Abstract: The invention concerns a flow metering techniques, which applies acoustic means and/or methods as embodied for an acoustic flow metering arrangement according to the invention. The flow metering can be implemented with an acoustic flow meter arrangement, which comprises a measuring section (1) provided with a sound source, at least two sound sensors and a reflector, being arranged so that the sound source of the acoustic flow meter arrangement and at least two sound sensors are arranged to mutual pre-defined distances in the measuring section. So, the determination of the sound velocity at rest (c) and the flow velocity (v), from the known distance between the sound source and the sensors, is obtainable from the values for sum (TSUM) and difference (TDIF) of the upstream and downstream transit times of sound between the sensor locations, by utilising the sound as directly propagated and as reflected.

Abstract: An acoustic measurement method of flow measurement for determinating fluid flow velocity or other variables in a measurement pipe section by determining over an exactly determined length (.+-.a.+-.b) the downstream propagation time T.sub.D and upstream propagation time T.sub.U for naturally occurring or artificially generated acoustic wave modes propagating upstream and downstream in the pipe section in the form of plane wavefronts at a low frequency. Pairwise opposed transducers mounted on the pipe are configured by delaying, scaling and summing the signals into at least two direction-discriminating filters, of which one eliminates signals related to downstream acoustic waves while the other eliminates signals related to the upstream acoustic waves in the pipe.

Abstract: The invention concerns a moderator material used for moderation of high-velocity neutrons, in particular of fission neutrons, to epithermal neutrons. The principal components of the moderator material are aluminum fluoride and aluminum metal, which have been formed into a dense composite substantially free of pores, wherein the material contains 20 to 50%-vol. of aluminum metal and 80 to 50%-vol. of aluminum fluoride. Further, the use of the moderator material in accordance with the invention in neutron capture therapy of cancer tumours is described, such as in boron neutron capture therapy (BNCT).

Abstract: Method and device of acoustic flow measurement for measurement of the flow velocities of gases and/or of quantities that can be derived from same. Long-wave sound is transmitted into a measurement pipe (10). The sound signals passing in the gas flow downstream and upstream are detected by means of two sound detectors (14a, 14b) placed in connection with the measurement pipe (10) at a specified distance (L) from one another, the flow velocity (v) of the gas that flows in the measurement pipe (10) being determined by making use of polarity correlation (15;15.sub.1, 15.sub.2) of said signals. The frequency of the sound that is transmitted into the measurement pipe (10) is scanned between a certain minimum and maximum frequency (f.sub.min, . . . f.sub.max). The measurement signals (f.sub.a (t), f.sub.b (t)) arriving from the two sound detectors (14a, 14b) are passed to a narrow-band filter (17a, 17b) or system of filters (17a.sub.1, 17a.sub.2, 17b.sub.1, 17b.sub.2).