FR2618605A1 - Device for detecting and amplifying small positive or negative ion currents - Google Patents

Abstract

Device for measuring small ion currents generated under vacuum in systems such as mass spectrographs for example, and allowing the collection of ions of both signs on conversion dynodes. The secondary particles torn from the dynodes bombard the entrance face of an electron multiplier. The exit electron current in turn bombards a fluorescent screen. The photons produced in the screen are channelled by a light guide to the photocathode of a photomultiplier tube placed outside the system. This type of ion->electron->photon->electron conversion allows near-simultaneous detection of positive or negative ions with a maximum amplification of the order of 10<13>, while in both cases receiving the signal at a potential close to earth. By varying the value of the various potentials, a dynamic range of 10<8> can moreover be covered easily. A discriminator moreover makes it possible to eliminate the parasitic pulses created downstream of the first multiplier, the inherent noise of which may be very small.

Description

DEVICE FOR DETECTION AND AMPLIFICATION OF LOW IONIC CURRENTS,
POSITIVE OR NEGATIVE.

The growing importance, in many fields of application, of the analysis of gases by mass spectrography, requires increased performance instrumentation, in particular: a) greater amplification adaptable to very wide dynamics; b) an even higher sensitivity and therefore an inherent noise of the detection and amplification system as low as possible. On the other hand, it is desirable to be able to identify, almost simultaneously, the negative and positive ions and thus obtain a complete spectrum: for obvious reasons, the signal must, in both cases, be processed at a potential close to the mass.

The present invention relates to a detection and amplification device meeting these requirements.

In this system, the ions bombard, at the output of the mass spectrograph, a target D (dynode) brought to a positive potential V for the detection of negative ions, and negative for the detection of positive ions.

Igta worth a few kV.

Depending on its polarity, target D attracts positive or negative ions.

Under the effect of this high energy bombardment, D generates secondary particles (positive ions, negative ions, electrons, photons !.

For a given value of g, the intensity of this secondary emission is proportional to the intensity of the primary bombardment: it is moreover rapidly increasing with the impact energy (absolute value iVi of the potential D).

An electronic multiplier M, with a very low noise level (a wafer of micro-channels for example) is placed opposite the target. The entry face of M is at a potential close to the trap.

The multiplier therefore receives positive secondary ions and photons from D when S is positive (detection of negative ions).

fl receives on the other hand negative ions, electrons and photons when the target D is at a negative potential (detection of positive ions).

At the output of the multiplier, we obtain an electronic current IZ given by
I, - 0GZ Ii (where IL is the primary ionic current, the conversion factor on the target D, ss the conversion factor on the input face of the multiplier M, GK the gain of the multiplication may L4 and 4 depend on YD while GM depends on the potential VM at the output of M. A fluorescent screen F, placed opposite the output of the multiplier M and fixed to a positive potential VF, greater by a few kV than V, receives the electrons from the multiplier.

The electrons thus have a kinetic energy equal to e (VF - V).

Part of this energy is found at the exit of the screen in the form of photons. The number n of photons produced per incident electron is given by n - t (VF - V) where r is a constant characteristic of the phosphor of the screen.

A light guide, in contact with the exit face of the screen, channels by total reflections the photons outside the vacuum system, while ensuring its tightness. The photons are finally received on the cathode of a photomultiplier tube placed outside, in the immediate vicinity of the end of the guide. The optical signal is transformed into an electrical signal with amplification.

The PM supply potentials are obviously quite independent of the potential distribution in the vacuum system. It is therefore possible in particular to fix the cathode of the PM at the high negative voltage VpH and to output the anode signal at a potential close to ground.

&alpha; et ss sont fonctions du potentiel de cible VD CM est fonction de V, et Gpm est fonction de la différence de potentiel V,, appliquée au pont
d'alimentation du PM. On dispose ainsi de quatre potentiels pour régler le
facteur d'amplification Ie à la valeur requise.For a primary ionic current Ii, we obtain a current of signal I given by: 1e = I # n 2 GpY where t is the quantum efficiency of the photocathode of the PM and GDM the gain of this PM. By replacing IF and n by their value, we finally have

&alpha; and ss are functions of the target potential VD CM is a function of V, and Gpm is a function of the potential difference V ,, applied to the bridge
power supply from the PM. We thus have four potentials to adjust the
amplification factor Ie to the required value.

II
For example, for VD 5 kV, Vm - 1 kV, VF r 5 kV and Vp ,, = 2 kV, we have

By reducing the values of these potentials, we can reduce this factor to, 10
The proposed system satisfies the conditions set
amplification can have a very high value while cn covering a
very broad dynamic.

On the other hand, the choice of the 1st amplification stage (wafer of
microchannels for example) was dictated by the following considerations: its
noise level is very low and can be further reduced by limiting the gain
GM. This limitation is quite acceptable given the very
important downstream (fluorescent screen ct PM). On the other hand, the value of GM (between
lO and lO) is sufficient to allow discrimination on the basis of
output of the system between the useful pulses (generated upstream of the
wafer) and parasitic pulses (generated downstream) which thus have a
amplitude between iol and 10 times smaller.
is therefore cn makes the very low noise level of the wafer, if we neglect the pulses due to accidental causes upstream of M (micro-clashes at the dynodes D, parasitic photons created in the mass spectrograph)
An appropriate treatment of the surfaces and a suitable arrangement of the electrodes moreover make it possible to practically eliminate these pulses.

The signal-to-noise ratio can thus reach significantly higher values than in conventional systems; sensitivity is greatly improved.

It should also be noted that thanks to this detection mode (conversion

<tb> ions <SEP>---><SEP> electrons <SEP>---><SEP> photons <SEP>-><SEP> electrons)
<tb> the currents only become important in the final amplification stage, placed outside the system (photomultiplier tube). Pollution by decomposition of hydrocarbons is thus avoided. On the other hand, the choice of the potentials inside the system, completely independent of the supply potentials of the final amplifier (PM outside), is here completely free and can be perfectly optimized.

Finally, the use of a system with two conversion dynodes, one positive, the other negative, allows the almost simultaneous detection of the ions of the two signs without having to switch the high voltage. It is moreover an embodiment of this modality that we will now describe by way of example and in order to better understand the invention (FIG. I).

A quadrupole-type mass spectrograph S selects, depending on the polarity of the potentials applied to the electrodes of the source, positive or negative ions s. These potentials have absolute values not exceeding a few hundred volts and can be switched very quickly, so it is possible to obtain, during the same scan, the complete mass spectrc of the positive and negative ions formed in the gas. gas to be analyzed.

On leaving the quadrupole, the ions cross the space E between two electrodes between which a low potential difference is established. The positive ions are thus deflected to the left for example, and the negative ions to the right. They are attracted respectively by the conversion dynode D4 brought to a potential of approximately - 5 kV and by the conversion dynode Dt brought to + 5 kV.

A wafer of microchannels M whose input face is grounded and the output face has a potential of 1 kV, is placed opposite the dynodes, in the axis of the system.

Positive ions bombard the D4 dynode. The negative secondary particles (negative ions, electrons) and the photons produced strike the entry face of the wafer.

Likewise, the negative ions bombard the Du dynode and the positive secondary particles produced (positive ions) and the photons also strike the entry face of the wafer.

The secondary electrons produced by the impacts multiply in the channels. At the exit, they are accelerated by the field created by a fluorescent screen F brought to a potential det + 5 kV.

These electrons lose their kinetic energy in the screen by creating photons.

This screen is for example of the P47 type emitting in the blue with a very short relaxation time. It is supported by a light guide G which directs the photons outwards by total reflections while ensuring the tightness of the system.

The photons are finally received by the cathode of a photomultiplier tube (PM) outside the system and placed in the immediate vicinity of the end of the guide G.

The choice of the potential values at the terminals of the supply bridge of the
PM is obviously quite independent of the potential distribution within the system. The cathode potential is fixed at about kV and the other end of the bridge to ground. The anode signal can thus be directly processed.

Let us also mention the presence between the dynodes Dd, DX of a conical electrode A at the potential O whose role is to absorb by multiple reflections the parasitic photons produced in the quadrupole while improving the characteristics of electronic optics.

As we saw above, the amplification factor is given by

With the values of the potentials fixed in this example, this factor is of the order of 3.10 and can be lowered to about 10 or less by decreasing these potentials.

The string noise current is very low; in fact, the choice of a P47 screen allows the use of a PM equipped with a bi-alkaline photocathode (maximum sensitivity in the blue).

These photocathodes have an extremely low dark current, less than inA. In any case, the parasitic pulses created downstream of the wafer M can easily wander off by a discriminator: in fact, the useful pulses are created upstream of M and therefore have an amplitude 1000 times greater.

The sensitivity limit is thus set by the rate of parasitic pulses in the wafer. This rate does not exceed 1 / cm / s.

It is also necessary to take into account micro-clamping at the level of the dynodes. They can be practically eliminated by appropriate treatment of the surfaces.

The new system described is intended for the measurement of the ionic currents of the two signs at the output of mass spectrographs, more particularly in the case of chromatography-mass spectrography coupling where the ionic currents can be very weak (detection of a single ion )

Claims (4)

Device for measuring weak positive (or negative) ionic currents produced in a vacuum system (such as a mass spectrograph for example) characterized by the fact that the positive (or negative) ions bombard a dynode brought to a high negative potential (or positive); by the fact that the secondary particles emitted by the dynode bombard the input face of a very low noise level electron multiplier (for example, a wafer of microchannels) brought to a potential close to ground; by the fact that the electronic current amplified in the multiplier bombards a fluorescent screen brought to a positive potential much higher than the output potential of the multiplier; in that this screen is supported by the entry face of a light guide which seals the assembly and directs the photons produced in the screen outwards; in that the outer end of the light guide is in the immediate vicinity of the entry face of the photomultiplier; in that the cathode of the PM is brought to a high negative voltage, the other end of the supply bridge being grounded; by the fact that the total amplification factor can vary by a factor> 108 by variation of the various potentials, the maximum value being able to reach or exceed 1013. 2) Device according to claim 1, characterized in that a auxiliary cone-shaped electrode is located opposite the ion beam incident and absorbs by multiple reflections the parasitic photons produced in the ion generating system. 3) Device according to claim 1 or 1 and 2 characterized in that two dynodes are used, one at a high positive potential, the other at a high negative potential, one allowing the detection of negative ions, the the other detection of positive ions without having to switch the high voltages of the dynodes; by the fact that a weakly polarized deflection electrode located at the exit of the primary ion beam deflects the positive ions towards the negative dynode and vice versa, thus ensuring a more complete collection of the ions of the two signs. 4) Device according to claims 1 or 1 and 2 or 1 and 3 or 1 and 2 and 3, characterized in that a discriminator processes the output signal and eliminates the noise pulses of significantly lower amplitude created downstream of the electron multiplier. (microchannel cake).