The Discovery of Radioactivity (1965)

[Music] [Music] in 1895 at wburg vilhelm runkin conducted Ed a series of experiments on the effect of cathode rays on certain crystals he discovered that some crystals became fluorescent as soon as they were subjected to the Rays of a cathode tube to establish a definite connection between the cathode rays and the fluoresence of the crystals rkin decided to see what would happen when the light from the tube was screened off he covered the tube with black paper and switched on the crystals immediately became fluorescent again it was now clear that he was dealing with invisible Rays capable of causing fluoresence in crystals and also of penetrating lightproof paper runken realized he was dealing with a new phenomenon he called these mysterious Rays x-rays soon he discovered these Rays had wholly unexpected characteristics in one experiment he wrapped a photographic plate in black paper he removed the red filter from the light then he placed his hand on this plate and exposed it to the Rays the plate was developed the negative showed not just the outline of his hand but also the outline of its bone structure a metal ring on one of the fingers appeared as a bright glare runken had taken the first x-ray since the bones were much better defined on the negative than was the tissue the Rays apparently found it easier to penetrate tissue than bone the metal had resisted the Rays in Paris hry beel working along the same lines as rkin started with the known phenomenon that certain chemical substances became fluorescent after being exposed to sunlight runken had shown that exposure to x-rays caused a similar fluorescence was there Beckel asked some connection like runken he used a photographic plate wrapped in lightproof paper for his experiment on this plate he placed fluorescent uranium salts which had been exposed to sunlight when developed the negative from this plate showed dark spots where the salts had been again it was clear that the paper around the plate had been penetrated by rays but other samples which had not been exposed to the Sun and were not therefore fluorescent were equally effective in blackening photographic plates the Rays could not be due to fluoresence the only explanation was that the uranium salts themselves must be throwing off invisible and hitherto unexplored Rays another another important step forward had been taken beel now turned his attention to uranium salts he placed some salts between two charged condenser plates the indicator of the electroscope went back it had been discharged the charge had leaked away the Rays from the uranium must have caused the air in the condenser to conduct electricity the air had become ionized Beckel had made a completely new discovery that minerals exist which give off invisible Rays capable of discharging a condenser and blackening photographic plates Mari curi a young polish chemist decided to investigate the phenomenon further and began a syst systematic search for other minerals which would give off these Rays she was in fact looking for what we now know as radioactive elements she discovered that the uranium or pitch blend discharged a condenser considerably faster than other minerals did clearly pitch blend must contain a radioactive element much more powerful than those found in the other substances but pitch blend consists of many compounds and contains many other substances beside uranium so Mari curi and her husband Pierre set out to isolate the substance in Pitch blend which was causing the radiation this meant processing large quantities of the mineral and to do this the curies set up their laboratory in an old shed here they began the long job of dividing pitch Bland into its components and analyzing them first it had to be pounded into a more convenient powdered form which was dissolved in acid and various techniques were used to isolate the radioactive elements contained in the original pitch blend the final solution was allowed to crystallize and eventually two powerful radiating substances were found one the C is called polonium after Mari C's native country Poland and the other radium only a minute quantity of radium could be obtained from one ton of pitch blend but the Rays from radium Pro to be millions of times more powerful than those from uranium these they called radioactive Rays Julius ster and Hans GLE two German scientists measured these Rays on on an electroscope similar to this one a positively charged Rod is brought up to the cap of the electroscope where a positive charge is induced in the cap this flows down to the fixed electrode and to an attached but movable Gold Leaf the leaf is repelled but is discharged on contacting the negative electrode on the left in their experiment instead of the charged Rod they connected an I ization chamber to the electroscope the current was switched on radioactive gas formed from the element thorium was pumped into the chamber causing the electroscope to react as it did to the charged Rod the leaf is again repelled discharging itself when it contacts the negative electrode the charge is measured at regular intervals these units of measurement can be joined to form a graph it showed that the radioactivity of a substance gradually diminishes and that radioactive atoms while giving off energy in this way gradually cease to be radioactive now that the existence of these Rays had been discovered and ways of measuring them had been found Ernest Rutherford began to analyze the whole phenomena in more detail he enclosed a radioactive substance in a lead case since radioactive rays are absorbed by lead the Rays could only Escape through a hole in the lid the emerging Rays could be detected by a fluorescent screen Rutherford now discovered that in the presence of a powerful magnet the screen was illuminated at three separate points suggesting that there were three kinds of Ray each responding differently to the magnetic field he called the Rays alpha beta and gamma from the ways in which the Rays had been deflected ruford concluded that Alpha rays were positively charged and beta Rays negatively charged the gamma rays were not deflected so he decided that these were not charged particles but Electro magnetic radiation at this time the atoms of radioactive material were thought to look like this here the uranium is casting off Alpha Rays the moving red particles it was discovered that the radioactivity of a substance diminished in an unusual way for example the radioactive atoms of a gram of thorium Decay by half in 10 seconds after further 10 seconds half of the remaining radioactive atoms Decay this time period in the case of thorium 10 seconds is called the halflife the halflife is the length of time taken for a given amount of substance to lose half its radioactivity the graph of ster and GLE had earlier Illustrated the same theory on this graph we consider an arbitrary Mass n of a substance from the graph we can find the value of no over two and on the time AIS the corresponding time taken to reach this point of De Decay the process can be repeated to find the time to Decay from n/2 to no over4 we can clearly see that the Decay period is the same in each case each substance has its own particular rate of Decline and from these four examples it can be seen that as we have already stated the halflife is the length of time taken for a given amount of substance to lose half its radioactivity when a radioactive atom casts out a ray the atom is changed to one of another element the new element will then cast out another Ray and be changed to yet another element so a definite Decay series may be obtained for the radioactive elements the uranium radium series the actinium series and the thorium series these figures give the atomic weights and positions in a periodic table of elements of a thorium at the atom gives off a positive alpha particle the thorium is changed to the new element mesothorium 1 when mesothorium 1 gives off a negative beta particle it becomes mesothorium 2 it was discovered that the new materials were formed in a definite series radi thorium thorium X thoron thorium a thorium B thorium C thorium C2 and finally thorium d a type of lead which could not be transformed further these inter mediate materials were not new elements but isotopes of known substances The Continuous decay of radioactive elements means that every naturally radioactive substance contains a mixture of the different elements of the Decay series also the various types of Ray can appear simultaneously these two facts meant that the atom could no longer be regarded as stable and indivisible the atom can Decay and in releasing energy gives off alpha beta and gamma rays the discovery of radioactivity led to a new conception of the structure of the atom and of matter [Music]