Possibility of indirect CW detection of two-frequency NQR
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Possibility of indirect CW detection of two-frequency NQR J. P irnat a , J. Lužnik a , and Z. Trontelj a, b a) Inst. of Mathematics, Physics and Mechanics, b) Fac. of Mathematics and Physics, Univ. of Ljubljana, Slovenia. Measurement procedure:. Proposed CW two-frequency NQR

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Possibility of indirect cw detection of two frequency nqr j pirnat a j lu nik a and z trontelj a b a inst of ma

Possibility of indirect CW detection of two-frequency NQRJ. Pirnat a, J. Lužnik a, and Z. Trontelj a, ba)Inst. of Mathematics, Physics and Mechanics, b) Fac. of Mathematics and Physics, Univ. of Ljubljana, Slovenia

Measurement procedure:

Proposed CW two-frequency NQR

As a detector of RF transition a super-regenerative oscillator-detector (SRO) can be used. It has a characteristic transfer function and a typical record of a single sharp spectral line is shown below.

FIG.2. SRO spectrum of a sharp line.

FIG.3. Schematic set-up of our “CW” two-frequency NQR (using SRO and synchronous quenching of 1 and 2).

Tuning of the SRO to any of the stronger side-bands 1n.fq ; output offset indicates NQR

Adjusting RF amplitude and quench timing of the second RF field 2 ; step-wise increasing/decreasing of 2 through the connected transition.

Introduction

Multi-frequency pulse NQR as a measuring technique introduces new possibilities in the field of magnetic resonances. In our contribution development and testing of a simple CW parallel to known pulse versions is presented: two RF fields are applied simultaneously at two suitable frequencies to a multilevel quadrupole nucleus (I 1, electric field gradient asymmetry  0).

In some special cases, for instance in the case of detecting a possible presence of some discrete known spectral lines or by evaluating their intensity or shift, the CW techniques based on modern IC components might be useful. The aim of our investigation is to test the feasibility of a technically nondemanding and energy saving CW technique and to improve sample specific NQR detection in this case.

FIG.1. Let the observed quantum system consist of more than two energy eigenstates with several transitions allowed (respective example: quadrupole 121Sb nucleus with I=5/2). When observing the intensity of a chosen line 1, temporary simultaneous RF irradiation (partial saturation) of any other connected transition 2(or(1 +2))can change the former line’s intensity.

Pound’s experiments with quadrupole perturbed NMR from 19501 can be regarded as the basis of this technique.

Recording the output offset of the SRO to see the change when 2 passes the connected resonance.

  • Phenomenon size estimation -

  • - assuming 3 levels and proportionality of the line intensity and the

  • corresponding population difference (linearized Boltzman f.).

  • Population differences at thermal equilibrium…,

  • during saturating trans. E3-E2

  • (equalizing the coresp. pop.)

  • ======================

  •  

  • E2-E1 pop.difference increase E3-E1 pop.difference decrease

  • The 1 signal change at double resonance is overestimated (at least 2x) because of orthogonal orientation of both RF coils and because of neglecting other influences.

  • Relaxation is important and should be considered as well, but it will be done in future. Efficiency of the saturating RF field (magnetic coupling to NQ probes) should be considered too.

  • Technical difficulties:

  • Perturbation of the SRO by the second RF field 2;

  • SRO output fluctuations due to small temperature variations (NQR thermal coefficient) and other instabilities.

  • Accomplished improvements:

  • 1. Empirical adjustment of orthogonality of the saddle coil.

  • 2. Coherent quenching of the second frequency to eliminate its

  • coupling to SRO in the detecting intervals of SRO.

  • 3. Series capacity tuning of the saddle coil to maximize RF at 2.

  • 4. Magnetic modulation and lock-in detection of NQR.

  • 5. Replacement of the quench servo system by fixed grid voltage at

  • SRO.

Experiments with a test sample Sb2S3

We hoped the improvements would surpass

our previous 2-frequency experiments with

nonmodulated 2 and 14N in RDX (~5 MHz),

where often spurious resonances were observed.

FIG.4. Sb2S3 molecule in the crystal

structure Pbnm (2/m2/m2/m).

TAB. I. Two sets ( ~ 0 and  ~ 0.38) of NQR lines [MHz] for 121Sb (5/2) and 123Sb (7/2) at 300K 2

~0: 121Sb(1) 123Sb(1);~0.38: 121Sb(2)123Sb(2)1/23/2 |44.3526.93| 40.9930.243/25/2| 88.6953.85| 69.5940.99

5/27/2 | 80.86| 64.18

Stronger lines, applicable as 1 are those at 26.93, 40.99 and 44.35 MHz. Magnetic moment and its coupling to RF (saturation) are higher by 121Sb, what favors the lines 40.99 and 44.35 MHz. But the range of our RF source is 80 MHz, which leaves us with three 2-fr. combinations: (40.99-69.59)MHz (more promising), (26.93-53.85)MHz and (40.99-71.23)MHz (71.23 = 30.24+40.99). For 2 it should hold: the wider the separation of levels, the stronger is the relative change of populations. However, our RF source’s output power was limited and insufficient for complete saturation. Estimated upper limit of the effect is line intensity change 42% for (40.99-69.59)MHz.

The quadrupole relaxation times T2 of two 121Sb lines at r.t. have been measured as ~80 s and ~100s 2.

___________________________

1. R.V. Pound, Phys.Rev. 79, 685 (1950); A. Abragam, Princ .of Nucl. Magnetism, Oxford Univ.Press, London 1961, p.p.411.

2. I.A. Safin, I.N. Pen’kov, Dokladi ak.nauk SSSR, Fiz.him., T.147, 410-413 (1962).

FIG.5. Time dependence of the SRO oscillations (upper) synchronized with the gated RF radiation 2 (lower) in orthogonal direction .

FIG.6. 121Sb NQR signal near 69.59 MHz (2) recorded indirectly as 1 intensity change of 40.99 MHz NQR.

FIG.7. 123Sb NQR signal near 53.85 MHz (2) recorded indirectly as 1 intensity change of 26.93 MHz NQR.

  • Summary

  • An alternative CW method of indirect detection of interacting multiple NQR transitions is proposed and tested.

  • Present sensitivity is disappointing, but it might be improved by development of new SRO detectors “on chip” or by applying other type CW detectors.

  • The orthogonal coil system should be improved.

  • Expected instrumentation price and energy consumption of such spectrometer are low.

  • The method should be more efficient at high frequencies (higher population differences). Commercial SROs - chips from ~300 MHz to >1 GHz are available (communication applications).

  • Further work is in progress.


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