WO2004048964A1

WO2004048964A1 - Pyro-electric electron source

Info

Publication number: WO2004048964A1
Application number: PCT/IT2003/000745
Authority: WO
Inventors: Corrado Carretti; Giorgio Longoni; Bruno Ferrario
Original assignee: Saes Getters S.P.A.
Priority date: 2002-11-22
Filing date: 2003-11-18
Publication date: 2004-06-10
Also published as: TW200419618A; AU2003288731A1; ITMI20022484A1

Abstract

The present invention relates to a pyro-electric electron source, comprising a hermetic chamber (1), provided with at least a beryllium window (3) and internally containing a crystal (2) of pyro-electric material arranged in front of said beryllium window (3), a heating or cooling element (6) for said crystal (2), an inert gas at low pressure and a device (4) comprising a non-evaporable getter material. The source of the invention has particular application as ionizing device for an ion mobility spectrometer.

Description

"PYRO-ELECTRIC ELECTRON SOURCE"

The present invention relates to a pyro-electric electron source, particularly useful as a ionizing device for ion mobility spectrometers, and to a spectrometer comprising said ionizing device.

In the present description, with the term electron source it is to be intended a device capable of producing an electron flow in a gas or in the vacuum. The electron sources have a number of different applications in technology, for example in cathode-ray tubes. A particular application is the utilization of the electronic flow to ionize a gas to be analyzed with the technique of ion mobility spectrometry. Although the invention relates to electron sources and the general use thereof, in the following it will be particularly referred to the use as gas ionizing element in said analytic technique.

Ion mobility spectrometry is generally known in the chemical analysis sector with the abbreviation IMS, which indicates also the instrument through with the analytic technique is carried out (indicating in this case Ion Mobility Spectrometer).

The interest for the IMS technique is due to the very high sensibility thereof, together with the low dimensions and costs of the instrument. Working in suitable conditions it is possible to detect gas or vapor species in a gas mixture in quantities of the order of picograms (pg, i.e. 10^"12 g), or in concentrations of the order of parts per trillion (ppt, equivalent to a molecule of substance in analysis per 10 molecules of gas of the sample) .

The fields of application of the technique are numerous, both in civil sectors (particularly, in industry for detecting inorganic or organic contaminants in clean chambers or harmful species in the industrial emissions) and in military sectors

(particularly, for detecting the presence of explosive or toxic substances, as nervine gases).

In the IMS technique, the species to be analyzed are ionized by a suitable ionizing element; the ions are accelerated in the chamber of the instrument by electric fields generated by a series of electrodes and simultaneously slowed down by a static gas or more commonly counter-flow gas, with respect to the direction of the ions, being present in the chamber; and detected by a detector of charged particles arranged at the end of the chamber opposite to that wherein the ionization takes place. Because of the slowing down effect of the gas in the chamber, the different ions arrive on the detector at different times, and the analysis is based on detection and attribution to the different species of charge peaks measured by the detector in function of the time. For further details about the technique and the IMS instruments it is to be referred to the literature of the sector, for example to the book "Ion Mobility Spectrometry" by G.A. Eiceman and Z. Karpas, published in 1994 by CRC Press or to the USA patents 5,955,886 and 6,229,143.

In order to assure the correct operation of the ion mobility spectrometer, and thus the reliability of the analysis, it is necessary to carry out a stable and constant gas ionization. For this purpose, as ionizing element is commonly used a beta rays source provided by ⁶³Ni, whose emissions have currents of about 60 picoampere (pA) and an energy between about 17 and 66 Kiloelectronvolt (KeV).

However, the use of this radioactive isotope obviously implies some serious safety problems, and make the transport phases and the maintenance of the spectrometry considerably complicated. It is further known since long that some materials having a crystalline structure which is asymmetric along one axis (defined polar axis), as for example the crystals of lithium niobate (LiNbO ), of lithium tantalate (LiTaO₃) or of barium titanate (BaTiO ), are pyro-electric, in other words between two opposite faces of said crystals and perpendicularly to said axis it may be developed a high potential difference when the same crystals are subjected to heating or cooling; very high electric fields are produced around said faces.

From patent US 3,840,748 it is known a beta-rays and X-rays generator comprising an evacuated chamber inside which a crystal of pyro-electric material is arranged. This crystal is heated to a temperature sufficient to generate the polarization of the material and the consequent emission of electrons from the material itself. When the generator is used as electron source, inside the chamber there is also arranged an earth potential electrode, for example a Lenard window, which has the function to attract the electrons emitted by the crystal.

However, this type of generator has found scarce practical applications, and in particular can not substitute an ionizing element based on ⁶³Ni, since for its working a considerable power supply from the outside is required, which makes it definitely disadvantageous from an economical point of view.

As a matter of fact, a first energetic contribution is obviously necessary to heat the crystal, thus generating the pyro-electric phenomenon. Once the electrons have been generated by the crystal, it is then necessary a further energetic contribution to supply the electric circuits which aim to accelerate them so that they may give rise to the desired effects, as for example the emission of X-rays.

It has been recently found that if there is a gas at low pressure around the crystal, the high electric fields produced on the surfaces of the crystal owing to the heating or cooling thereof are capable of ionizing the same gas, with formation of positive ions and electrons, which are rejected or attracted by the surface of the crystal depending on the charge thereof. The electrons emitted by the gas are highly accelerated by the electric field near the crystal, without the necessity to use accelerating electrodes as in the case of the patent US 3,840,748 described above; therefore, the presence of a gas near the crystal allows to obtain a high energy electronic emission.

The just described principle is utilized by a X-rays generator produced and commercialized by the US company Amptek Inc. This generator comprises a hermetically closed chamber, inside which there are arranged a pyro-electric crystal immersed in a gas at low pressure and a device for heating and cooling the same crystal. The surface of the chamber opposite to the crystal is made up of one copper plate and one beryllium plate, placed one over the other.

Thanks to the heating and cooling device, the crystal of pyro-electric material is subjected to short thermal cycles. The electrons cyclically emitted by the gas surrounding the surface of the crystal are accelerated toward the cupper plate, which in its turn emits X-rays which are transmitted through the beryllium plate. However, this generator is used to produce X-rays, but not electrons necessary for ionizing a sample gas in an IMS instrument.

A possible problem of this generator is the lost of efficacy during time, because the pressure inside the chamber increases in a few thermal cycles, in spite of being hermetic, owing to the degassing of the internal components of the chamber. When the gas pressure in the chamber becomes greater than certain limit values, the ionization of the gas produces, in addition to the electrons, also a high concentration of positive ions; these are attracted by the negative surface of the crystal, neutralizing or considerably reducing the charge thereof, with the final result to depress the electric field around the crystal and thus the electronic emission. Another problem related to the pressure increase is that one portion of the electrons are deflected from their trajectory toward the copper plate, obviously determining a decreasing of the generation of X-rays.

An object of the present invention is therefore to provide an electron source whose operation is not based on the use of radioactive material. Another object is to provide a gas ionizing element to be used in an instrument of ion mobility spectrometry which allows a stable ionization of the sample gas. Said objects are achieved through an electron source whose main features are specified in the first claim and other features thereof are specified in the subsequent claims. Advantages and features of the electron source according to the present invention will be clear to those skilled in the art from the following detailed description of one embodiment thereof with reference to the drawings, wherein:

- figure 1 shows a schematic view of the electron source of the invention in the most generic embodiment thereof; - figure 2 shows a preferred embodiment of the source of the invention.

Referring to figure 1, there is shown that the electron source according to the present invention comprises a hermetically closed chamber 1 inside which a crystal 2 of pyro-electric material is arranged, made for example of LiNbO or LiTaO₃. A portion of the surface of chamber 1 is made up of a hermetically sealed beryllium window, 3, having the characteristic that it can be crossed by a fraction of the electrons emitted by the gas, in a measure depending on the thickness thereof and on the energy of the electrons incident thereon. Window 3 is placed in chamber 1 opposite to a surface of crystal 2, in such a way to be crossed by the ideal line defined by the polar axis of the crystal (with broken line in the drawing). Moreover, although it -is not strictly necessary, the surface of the crystal facing the window and this latter are preferably perpendicular to said axis. In figure the arrows inside the chamber and the symbol "ef" indicate the beam of electrons that are accelerated towards window 3 by the electric field that is present around crystal 2, while the arrows outside chamber 1 and the symbol "e_t ^"" indicate the electron beam emitted by the source of the invention. Other geometric characteristics of the system are not critical: the crystal 2 has preferably a regular shape, for example parallelepipedal or cylindrical and may have a thickness comprised between fractions of millimeter and a few millimeters, for example about 1 mm; the distance between crystal 2 and window 3 may be for example of the order of a few tens millimeters; the window may have a thickness comprised between about 10 and about 150 micrometers (μτή), and preferably less than 100 μm. These values of the thickness of window 3 assure that at least 50% of the electrons accelerated towards the window are capable of passing through it.

In chamber 1 there is present an inert gas, such as nitrogen or a noble gas. The pressure of the inert gas inside the chamber is low, and preferably comprised between 0.05 and 1 Pascal (Pa). For pressures lower than those of such an interval the electronic current of the chamber decreases, while for higher pressures there are the above mentioned problems of compensation of the charge on the faces of the crystal, with reduction or dissolution of the electric field around it, and reduction of the energy with which the electrons arrive on the beryllium window because of the gas diffusion: in both cases, the source reduces the electronic emission under the useful values in the practical applications.

Inside chamber 1 there is also a device 4 comprising a getter material of the non evaporable type. The non evaporable getter material, also known as NEG, are generally zirconium- or titanium-based alloys, commonly with other transition metals or aluminum; these materials are well known in the art and do not require a detailed description. They are capable of removing from an enclosure some gases such as hydrogen, oxygen, water and carbon oxides, even when there are low pressures of noble gases in the same enclosure. Device 4 may be made of the sole getter material, e.g. a pellet of sintered powders of the material, but more commonly the device is made of an open metal container in which the getter material is arranged. Device 4 avoids the pressure of the chamber 1 to undesirably increase, due to the gas release by the inner components of the source, thus ensuring an efficient operation thereof in the time. A heating element 5 is preferably associated to getter device 4, in order to keep said getter material at an optimal temperature for its operation. As mentioned, the effect of generating intense electric fields on the surface of the crystal takes place during the heating or cooling phases of the same. The phenomenon is observed with temperature variations of some tens or a few hundreds degree, generally with temperature variations comprised between about 50 and 200 °C, preferably of about 100°C. These temperature variations of the crystal are achieved with at least a heating or cooling device, 6, arranged in the chamber 1 very close to the crystal, preferably in contact with the surface of the crystal opposite to that facing the beryllium window.

In the case element 6 is a heating element, it may be for example of the electric resistance type; in this case, the heating phase of the crystal is achieved through the passage of current in the resistance, while the cooling phase is due to natural cooling. As an alternative, element 6 may be a heat exchanger of Peltier type, which as known may act as heating or cooling element depending on the polarity applied to its ends. Finally, in one preferred embodiment of the invention, it is possible to use a combination of a resistance heater and a Peltier element to be used for cooling: with this configuration the maximum velocities are obtained both in heating and cooling, allowing a high frequency of the electronic emission cycles.

This preferred embodiment is shown in figure 2 wherein, besides the elements already described with reference to figure 1, there are the resistance heater 7, fed through contacts 8 and 8', and the Peltier element 9, fed through contacts 10 and 10'. Preferably a temperature sensor 11, such as for example a thermocouple, is associated to element 6, for checking the thermal conditions and thus the intensity of the electronic emission. Furthermore, the face of the crystal 2 opposite to that facing the window 3 is preferably electrically earth connected (through element 12) to discharge the strong electron charge which accumulates on this face during one of the heating or cooling phase of the crystal, according to its orientation in chamber 1. Since it has been observed that the electronic emission is more stable and longer during the cooling phase, it is preferable that crystal 2 is oriented in such a way to have directed toward window 3 the surface which has the negative polarity during the cooling. Finally, in another variant of the source of the invention, at least the surface of crystal 2 facing the window 3 is covered with a layer of a plastic material, for example an epoxy resin. The presence of this layer avoids that, when the surface of the crystal facing the beryllium window has a positive charge, the electrons hit said surface causing the emission of X-rays, in particular by the heavy elements forming the crystal 2, as the niobium in the case of LiNbO₃ or the barium in the case of BaTiO₃. The deceleration of the electrons in a plastic material, made up of relatively light atoms, causes only the production of X radiations of small entity.

As previously said, although allowing different utilizations, the electron source of the present invention is particularly useful as an alternative to the use of a radioactive material for the ionization of a sample gas in an IMS instrument. In fact, the source of the invention allows to generate electron beams with energies up to 150 KeV and electronic currents of the order of some hundreds of pA, thus comparable or even greater than those generated by the traditional ionizing elements based on Ni. Another advantage of the source according to the present invention is in the fact that it allows to produce a stable and constant ionization in time, and is not subject to damages due to the repeated thermal cycles.

Possible variations and/or additions may be made by those skilled in the art to the embodiment here described and illustrated without departing from the scope of the invention itself.

Claims

1. An electron source comprising a hermetic chamber (1) provided with at least a beryllium window (3) and internally containing a crystal (2) of a pyro- electric material provided with a polar axis, an inert gas at low pressure, a device (4) comprising a non- evaporable getter material and at least an element (6) for heating or cooling said crystal (2), said window (3) being arranged in such a way to be crossed by the ideal line defined by the polar axis of the crystal (2).

2. An electron source according to claim 1, characterized in that said pressure of the inert gas inside the chamber (1) is comprised between about 0.05 and 1 Pa.

3. An electron source according to claim 1 or 2, characterized in that said inert gas is chosen in the group formed of nitrogen and noble gases.

4. An electron source according to any of the previous claims, characterized in that said beryllium window (3) is formed of a beryllium plate of thickness comprised between about 10 μm and 150 μm.

5. An electron source according to claim 4, characterized in that said beryllium plate has a thickness lower than 100 μm.

6. An electron source according to any of the previous claims, characterized in that said heating or cooling element (6) is capable of causing a temperature variation in said crystal (2) of pyro-electric material comprised between about 50 °C and 200 °C.

7. An electron source according to claim 6, characterized in that said temperature variation is of about 100 °C.

8. An electron source according to any of the previous claims, characterized in that said heating or cooling element (6) is a resistance heater.

9. An electron source according to any of claims 1 to 7, characterized in that said heating or cooling element (6) is a heat exchanger of Peltier type.

10. An electron source according to any of the previous claims, characterized in that it comprises both a resistance heater (7) fed through electric contacts (8, 8') and a heat exchanger of Peltier type (9) fed through electric contacts (10, 10').

11. An electron source according to any of the previous claims, characterized in that said heating or cooling elements (6; 7, 9) are in contact with the surface of said crystal (2) opposite to the beryllium window (3).

12. An electron source according to any of the previous claims, further comprising an element (5) for heating said getter material (4).

13. An electron source according to any of the previous claims, characterized in that it further comprises a temperature sensor (11) to detect the temperature of said heating or cooling element (6; 7, 9).

14. An electron source according to any of the previous claims, characterized in that at least the surface of said crystal (2) facing said window (3) is covered by a layer of plastic material.

15. An electron source according to any of the previous claims, characterized in that said crystal (2) has a substantially cylindrical or parallelepipedal shape.

16. The use of an electron source according to any of the previous claims, as ionizing device in an ion mobility spectrometer.

17. An ion mobility spectrometer comprising a ionizing device according to claim 16.