Measurements with a small (M=24.4 kg) cryogenic gravitational-wave antenna atT=1.5 K

Summary New measurements obtained at the University of Rome by means of a cryogenic gravitational-wave antenna of massM=24.4 kg operated atT=4.5 and 1.5 K are reported. The operation was interrupted after 90 hours of data taking, mainly during successive nights, although the antenna could continue to function very satisfactorily for a much longer time. The rate of disturbances is so small that it can be easily eliminated by coincidences with other antennas. The overall effective temperature obtained with an appropriate data analysis algorithm atT=1.5 K isT _eff∼=0.79 K. This value is mostly due to the noise temperature of the FET preamplifier which constitutes the main limiting factor to the sensitivity of present gravitational-wave resonant antennas.

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Riassunto Si presentano i risultati di nuove misure eseguite all'Università di Roma con un'antenna gravitazionale criogenica di massaM=24.4 kg funzionante aT=4.5 e 1.5 K. Le misure sono state interrotte dopo 90 ore di raccolta di dati, fatte prevalentemente durante la notte, nonostante che l'antenna potesse continuare a funzionare molto soddisfacentemente per tempi assai più lunghi. La frequenza statistica dei disturbi è tale da poterli facilmente eliminare a mezzo di coincidenze con altre antenne. La temperatura efficace dell'antenna funzionante aT=1.5 K, ottenuta impiegando nell'analisi dei dati un opportuno algoritmo, èT _eff=0.79 K. Questo valore è dovuto prevalentemente alla temperatura di rumore di fondo del FET del preamplificatore che costituisce il principale fattore limitante la sensibilità delle attuali antenne risonanti.

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Резюме Приводятся результаты новых измерений, проведенных в Римском Университете с помощью антенны гравитационных волн с массойМ=24.4 кг, работающей при низких температурахТ=4.5 и 1.5 К. Действие антенны было прервано после 90 часов работы, хотя антенна могла продолжать фынкционировть очень удовлетворительно более продолжительное время. Интенсивность возмущений является настолько малой, что их влияние может Быть легко исключено посредством совпадений с другими антеннами. Полная эффективная температура, полученная с помощью анализа соответствующих данных приТ=1.5 К, составляетТ _эфф=0.79 К. Эта величина, главным образом, обусловлена шумовой температурой FET предусилителя, который представляет основой ограничивающий фактор для чувствитеительности резонансных антенн гравитационных волн.

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Research supported by the Faculty of Natural Sciences of the University of Rome and by the C.N.R.     

Initial operation of theM=390 kg cryogenic gravitational-wave antenna

Summary The intermediate gravitational-wave antenna ofM=390 kg and ν₀=1795 Hz has been operated at 4.2 K in the Laboratorio Plasma Spazio of CNR from June 10 to July 18, 1978. The data collected during more than 50% of the time show an effective temperature of 1.5 K on the average, with periods of the order of a few hours during which it goes below 1 K. This corresponds to an instrumental sensitivity of 2 GPU=200 J/m²·Hz. The observed difference between observed and computed equivalent temperature is due to disturbances of mechanical and thermal origin which we believe can be further reduced.

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Riassunto L'antenna gravitazionale intermedia di massaM=390 kg e frequenza ν₀=1795 Hz è stata posta e mantenuta in funzione a 4.2 K nel Laboratorio del Plasma nello Spazio del CNR dal 10 Giugno al 18 Luglio 1978. I dati, raccolti per più del 50% del tempo, corrispondono in media ad una temperatura efficace di 1.5 K, con periodi dell'ordine di alcune ore durante i quali essa scende al di sotto di 1 K. Questa corrisponde a una sensibilità strumentale di 2 GPU-200 J/m²·Hz. La differenza fra il valore osservato e quello calcolato della temperatura efficace è dovuto a disturbi di origine meccanica e termica che noi riteniamo di poter ulteriormente ridurre.

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Резюме Промежуточная антенна гравитационных волн сM=390 кг и ν₀=1795 Гц действовала при 4.2 К в лаборатории плазмы Национального Исследовательского Центра с 10 июня по 18 июля 1978 г. Данные, полученные в течение более 50% времени, указывают эффективную температуру 1.5 К в среднем, с периодами порядка нескольких часов, в течение которЫх эффективная температура опускалась ниже 1 К. Это соответствует инструментальной чувствительности 2 GPU=200 Дж/м²·Гц. Обнаруженная разница между наблюденной и вычисленной эквивалентной температурой обусловлена возмущениями механическиой и тепловой природы, которые, как мы считаем, могут быть в дальнейшем уменьшены.

     

Noise measurements on a tunnel junction d.c. SQUID for a gravitational radiation detector at ultralow temperature

Summary We present noise measurements made at temperatures down to 200 mK on a tunnel junction d.c. SQUID, to be used as amplifier for the gravitational-wave experiment Nautilus of the Rome group. We find that the flux noise decreases with temperature, as expected, with a best measured value of at 0.2 K. The presence of an excess noise term independent of temperature is discussed.     

Magnetic study of the spontaneous brain activity of normal subjects

Summary An investigation of the spontaneous brain activity from normal subjects has been carried out by means of the neuromagnetic method. Two different systems were used for magnetic measurements. Both used 2nd-order gradiometers specifically designed for brain studies: 2.9 cm diameter, 5 cm baseline. A r.f. SQUID and a d.c. SQUID were coupled to the gradiometers. The measured noise levels are and , respectively. Thorough magnetic mapping of the spontaneous activity was performed on a few subjects. By mapping the average amplitude of the α-rhythm signals two maxima of activity over two symmetric regions of the scalp were clearly observed. Despite a general symmetry between hemispheres, however, the complexity of the measured maps suggests a corresponding complexity of the generators responsible for α-activity. Partially supported by Progetto Finalizzato Superconduttività     

A quadratic interaction estimate for conservation laws: motivations,techniques and open problems

In a series of joint works with S. Bianchini [3, 4, 5], we proved a quadratic interaction estimate for general systems of conservation laws. Aim of this paper is to present the results obtained in the three cited articles [3, 4, 5], discussing how they are related with the general theory of hyperbolic conservation laws. To this purpose, first we explain why this quadratic estimate is interesting, then we give a brief overview of the techniques we used to prove it and finally we present some related open problems.