Possibility of indirect CW detection of two-frequency NQR J. Pirnat a, J. Lu

1
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

• 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 I5/2). 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 lines
  intensity.
• Pounds experiments with quadrupole perturbed NMR
  from 19501 can be regarded as the basis of this
  technique.

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/decreasi
ng of ?2 through the connected transition.
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

• 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
  44.35 26.93 40.99 30.24 3/2?5/2
  88.69 53.85 69.59 40.99
• 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
  30.2440.99). For ?2 it should hold the wider
  the separation of levels, the stronger is the
  relative change of populations. However, our RF
  sources 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.

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 gt1 GHz
  are available (communication applications).
• Further work is in progress.