Presentation Abstract

Recently, the possible existence of genuine spin radiation and some of its expected properties have been outlined [1] within the framework of speculations about possible remote spin-radiation detection & excitation at distances ranging from sub-planetary to astronomical [2,3]. Whether these dreams might come true depends upon the following factors:

(1) Existence of detectable genuine radiation generated by spinning magnetic particles in a magnetic field. It has been estimated that spontaneous spin radiation from isolated spins is too weak to be observed. Most of our experiences regard experimental arrangements based on resonant induction coils and cavities and, as far as we can say so far, they do not emit/absorb any radiation into/from the outside space. In order to explain MR spectra, however, we must bring into picture at least virtual electromagnetic quanta [4] and it would be very strange if virtual quanta were effective, while true radiation were not. The author contends that spin radiation does exist and, once its properties are fully understood, can be investigated by standard spectroscopic techniques employing remote radiation detectors and sources. As far as intensity is concerned, let it be said that even in standard MR techniques, a sample composed of truly isolated spins can not generate any signal. In order to observe a signal, one needs a thermodynamic 'sink' or, in other words, spin-lattice interactions. It follows that one should expect emission/absorption of spin radiation to be appreciable only in the presence of spin interactions with the same random local magnetic fields which cause relaxations. Consequently, the QED model of the emission/absorption of electromagnetic quanta by a spin-endowed particle needs to be revisited, incorporating the presence of random magnetic fields (in a sense, this is a spin analogy of thermally excited atomic radiation emission). Once done, the intensity of spin radiation emission from a real sample turns out to be all but negligible.

(2) Existence of characteristic properties of spin radiation which might distinguish it from any unrelated background radiation. The author contends that such properties exist and are quite striking. They consist in (i) the well known frequency dependence upon the magnetic field intensity, (ii) perfect circular polarization (chirality), and (iii) nearly perfect directionality (beaming). The most striking of these is the last one. It turns out that a sample emits spin radiation exclusively into a narrow cone whose angular aperture is approximately given by the ratio between the homogeneous linewidth and the Larmor frequency of the resonance peak.

This presentation comprises four parts. The first part is dedicated to a detailed theoretical discussion of points (1) and (2) and to how the expected properties of spin radiation could be experimentally exploited in order to prove its existence. The second part then proceeds to discuss the design of actual passive and active remote MR experiments, including receiver and transmitter dislocation, gating, differential chiral arrangements, and phase and chirality cycling.

The third part speculates about possible MR applications on scales ranging from sub-planetary to astronomical. Sub-planetary applications might include studies of Earth atmospheric phenomena (remote ESR of storm systems), water masses, polar ice, and even large-scale petroleum prospecting. The most likely planetary-scale applications concern the Jupiter system with its strong and extensive magnetic field ( ~0.1 T) and complex atmosphere and ionosphere. In all these cases one can envisage a number of actively stimulated experiments involving single or multiple spacecrafts. In other cases, such as investigations of sunspots (fields up to ~0.2 T), the scope might be restricted to passive analysis of the spin radiation they emit. Even such passive techniques, however, could be quite sophisticated.

Finally, we can just dream about spin phenomena occurring on objects like pulsars, where magnetic fields of the order of 100 MT drive the Larmor frequencies of nuclides (including neutrons) up to, and beyond, the optical region and make their spins nearly totally polarized. And, of course, there are magnetic stars with all kinds of intermediate field strengths and magnetars with fields up to 100 GT ... Should the methods designed to distinguish spin radiation from the rest work, we might be in for some surprises once we apply them to the radiation emitted by such strongly magnetic objects.

References:

[1] Talk: Magnetic Resonance in Astronomy: Feasibility Considerations,
      XXXVI-th National Congress on Magnetic Resonance of GIDRM, Salerno, Italy, September 20-23, 2006.
[2] Talk: Perspectives of Passive and Active Magnetic Resonance in Astronomy,
      22nd NMR Valtice meeting of Central European NMR Discussion Groups, Valtice, Czech Republic, April 15-18, 2007.
[3] Poster: Spin Radiation: Properties and Suggested Experiments,
      EUROMAR 2008, Saint Petersburg, Russia, July 6-11, 2008.
[4] D.I.Hoult, N.S.Ginsberg, The Quantum Origins of the Free Induction Decay Signal and Spin Noise,
      J.Magn.Reson. 148, 182 (2001).