A b s h d . ?%e nucleus '"Hf, with its long-lived (T,n = 31 y) high-spin isomeric state i"= 16'. is a chaiienge for new and exotic nuciear physics studies. w e describe the first experiments we have performed in order to produce a reasonable microweight quantity of this hafnium isomer with an isomeric to ground-state ratio as high as possible (here 5%). The reaction '"Yb (4He,2n) using an enriched target has been studied by measuring the excitation functions and the isomeric to ground-state ratio. About 3 X 10' ' isomeric atoms have been produced up to now in irradiations with high-intensity beams (-100pA) at the U-200 cyclotron in Dubna. Chemical separation methods could be checked using about 10'" atoms of the isomer. Isotopic separation experiments have been performed in Orsay and preliminary results are given for the separation efficiency.
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An equivalent circuit of a pickup with a piezoceramic transducer, which takes its electric properties into account more fully, is proposed. Experimental d a~ are presented.Pickups of mechanical quantities based on polarized ferroelectrie ceramics form the main part of the assembly of modern piezoelectric means of measurement. Their fundamental properties are described fairly completely in the elassical theory of piezoelectric transducers [1], which is used by practically all workers in this field. However, with the increasing requirements imposed on measurement accuracy, it has become necessary to supplement it in each special case. We are talking here of the effect of conversion nonlinearity, since this is unimportant in the majority of the pickups that are used.A ferroelectrie ceramic is a material in which the electrical polarization processes are more complex than in an ordinary dielectric. This is characterized, in particular, by the fact that its permittivity e is represented by the sum of two terms [2]where e** is the permittivity due to fast polarization processes, g and 0 are parameters of the material, where 0 has the dimensions of time, and a, is the angular frequency. The resistive component of the currents through a ferroelectric material, situated in a field, is determined by these parameters, and we can therefore assume that the permittivity is a complex quantity. Its real part is described by Eq. (1), and its imaginary part is given by 4~o~o(2) Ect-1+r~202-According to these formula.~, the electric circuit of a ferroelectric ceramic element must have the form shown in the dashed frame in Fig. 1 (the resistance of the insulation is ignored --it is very high). Here 0 = re'. The circuit of a "classical" piezoelectric transducer under the same conditions is simply the capacitance C. The extent to which both circuits differ is mainly determined by the ratio C'IC. According to experimental data obtained on samples of TsTS ceramic in the audiofrequency range it has a value of the order of 0.01-0.03. The conductive component of the admittance is small (the loss tangent is of the order of 0.01) and can be ignored. The same applies to the relative reduction in the capacitance with frequency-over this range.This lack of constancy of the capacitance should affect the property of a piclo_,p with a ceramic piezoelectric transducer. In fact, according to the data'of companies well known for the care they take in developing apparatus (Bruhl and Kjar and Endevco), the charge conversion factor of such pickups over this frequency band, where neither the finite resistance of the load nor the resonance drop has an effect, falls with frequency by 2-3 % per decade. At the same time, the voltage conversion factor under these conditions remains constant.On the basis of existing experimental data, we can assume that for ferroelectric ceramic piezoelectric materials the piezoelectric moddus d depends on the frequency in the same way as the permittivity. This is quite logical since in both cases it is polarization of the mat...
We discuss reasons j'br determining the mounted resonant Aq'equencv of piezoelectric sensors by monitoring the frequency dependence of the electrical impedance of the sensors. We present data for experimental verification of the method.The mounted resonant frequency of a vibration monitoring transducer determines one of the limits of the working frequency range. A procedure using an electrodynamic or piezoelectric vibration tester is used to determine it as a required metrological parameter [ 1 ]. For a mounted resonant frequency higher than 8-i 0 kHz, the only suitable commercial vibration tester is the 4290 tester from Bruel & Kjaer with frequency range up to 50 kHz, maximum vibrational acceleration 1 m/sec 2, and permissible load up to 30 g. An adapter is required to test sensors not having a hole under the M5 stud; in this case the mounted resonant frequency changes, since the sensor together with the adapter is a mechanical system with different parmneters. All this limits the capabilities of this technique. Other devices, such as piezoelectric excitation sources or devices with shock excitation of damping vibrations, also have their disadvantages.In this comaection, we should note one more method suitable for the piezoelectric sensors which are part of most high-frequency devices for measuring mechanical quantities. This method is based on the reversibility of the piezoelectric effect: the electrical impedance of the transducer is a function of its mechanical state, abruptly changing at resonance as a function of frequency.This method is commonly accepted for testing piezoelectric resonators and determining the properties of piezocermnic materials [2]. The test object is included as a coupling element in a resistive four-terminal network powered by a generator with the required frequency range; the frequencies are determined at which the output voltage of the four-terminal network has an extremum, and the maximum corresponds to the resonant frequency. This technique is not effective when used with piezoelectric sensors, since because the mechanical Q of the sensor is small compared with the resonators, the resonant change in the impedance is also small and has a weak effect on the output signal of the tour-terminal network. But if the sensor is included in an ann of a capacitive bridge mad the latter is balanced at a frequency lower than the expected mounted resonant frequency, where the impedance of the sensor does not have any distinctive features, then the desired frequency can be clearly detected. Its value may be read from the maximum of the output signal of the bridge or from the phase shift of the latter relative to the bridge supply voltage as in [ 1]. The readings from both methods practically coincide.In Fig. 1, we show the results obtained when testing the impedance method on Bruel & Kjaer 4370 and 4384 accelerometers with certificate values for the mounted resonant frequency of respectively 17 kHz and 42 kHz. The lines of the spectrum belonging to the different accelerometers ai'e shown on the s...
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