Patent Description:
Electron paramagnetic resonance (EPR) spectroscopy is also known as electron spin resonance (ESR) spectroscopy; the terms will be considered as synonymous herein, and will be referred to simply as 'EPR' for conciseness. EPR is a powerful tool used in numerous branches of science.

However, in practice, the EPR signal can be relatively small and be subject to noise, such as thermal noise, so that the signal-to-noise ratio (SNR) is low. This means that there is a problem detecting the EPR signal, or there is a problem that long integration times are required to obtain useable data.

The apparatus used to deliver microwave power to a sample being studied by EPR is known as a probehead. The probehead is subjected to an applied magnetic field as part of the EPR, so there is also a problem that any modifications made to the probehead must not be negatively affected by a magnetic field. Furthermore, the probehead is typically inserted into a cryostat to control the temperature of the sample, so there are the problems that the space available on the probehead is limited, and the probehead must potentially operate at cryogenic temperatures.

The present invention aims to alleviate, at least partially, some or any of the above problems.

<NPL> is considered to represent the closest prior art and its disclosure forms the basis of the precharacterizing portion of claim <NUM>.

<NPL>, discloses an EPR apparatus in which a circulator and a preamplifier are mounted in the variable temperature insert of a cryostat.

A first aspect of the present invention provides an insert for an EPR probehead as defined in claim <NUM>.

Another aspect of the present invention provides a probehead as defined in claim <NUM>.

A further aspect of the present invention provides a method as defined in claim <NUM>.

Further optional aspects of the invention are defined in the dependent claims.

Embodiments of the invention will now be described, by way of non-limiting example, with reference to accompanying drawings, in which:.

A conventional EPR apparatus typically comprises a microwave bridge that contains a microwave source and a microwave detector. Microwave power is conveyed via one or more waveguides between the bridge (source and detector) and a probehead. In some cases a single waveguide is used both for the microwave power from the bridge (source) to the probehead and for the reflected microwave power returning to the bridge (detector), in which case a circulator in the bridge directs the returning microwaves to the detector and not to the source. Additionally, the bridge can have separate output and input ports for microwave waveguides (such as for transmission measurements).

The probehead comprises a microwave resonator (resonant cavity), for example of metal or dielectric, that includes a sample space and access thereto. A separate electromagnet applies a magnetic field across the sample space. Depending on the measurement, the magnetic field can be constant, swept or modulated as required. The bridge is coupled to electronics to drive the microwave source and magnet; to collect data from the detector; and to process the data e.g. to perform spectroscopy.

<FIG> illustrates schematically an EPR probehead comprising an insert <NUM> according to a first embodiment of the invention, and a resonator <NUM>. The insert <NUM> is located (or inserted) in the microwave path(s) between the resonator <NUM> and the bridge (not shown). In the preferred embodiment, the probehead is removably insertable into a cryostat <NUM> to enable the temperature of the resonator <NUM> (with sample space and any sample therein) to be controlled, and optionally cooled to cryogenic temperatures e.g. below <NUM>, for example using liquid nitrogen and/or liquid helium. The microwave bridge (not shown) and other electronics can be at room temperature outside the cryostat <NUM>. The probehead can also be operated at room temperature, either inside or outside a cryostat, and a cryostat is an optional feature.

The insert <NUM> has semi-rigid input line <NUM> to convey microwave power from the bridge (not shown) to a directional coupler <NUM>. The input line <NUM> delivers the microwave power to a first port <NUM> of the coupler <NUM>. Most of the input microwave power is transferred to a `transmitted port' <NUM> of the coupler <NUM>. The transmitted port <NUM> is terminated, for example with a 50Ω load <NUM>. The remaining portion of the microwave power is coupled to a second port <NUM> (also known as a coupled port), from which it is conveyed via a microwave path <NUM> (such as a waveguide) to the resonator <NUM>. The proportion of power that is coupled to the second port <NUM> is given by the coupling factor C, which is defined as: <MAT> where PI is the power at the first port <NUM> (input port) and PC is the power at the second port <NUM> (coupled port). The convention of a minus sign in the definition of the coupling factor is used herein, such that, for example, a directional coupler with C = 6dB (referred to as a "6dB coupler") means that approximately <NUM>% of the power is transferred to the coupled port [in a convention without the minus sign in the definition, this would be called a "-6dB coupler"].

Following interaction of the incident microwave power with a sample in the sample space (not shown) associated with the resonator <NUM>, an EPR microwave signal (such as a spin echo) is returned via the microwave path <NUM> to the second port <NUM> of the coupler <NUM>. Because the coupler <NUM> is a directional coupler, the second port <NUM> now acts as the input port, and most of the microwave signal is directed (transmitted) to a third port <NUM> of the coupler <NUM> (reduced by a portion of microwave power that is coupled back to the first port <NUM>, as determined by the coupling factor C). The microwave signal from the third port <NUM> is conveyed to an input <NUM> of an amplifier <NUM> in the probehead. The amplifier <NUM> generates an amplified version of the microwave signal which is output on an output <NUM> and is conveyed from the insert <NUM>, away from the probehead, out of the cryostat <NUM>, and to the microwave bridge (not shown). Preferably the amplified microwave signal is conveyed directly to an input port of the bridge or directly to the microwave detector, by-passing any circulator in the bridge.

The function of the directional coupler <NUM> is to supress thermal noise coming down the input line <NUM>, from the room temperature environment outside the cryostat <NUM>, because most of the noise power is directed to the termination load <NUM> and so does not reach the resonator <NUM>. Hence the SNR is improved. The cost of this is that the maximum power (and hence bandwidth) of applied microwave pulses reaching the resonator <NUM> is also reduced, but the improvement in SNR more than compensates for this as shown later in the results section. One can also use higher power microwave amplification to compensate for the power loss. An embodiment of the invention could use a 3dB coupler (<NUM>:<NUM>), but the preferred range of coupling factor C is from 6dB to 30dB. If C is smaller than 6dB then the noise reduction benefit is relatively small, and of course a significant portion of the echo signal returning from the resonator is coupled back to the input line <NUM> so does not reach the amplifier <NUM> so useful signal power is lost. If C is greater than 30dB, then the applied microwave excitation power reaching the resonator <NUM> starts to become too small. A preferred range of coupling factor C of the coupler <NUM> is from 6dB to 15dB. Exemplary couplers include the Pasternack PE2CP series.

The amplifier <NUM> amplifies the microwave signal before it leaves the insert <NUM>, and it is supplied with electrical power via wires (not shown) from a power supply in the external environment. In some contexts the amplifier may be referred to as a 'preamplifier' because further amplification is usually provided in the bridge or spectrometer, so the terms 'amplifier' and 'preamplifier' used herein should be seen as synonymous. The amplifier preferably has a very low noise temperature, will handle the required microwave power without saturating, and operates over a desired signal frequency range, such as from <NUM> or <NUM> ideally up to <NUM>, incorporating the widely used X-band of microwaves at around <NUM>. The amplifier can be selected to operate over a desired frequency range of interest within the overall microwave band, and different amplifiers can be used for inserts intended for different frequency ranges. Suitable amplifiers can comprise a field-effect transistor (FET). A particularly suitable FET is the high electron mobility transistor (HEMT), comprising a semiconductor heterostructure. The high mobility of the electrons in the structure means that the device has low noise. An exemplary amplifier for use in an embodiment of the invention is the Low Noise Factory LNF-LNC6_20C cryogenic HEMT preamplifier (34dB gain; noise temperature of <NUM> at <NUM>, and <NUM> at room temperature). In a preferred arrangement, the amplifier <NUM> is thermalized via a copper arm extending below the resonator <NUM>.

There are a number of advantages in having the amplifier <NUM> as part of an insert <NUM> for a probehead. When the sample at the resonator is cooled, then the amplifier is also cooled, which lowers the noise temperature of the amplifier (and which wouldn't occur if the amplifier where located for example in the bridge). As already explained, the directional coupler isolates the detection circuit from room temperature noise, but if the amplifier were located in the bridge, then one would add in room temperature noise even if the sample were cold. Placing the amplifier <NUM> in close proximity to the resonator <NUM> also avoids signal losses and the introduction of noise along the microwave path before amplification of the signal. For this reason, a SNR improvement in collected EPR data is even achieved when the probehead and sample are at room temperature (see results). In a preferred embodiment of the invention, the microwave path between the resonator <NUM> and the amplifier <NUM> is less than <NUM>, and can be less than <NUM>, such as down to around <NUM>.

A second embodiment of an EPR probehead insert <NUM> will now be described with reference to <FIG>; like parts will be indicated with like reference numerals as used in <FIG>, and a detailed description of the parts in common that have already been described in the embodiment of <FIG> will be omitted. All of the features of the first embodiment can be imported into this second embodiment, individually or in any combination.

In some circumstances it can be desirable to restrict the microwave power reaching the input <NUM> of the amplifier <NUM> to avoid damage to the subsequent microwave components and damage to the amplifier itself (which is typically the most expensive component of the insert). This is particularly true for measurements using pulsed microwave excitations, which can be very high power, for example <NUM> kW at X-band. A portion of the excitation pulse power may be reflected from the resonator and directed to the amplifier, followed by the spin echo signal, which can be lower power.

A first feature to restrict the power reaching the amplifier is a power limiter <NUM> between the third port <NUM> of the coupler <NUM> and the amplifier <NUM>. The limiter <NUM> can be a single device or can be two or more devices in series. An exemplary limiter <NUM> has a <NUM> W peak power, <NUM> mW flat leakage, recovery time <<NUM> ns, and <NUM>% duty cycle, for example a Narda LIM-<NUM> limiter.

A second feature to restrict the power reaching the amplifier <NUM> is a switch <NUM> to divert microwave power away from the amplifier input <NUM> during the high power microwave excitation pulse, and to switch the spin echo (EPR) microwave signal to the amplifier input <NUM> at other times. <FIG> shows diagrammatically: an input pulse on the input line <NUM> on the left-hand side; the reflected pulse diverted by the switch <NUM> in one state to a termination <NUM>, such as a 50Ω load; the spin echo microwave signal directed by the switch <NUM> in another state to the amplifier input <NUM>; and the amplified spin echo microwave signal at the amplifier output <NUM>. In a preferred embodiment of the invention, the switch <NUM> is a solid-state, fast, non-reflective switch, for example Analog Devices HMC547ALP3E (<<NUM> ns switching time; <NUM> dB isolation). The operation of the switch <NUM> can be controlled by a signal from the electronics associated with the EPR spectrometer and bridge (which also controls the microwave source pulses).

Although <FIG> shows the preferred embodiment, depending on requirements, the limiter <NUM> could be omitted, or the switch <NUM> could be omitted, or the sequence of the limiter <NUM> followed by the switch <NUM> could be swapped. For continuous wave (CW) operation and for sufficiently low pulse power operation, both the limiter <NUM> and switch <NUM> could be omitted, as in the embodiment of <FIG>.

<FIG> shows a further optional feature of the insert <NUM>, namely a temperature sensor <NUM> (such as a resistance thermometer) in thermal contact with the amplifier <NUM> for measuring the temperature of the amplifier. The temperature of a sample can be inferred from a sensor built into the cryostat. Optimal measurements are obtained when the sample temperature and the amplifier temperature are equalized, to ensure proper thermalization of the microwave components of the insert <NUM>. The temperature sensor <NUM> can be used in the first embodiment of the invention of <FIG> i.e. without the limiter <NUM> and switch <NUM>.

The insert <NUM> of any of the above embodiments can be provided as an integrated, compact, three-port package for use with any suitable probehead. Typical insert dimensions are approximately <NUM> x <NUM> x <NUM>.

X-band EPR measurements were performed using a modified Bruker ER 4118SPT probehead equipped with a Bruker X-band ER 4118X-MD5W microwave resonator and using a probehead insert according to the preferred embodiment of <FIG> using the exemplary microwave components as described above for the insert. The probehead was connected to a Bruker ELEXSYS E580 EPR spectrometer, equipped with a <NUM> kW traveling-wave tube (TWT) amplified microwave source.

<FIG> shows the results of measurements of the Hahn echo of a standard coal sample, in a <NUM> outer diameter EPR tube, using pulsed EPR, comparing the signal obtained using an insert according to an embodiment of the invention, and using a conventional setup without the insert (in the conventional setup, the input line is directly connected to the resonator, bypassing the coupler, but the position of the resonator and sample were kept fixed, and all other experimental parameters were kept essentially constant, except the microwave power was adjusted to yield the same duration of the π-pulse with and without the insert). The Hahn echo traces have been shifted to remove any background offset, and the amplitudes have been normalized with respect to the noise level calculated for each trace (i.e. the traces have been scaled such that the noise level is the same with and without the insert).

As can be seen, the sensitivity of the measurement is improved in Figure 3a using a 6dB coupler at <NUM>, with a voltage SNR increase by a factor of approximately <NUM>. This represents a potential reduction in measurement time by a factor of <NUM>. Using a 30dB coupler also at <NUM> in Figure 3b shows an increase in SNR by a factor of approximately <NUM>, representing a remarkable <NUM>-fold decrease in measurement time. This means that experiments that would normally take a full day could be performed in less than <NUM> minutes. The gain in sensitivity can also be used to reduce the spin concentration or sample volume, enabling studies on systems that are conventionally not possible, or not possible at X-band. Figure 3c shows the corresponding results at <NUM> with a 6dB coupler, giving a significant improvement even at room temperature, with the SNR increased by roughly <NUM>, so a <NUM>-fold reduction in measurement time.

<FIG> shows the improvement obtained using an insert embodying the invention with CW EPR. The measurements were performed at <NUM> using a powder sample of [(CH<NUM>)<NUM>NH<NUM>][Zn(HCOO)<NUM>] metal-organic framework, where <NUM> mol% of Zn(II) was replaced by Cu(II). The spectra are normalized to the Cu(II) signal, and in the figure the graphs are vertically offset for clarity. Using an insert with a 6dB coupler, the SNR is enhanced by a factor of about <NUM>.

<FIG> shows the results of HYSCORE experiments at <NUM> and <NUM> mT on a respiratory complex I sample. The <NUM>H HYSCORE spectra (contour plots with skyline projections) obtained in <NUM> hour of signal averaging with and without the insert are presented in <FIG> respectively. The spectrum obtained with the insert shows strong <NUM>H ridges peaked at (<NUM>, <NUM>) and (<NUM>, <NUM>) MHz (<FIG>). The SNR is much worse using the standard setup (<FIG>). The increase in the SNR is better revealed in the corresponding 3D plots presented in <FIG> (with and without the insert, respectively). The SNR is improved by a factor of approximately <NUM> with the insert.

Nitroxide and Cu(II) molecular rulers, with lengths of approximately <NUM> and <NUM>, respectively, were used to assess the SNR improvement for DEER experiments comprising dipole spectroscopy at X-band. The experiments were performed at <NUM> for nitroxide and <NUM> for Cu(II). The primary DEER data was background-corrected to yield the form factor plots of <FIG>. The corresponding distance distributions obtained by Tikhonov regularization are shown in <FIG>. The indicated distances correspond to the maxima of the distributions. The nitroxide results are in <FIG>, and the Cu(II) results are in <FIG>. In all cases, the graphs obtained with an insert embodying the invention (with 6dB coupler), and without an insert, have been displaced vertically from each other for clarity to avoid overlap, with the upper graphs in each figure being those obtained with the probehead insert.

Claim 1:
An insert (<NUM>) for an EPR probehead, the insert (<NUM>) comprising:
a directional coupler (<NUM>), wherein the directional coupler (<NUM>) is configured to receive microwave power from a source at a first port (<NUM>) and to transfer a portion of the received microwave power to a second port (<NUM>) for transmission to a sample space, and wherein the directional coupler (<NUM>) is configured to receive a microwave signal from the sample space at the second port (<NUM>) and to pass the majority of the received microwave signal to a third port (<NUM>);
characterized by further comprising:
an amplifier (<NUM>) having an input (<NUM>) and an output (<NUM>), wherein the input (<NUM>) is arranged to receive the microwave signal from the third port (<NUM>) of the directional coupler (<NUM>) and to produce an amplified version of the received microwave signal at the output (<NUM>) for transmission to a detector.