamber. The overall length of the pipe is quoted in the Pos. C.
Second Lateral Beam Interceptor
This component (Pos. C – Level 8558) is mounted on a DN 200 cross piece with two linear X,Y manipulators, each with a 200 mm stroke controlled by stepping motors, to collimate the X-ray beam.
Extra flanges allow the mounting of vacuum gauge heads, and a by-pass for a “Varian” 1000 l/s turbomolecular pump, coupled to an “Alcatel” rotating vane pump (two stage, 65 m3/h) DN 40 with zeolite filter, to maintain a vacuum in the pipes below to 10-6 mbar.
Sample Holder Chamber
The Sample Holder Chamber is essentially made of:
Stainless steel chamber;
The chamber consists of a 6 way cross with a spherical body with a CF 200; Four CF 35 flanges are added for possible services (vacuum gauges, by-pass lines, etc.). The surface have been treated for UHV requirements.
- Five degree manipulators with three micrometric linear X,Y,Z axes movements and two angular movements around to Y axis and any axis in the X,Z plane. Z is the axis of the beam line.
- Introduction valve; The valve is a gate valve with CF 200 flange.
The Detection Chamber (Pos. D – Level 10169) for the X-ray beam is made of stainless steel with UHV specifications. It essentially consists of a horizontal cylinder, with an external diameter of 1300 mm, thickness 10 mm and length 1300 mm with a curved base. Both chamber and curved base are flanged. The curved base is hinged to the chamber for easy opening. A seal system, realized with two concentric Viton O-rings, can be differentially pumped. Four supports are welded inside the chamber upon which the detection system is fixed. Outside four supports are designed to allow the levelling of the chamber.
The central cylinder is equipped with the following flanges: 10 flanges CF 200, 3 flanges CF 100, 4 flanges CF 35, 2 view ports CF 150. Two CF 200 flanges are mounted on the two covers.
Inside a three degree UHV micrometric table with two linear X,Y axes movements and a one angular movement around the Y axis is placed and centred along the beam line axis (Z axis). The dimension of the table is 200 x 200 mm and it can support a load up to 10 kg.
The chamber is He leak tested to a maximum leak rate of 1 x 10-9 atmcm3 s-1.
The pumping system consists of one “Alcatel” rotating vane pump (two stage, 65 m3/h) DN 40 with zeolite filter, a 1000 l/s pumping speed “Varian” turbomolecular pump and a “CTI-Cryogenics” cryogenic pump with a nominal pumping speed of 1500 l/s for air and 4000 l/s for water. The operative pressure is below 10-6 mbar. A combined Pirani-Penning gauge is used for vacuum measurement.
The adjustable supports for X-ray chamber and connection pipe are made of painted iron with fine regulation screws (1/10 mm). They are used to support the weight of the X-ray chamber, the ion pump and the channelling.