B. van den Brandt

Long-lived states to sustain hyperpolarized magnetization

Major breakthroughs have recently been reported that can help overcome two inherent drawbacks of NMR: the lack of sensitivity and the limited memory of longitudinal magnetization. Dynamic nuclear polarization (DNP) couples nuclear spins to the large reservoir of electrons, thus making it possible to detect dilute endogenous substances in magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI). We have designed a method to preserve enhanced (“hyperpolarized”) magnetization by conversion into long-lived states (LLS). It is shown that these enhanced long-lived states can be generated for proton spins, which afford sensitive detection. Even in complex molecules such as peptides, long-lived proton states can be sustained effectively over time intervals on the order of tens of seconds, thus allowing hyperpolarized substrates to reach target areas and affording access to slow metabolic pathways. The natural abundance carbon-13 polarization has been enhanced ex situ by almost four orders of magnitude in the dipeptide Ala-Gly. The sample was transferred by the dissolution process to a high-resolution magnet where the carbon-13 polarization was converted into a long-lived state associated with a pair of protons. In Ala-Gly, the lifetime TLLS associated with the two nonequivalent Hα glycine protons, sustained by suitable radio-frequency irradiation, was found to be seven times longer than their spin-lattice relaxation time constant (TLLS/T1 = 7). At desired intervals, small fractions of the populations of long-lived states were converted into observable magnetization. This opens the way to observing slow chemical reactions and slow transport phenomena such as diffusion by enhanced magnetic resonance.

A 140 GHz prepolarizer for dissolution dynamic nuclear polarization.

Apart from their very classical use to build polarized targets for particle physics, the methods of dynamic nuclear polarization (DNP) have more recently found application for sensitivity enhancement in high-resolution NMR, both in the solid and in the liquid state. It is often thought that the possible signal enhancement in such applications deteriorates when the DNP is performed at higher fields. We show that for a dissolution-DNP method that uses conventional (2,2,6,6-tetramethylpiperidine 1-oxyl) radicals as the paramagnetic agent, this is not the case for fields up to 5 T.

Producing over 100 ml of highly concentrated hyperpolarized solution by means of dissolution DNP

New low-temperature inserts compatible with an existing hyperpolarizer were developed to dynamically polarize nuclei in large samples. The performance of the system was tested on 8 ml glassy frozen solutions containing 13C-labeled molecules and doped with nitroxyl free radicals. The obtained 13C low-temperature polarization was comparable to the one measured on 20 times smaller sample volume with only 3-4 times higher microwave power. By using a dissolution insert that fits to the new design, it was possible to obtain about 120 ml of room-temperature hyperpolarized solution. The polarization as well as the molecule concentration was comparable to the values obtained in standard size hyperpolarized samples. Such large samples are interesting for future studies on larger animals and possibly for potential clinical applications.

A compact versatile dilution refrigerator with top-loading capability

Abstract The design and characteristics of a compact versatile 3 He evaporation/ 3 He 4 He dilution refrigerator with central access to the mixing chamber are described. The sample holding device and /or the refrigerator unit can be removed from or inserted into the cryostat at 4 K. The apparatus is reliable and service-friendly. The version with a continuous heat exchanger reaches 50 mK, and at 100 mK its cooling power is 1 mW. A version with two sintered silver heat exchangers reaches around 30 mK.

Detection of nuclear recoils in prototype dark matter detectors, made from Al, Sn and Zn superheated superconducting granules

Abstract This work is part of an ongoing project to develop a superheated superconducting granule (SSG) detector for cold dark matter and neutrinos. The response of SSG devices to nuclear recoils has been explored irradiating SSG detectors with a 70 MeV neutron beam. The aim of the experiment was to test the sensitivity of Sn, Al and Zn SSG detectors to nuclear recoil energies down to a few keV. The detector consisted of a hollow teflon cylinder (0.1 cm 3 inner volume) filled with tiny superconducting metastable granules embedded in a dielectric medium. The nuclear recoil energies deposited in the SSG were determined measuring the neutron scattering angles with a neutron hodoscope. Coincidences in time between the SSG and the hodoscope signals have been clearly established. In this paper the results of the neutron irradiation experiments at different SSG intrinsic thresholds are discussed and compared to Monte Carlo simulations. The results show that SSG are sensitive to recoil energies down to ∼ 1 keV. The limited angular resolution of the neutron hodoscope prevented us from measuring the SSG sensitivity to even lower recoil energies.

Dynamic nuclear polarization in thin polymer foils and tubes

Abstract First results of DNP at 2.5 T and below 0.3 K in thin polymer foils and tubes with the chemical composition (CX 2 ) n , [ X = 1 H, 2 D, 19 F ], doped with TEMPO, are presented. Appreciable polarization of protons, deuterons, and 19 F-nuclei were obtained. The samples can be handled at room temperature for several hours, and therefore they are suitable for new applications.

Polarized scintillator targets

Abstract The hydrogen nuclei in an organic scintillator have been polarized to more than 80% and the deuterons in its fully deuterated version to 24%. The scintillator, doped with TEMPO, has been polarized dynamically in a field of 2.5 T in a vertical dilution refrigerator in which a plastic lightguide transports the scintillation light from the sample in the mixing chamber to a photomultiplier outside the cryostat. Sizeable solid samples with acceptable optical properties and light output have been prepared and successfully operated as “live” polarized targets in nuclear physics experiments.

Full angular distribution of the analysing power in p elastic scattering at 697 MeV/c

Abstract Full angular distributions of the differential cross-section dσ/dμ and of the analysing power Ay in p p elastic scattering have been measured at 697 MeV/c. The results of Ay are compared with the predictions of various theoretical models.

Creating local contrast in small-angle neutron scattering by dynamic nuclear polarization

Low-resolution small-angle neutron scattering measurements can benefit from polarized protons to generate scattering contrast profiles. In a recently developed technique, time-resolved polarized SANS tries to make use of spatial polarization gradients created around paramagnetic centres at the onset of dynamic nuclear polarization. The time constants which describe the build-up of polarization around the paramagnetic centre and the subsequent diffusion of polarization in the solvent were determined by analysing the temporal evolution of the nuclear polarization. The possible use and the limitations of this technique as a spectroscopic tool are discussed.

Search for the hypothetical π → μx decay

Abstract The KARMEN collaboration has reported the possible observation of a hitherto unknown neutral and weakly interacting particle x, which is produced in the decay π+ → μ+x with a mass mx = 33.9 MeV. We have searched for this hypothetical decay branch by studying muons from pion decay in flight with the LEPS spectrometer at the πE3 channel at PSI and find branching ratios BR(π − → μ − x ) −7 and BR(π+ → μ+x)