R. Querzoli

Sullo spettro dei raggi gamma di bassa energia, nella radiazione cosmica

RiassuntoCon uno spettrometro a scintillazione a un canale, munito di un cristallo di ioduro di sodio, si studia lo spettro energetico della componente ultra-molle della radiazione cosmica, all’aperto (al livello del mare) e sotto circa 100 g cm−2. Si trova che tale componente è sostanzialmente costituita da fotoni secondari per la maggior parte di energia compresa tra 50 e 200 keV. La banda ha un massimo molto marcato attorno a 80 keV all’aperto e a 90 keV sotto 100 g cm−2. L’intensità incidente all’aperto viene valutata grossolanamente a 100 quanti al minuto, per centimetro quadrato e steradiante attorno alla verticale. Ciò corrisponde, nell’ipotesi (vera in prima approssimazione) che la radiazione incidente sia isotropa, a un flusso di energia pari all’1% circa dell’energia totale incidente sulla superficie del suolo per effetto della radiazione cosmica. Mediante misure di assorbimento in piombo, si conferma la natura fotonica della componente ultra-molle, mettendo in evidenza la discontinuità dell’assorbimento ai due lati del limiteK di questo elemento.SummaryUsing a Nal-crystal and a one-channel scintillation spectrometer, we have studied the energy spectrum of the ultra-soft component of the cosmic radiation at sea-level, in the open air and below 100 g cm−2 concrete. This component is almost entirely made of secondary photons, mainly between 50 and 200 keV with a prominent peak near 80 keV in the air and 90 keV below concrete. The vertical incident intensity in the air is roughly 100 quanta/min · cm2 · sterad. The ultra-soft component is almost entirely isotropic, so the energy flux is about 1% of the total energy flux due to the cosmic radiation at sea level. The discontinuity of the absorption coefficient at theK critical absorption edge of Pb was put in evidence by means of absorption measurements in this element, confirming the fact that the ultra soft component is made of photons.

Sulle caratteristiche dei fotomoltiplicatori elettronici, usati come rivelatori di scintillazioni

RiassuntoAvendo realizzata la tecnica dei contatori a scintillazione, recentemente introdotta daBroser eKallmann, si è iniziato uno studio sistematico del nuovo dispositivo, allo scopo di individuarne esattamente le caratteristiche e le possibilità. Nella presente note si riferisce su una serie di misure aventi lo scopo di determinare la sensibilità di un fotomoltiplicatore tipo 931A a segnali luminosi estremamente deboli e la dipendenza di tale sensibilità dalle tensioni applicate ai vari elettrodi e dalla temperatura.SummaryThis Institute having constructed the apparatus for the technique of scintillation counters, recently introduced byBroser andKallmann, a systematic investigation of this new device has been started in order to determine exactly its characteristics and possibilities. The measurements carried out as set forth in this first paper served the purpose of studying the behaviour of photomultipliers type 931A under very little light intensities. It was first of all ascertained that under the experimental conditions chosen the leakage current was small as compared with the dark current due to thermionic emission of the photocathode and dynodes. Afterwards, a determination was carried out by means of a d.c. device of the trend of the ratio on the current caused by a standard constant light signal to the total dark current as a function of the feed voltage. Thus, a criterion has been found for classifying and selecting the tubes, for whose strong individual differences a compensation proves to be possible within certain limits through a proper regulation of the feed voltage. In a second series of measurements the effect was studied which is produced on the sensitivity of a photomultiplier by a change in the voltage between photocathode and first dynode, and a third series of measurements was carried out to study the effect of a sinking of temperature from the temperature of the surrounding air to that of solid CO2. Lastly, the results obtained were checked and confirmed by using as a signal the scintillation produced by the γ-rays of ThC″ in a crystal of napthalene.