Probe head for an NMR spectrometer

Details Probe head for an NMR spectrometer Probe head for an NMR spectrometer

2001-10-31

2003-09-09

Lefkowitz, Edward (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

C324S309000

Reexamination Certificate

active

06617851

ABSTRACT:

This invention relates to a probe head for a NMR-spectrometer comprising at least one transmitter unit for generating electromagnetic waves of high frequencies and at least one preamplifier unit for amplification of signals emanating from a sample which has been excited by means of the excitation waves, with a cryogenically cooled primary detection circuit, including at least a first antenna and a first waveguide, whereby said first antenna is connected to said preamplifier unit via said first waveguide.
The object of a NMR-experiment is to observe the electromagnetic radiation being produced by energy transitions of electrons and/or atoms in a sample irradiated by a high frequency field B
1
of frequency &ohgr;
0
in a time and space homogeneous magnetic field B
0
. The frequency &ohgr;
0
of the excitation field B
1
is preferably situated within or above the hf-range at or near about 300 MHz to 3 GHz. This electromagnetic radiation is being observed against the general background of electromagnetic noise associated with thermal fluctuations in the primary detection circuit and within the sample or in the environment, too.
One particular problem common to most NMR experiments is the faintness of the signals emanating from the sample being sometimes further attenuated by a reaction in progress or by some parts of the sample absorbing from the signal. In this respect it is continuously being attempted to improve the sensitivity of the probe head and the signal-to-noise ratio.
The field strength of the constant homogeneous magnetic field B
0
and the frequency &ohgr;
0
of its concomitant rf field B
1
have to be increased to their limits to maximize the signal-to-noise ratio. Simultaneously, as good as possiblke up to superconducting materials are being used in the primary detection circuit at as low as possible temperatures to minimize conduction losses and inherent electromagnetic noise. In addition the primary detection circuit has to be shielded well-proportioned from external noises.
A probe head of the above mentioned kind is known from U.S. Pat. No. 5,258,710. It comprises a first and a second resonator coupled with it, whereby said second resonator is coupled to a transmitter unit and said first resonator encloses the sample. A radio frequency signal is coupled for excitation of the sample through said second resonator to said first resonator and irradiates the sample being enclosed by said first resonator. Thereafter said first resonator acts like a receiving coil transmitting the received signals to said second resonator. The probe head is being cooled cryogenically on the one hand for improving the signal-to-noise ratio by reducing the thermal noise and on the other hand the conductivity of the probe head is increased by the utilization of superconducting materials for improving the strength of the signal.
Furthermore it is known from U.S. Pat. No. 5,751,146 a surface coil being open at one side fabricated of highly conductive material for NMR experiments, whose conductors are at least three to five times as thick as the skin depth. Thus, its high-frequency resistance is not influenced by the dimensions of the conductors.
Although already much effort was made to enhance the signal-to-noise ratio significantly, so far no NMR-spectrometers became known which achieved a gratifying signal-to-noise ratio, in particular from samples emitting negligible signal radiation.
It is therefore an object of the present invention to provide a probe head as described at the beginning with an optimized efficiency of its antennas and preamplifier unit and with a significantly improved signal-to-noise ratio.
This object is being achieved on the one hand by that at least said first waveguide operates in the range of the anomalous skin effect, whereby the mean free path of the charge carriers at least in said first waveguide being larger than the electromagnetic skin-depth, and whereby said primary detection circuit is provided with means for temporarily matching it with its own characteristic impedance.
The anomalous skin effect with its characteristic skin depth &dgr;
eff
, ensues if the mean free path of the charge carriers 1 is becoming larger than the electromagnetic skin depth &dgr;
em
of the electromagnetic field, 1>&dgr;
em
. Conduction electrons may achieve in particular at low temperatures a mean free path in the range of millimeters to centimeters. Under these circumstances an essentially dissipation free wave propagation becomes feasible regarding the material of the waveguide. Therefore the signal received by the antenna may reach the preamplifier unit with out significant reduction for further processing.
In order that such a waveguide with an extreme low resistance attenuation and extreme high quality factor operating as resonator may be charged with a wave in as short as possible time it has to be matched temporarily with its own characteristic impedance. The analogous has to be performed if the energy of the excitation wave has to be dissipated after termination of the excitation of the sample for initiation of signal receiving. For this e.g. the antenna may be transformed into its characteristic impedance by adding in parallel an impedance via a pin-diode.
The particular advantage of such waveguides operating in the range of the anomalous skin effect in comparison with superconducting ones is in particular due to the first having no problems with fluxoids/fluxquanta.
The utilization of waveguide materials, permitting wave propagation at the anomalous skin effect, is equally advantageous for said first as for any further feasible antennas.
Preferably at least said first waveguide should operate under condition of the extreme anomalous skin effect for minimization of the dissipation.
In this correlation metallic conductors with an inherent resistivity ratio of r
i
>≈10
3
, preferably of r
i
>≈10
4
have proven particularly qualified. The inherent resistivity ratio r
i
=&rgr;RT/&rgr;LT is defined as the ratio of the inherent resistivity of the conductor material at room temperature to the one at low temperatures, preferably at temperatures≦20 K, usually at 4,2K.
Furthermore, the inherent resistivity of the materials should depend as little as possible on the surrounding applied magnetic field, therefore
&Dgr;&rgr;/&rgr;=(&rgr;(
r
i
,T,B
≠0)−&rgr;(
r
i
,T,B
=0))/&rgr;(
r
i
,T,B
=0)
(with &rgr; the inherent resistivity depending on r
i
, T the absolute Temperature and B the applied magnetic field) should be as small as possible, in particular &Dgr;&rgr;/&rgr;<≈≦5 should not be considerably exceeded at T≦20 K and at an inherent resistivity ratio of r
i
≧10
3
.
Ultra pure aluminum has proven to be a particularly qualified metal. Aluminum is preferred with a purity of >99.9999% (6N-aluminum) with very low defect concentration.
However, on principle aluminum with a purity of ≈99.9% is still usable as material for conductors. However it is to be regarded that the surface-resistance R
s
of the conductors changes by orders of magnitude on transition from the anomalous to the electromagnetic skin effect.
Also it is of particular advantage if at least the internal and the external conductor surfaces of the first waveguide possess the same inherent conductivity as the interior of the conductor.
On principle this is valid for the entire primary detection circuit. To this aim the conductor material has to be relaxed completely by annealing and/or ageing, and the surfaces of the conductors for instance will be electro polished in order to remove completely the surface layer which was cold worked during fabrication of the waveguides. In addition the conductor surfaces can be passivated.
By these means a surface resistance in the range of 10
−7
&OHgr; or better can be achieved at operating conditions of T<4 K and at a magnetic induction of 11,744 T, just as in particular in ultra pure aluminum a resistivity ratio r
i
in the range of 10
5
.
In another preferred embodiment said preamplifier unit and sai

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