1913: The U.S. Curie Standard

The International Radium Standards Committee met in Brussels in September 1910 in connection with the International Congress on Radiology and Electricity.  The two most influential members of this committee were the Nobel prize winners Marie Curie and Ernest Rutherford. Rutherford had recently moved from McGill University in Montreal to the University of Manchester in the United Kingdom. Hahn, Boltwood, and RutherfordOther members of the committee were Bertram Boltwood, a radiochemist from Yale University and close colleague of Rutherford; André Debierne, a colleague of Marie Curie’s; Arthur S. Eve, a colleague of Rutherford’s from McGill; Hans Geitel, a German physicist from Wolfenbüttel; Otto Hahn, a German chemist who had spent 1905 – 1906 with Rutherford in Montreal; Stefan Meyer, an Austrian physicist who was head of the Institute for Radium Research in Vienna; Egon von Schweindler, a German physicist; and Frederick Soddy, another collaborator of Rutherford’s from McGill.

Marie Curie, who would soon receive a second Nobel prize, this time for chemistry, had strong ideas about the directions to take towards international standards.  The first order of business of this committee was to agree to the quantity and unit for radioactivity. It was quickly agreed that the unit for radioactivity would be the Curie in memory of Pierre Curie, but there was considerable discussion over the amount of activity that would correspond to 1 Curie. (A working amount of radium at the time was of the order of a few milligrams of the element.) Marie Curie felt that it was inappropriate to use the name Curie for an infinitesimal amount of material, so she insisted that a Curie correspond to a larger amount. Her definition was “la quantité d’émanation en équilibre avec un gramme de radium.” That is, that quantity of 222Rn (3.8 day half life) in equilibrium with one gram of its parent 226Ra.

The second order of business for the committee was to agree on an international standard that could be used to intercompare radium preparations from the principal laboratories in North America, UK, France, Germany and Austria. The preparation of a radium standard was assigned to Marie Curie, and in 1911 she prepared 21.99 milligrams of pure radium chloride in a sealed glass tube. The job of preparing multiple standards for distribution fell to a chemist, Otto Hönigschmid in Vienna, who was expert in gravimetric measurements and a leading authority on measurements of atomic weights.

First Solvay Conference in 1911In 1911, Marie Curie and Ernest Rutherford met again at the First Solvay Conference on physics in Brussels. According to Rutherford, one of the top items on the agenda was discussion of the new radium standard. In March 1912, the committee met again in Paris to intercompare the Curie standard with three Vienna standards of RaCl2 prepared by Hönigschmid. The intercomparison was made by comparing the gamma-ray emission rates of the four samples. The Curie standard was stored at the Bureau International des Poids et Mesures (BIPM) in Sèvres in the Paris suburbs. Hönigschmid was then charged with preparing another set of standards for distribution to the metrology laboratories of several countries. The account of the U.S. national standard is best described in Noah Dorsey’s own words:

Certificate for NBS "Curie" Radium StandardThe International Committee on Radium Standards also arranged for the preparation of standards for those governments that required them. These secondary standards are compared directly both with the International Standard in Paris and with the Vienna Standard. In December, 1913, the United States received its standard. It is designated by the Committee as “Secondary Radium Standard No. 6,” and is certified to have contained in the autumn of 1913, 20.28 mg. of radium chloride, equivalent to 15.40 mg. of radium. This radium standard is preserved at the National Bureau of Standards at Washington, D.C., and is the primary radium standard of the United States. The radium content of all the working standards at the Bureau are determined by comparison with this. Noah Dorsey

Stefan MeyerThe certificate was signed by Stefan Meyer from the Institut für Radiumforschung in Vienna, Marie Curie and Ernest Rutherford. The certificate has three columns and the text is replicated in German, French and English. The radium in this standard had been obtained from pitchblende from St. Joachimstal in Bohemia.

By the spring of 1914, the Bureau was calibrating radium preparations and radium emanations (222Rn) by comparison of submitted samples with the NBS Curie standard. The comparative gamma-ray measurements are described only in summary fashion in Dorsey’s book. It is clear from the Director’s Annual Reports from this period that the Radium Section performed a very large number of such calibrations annually. Director's Annual Report from 1922This large demand for calibrations was driven by the increasing use of radium and radon in radiation therapy. External beams of radium gamma rays were used in “radium helmets” in which several beams were focused on an isocenter for head and neck tumors. Radium needles and radon seeds (sealed glass bulbs containing the emanation from milligram quantities of radium) were used in what is still called in Europe, endocurietherapy. In the United States, it became known as brachytherapy (brachy- is Greek for “near”). One of the largest sources of radium in the United States was mineral deposits of carnotite from western Colorado and Utah. The Radium Company of Colorado and the Radium Refining Plant of the Standard Chemical Company of Pittsburgh were two of the commercial firms that relied on the Bureau for accurate measurements of their working standards.Certificate from Radium Company of Colorado

By the end of World War I, radium had increased in value to about $100,000 per gram, and, since it was so difficult to measure, there was a serious problem with fraudulent suppliers. One of the main jobs of the Bureau during those early years was to assay purported radium preparations to assure the buyers that the samples contained the stated amounts.

1921: Marie Curie visits the United States

By the end of World War I, Marie Curie was probably the most famous woman in the world. She had made a conscious decision, however, not to patent radium or its medical applications. As the price of radium escalated, she found that she did not have sufficient supplies for the radiochemical investigations that she wanted to undertake at the Institute of Radium in Paris.

As for the radium prepared by me out of the ore we managed to obtain in the first years of our work, I have given it all to my laboratory.The price of radium is very high since it is found in minerals in very small quantities, and the profits of its manufacture have been great, as this substance is used to cure a number of diseases. So it is a fortune which we have sacrificed in renouncing the exploitation of our discovery, a fortune that could, after us, have gone to our children. But what is even more to be considered is the objection of our many friends, who have argued, not without reason, that if we had guaranteed our rights, we could have had the financial means of founding a satisfactory Institute of Radium, without experiencing any of the difficulties that have been such a handicap to both of us, and are still a handicap to me. Yet, I still believe that we have done right. Marie Curie

Marie Curie in Pittsburgh photo 1Her benefactress was an American woman, Mrs. W.B. Meloney. Mrs. Meloney launched a campaign to have the women of America contribute $100,000 to buy a gram of radium for presentation to Marie Curie for her use at the Institute of Radium.

Thus, in 1921 Marie Curie made her first visit to the United States accompanied by her two daughters Irène and Eve. Irène Curie’s contributions to radium science will be mentioned later. The younger daughter, Eve, became an author, and her 1938 book, Madame Curie: A Biography by Eve Curie, gives a thorough account of their trip to the United States. Marie Curie in Pittsburgh photo 2One stop was the Radium Refining Plant in Pittsburgh, where Marie Curie toured the chemical extraction facilities used to prepare radium for the U.S. market. Two views of Mme. Curie on the tour are shown here.  The photograph on the right, from the Pittsburgh Sun, appeared at the National Bureau of Standards in Washington, probably about the time of her visit and hung for years in the Gamma Laboratory at the old Bureau site.

Marie Curie and President Harding on the steps of the White HouseOn May 20, 1921, Marie Curie visited the White House to receive the gift of the gram of radium from President Harding. The hazardous source itself was not brought to the ceremony. Instead, she was presented with a golden key to the coffer and a certificate. A replica of the coffer with dummy radium tubes was set on a table in the East Room of the White House during the ceremony. Certificate given to Marie Curie for 1 gram of radium As the document indicates, it was a Certificate for Radioactive Material submitted for measurement and certification to the National Bureau of Standards. It was signed by Samuel W. Stratton, the Director of the Bureau, which was then as now a part of the Department of Commerce. This was not a standard source, but was intended for research purposes. Accordingly, it was subdivided into ten, hermetically sealed, glass tubes of about 100 mg each. Marie Curie with President and Mrs. Harding and Mrs. W.B. MeloneyThe certification was presumably made by gamma-ray comparisons with the NBS radium standard (Secondary Standard No.6). “The residual uncertainty in the numerical value of the radium element equivalent of these several specimens does not exceed 0.7 of one percent.”

Cask used to ship the gram of radium to ParisThe radium was packaged for shipment at NBS and loaded on to the ship for the return trip to France. The box used for the shipment resides in the Curie Museum in Paris. Although Marie Curie’s visit to the White House is recorded in great detail, very little is known about her contacts with the NBS. There is, however, her own report in the biography of Pierre Curie (p. 117). “I have visited with special interest the Bureau of Standards, a very important national institution in Washington for scientific measurements and for study connected with them. The tubes of radium presented to me were at the Bureau, whose officials had kindly offered to make measurements, and to take care of the packing and delivery to the ship.”

1927: NBS gold leaf electroscope

Noah Dorsey gave a concise description of the electrometric methods used for relative measurements of radium in his book Physics of Radioactivity.

NBS gold leaf electroscopeAs an illustration of the electrometric methods, the general procedure followed in the determination by the simple electroscope of the radium content of a small sealed glass tube will be described.The simple electroscope consists of a metal case within which, and near its center, is supported in a vertical position a well-insulated metal strip to the top of which is attached a narrow strip of thin foil, preferably of gold leaf. This strip of foil is usually spoken of as the leaf. The strip of metal and the leaf constitute the insulated system of the electroscope.  When the insulated system is electrically charged by a suitable switch passing through the wall of the case, the leaf is repelled by the strip, and is deflected from its normal, vertical position. In opposite sides of the case are windows through which the position of the leaf can be observed. Such observation is usually made by means of a microscope having in its eyepiece a ruled scale.

When intended for gamma ray measurements, the electroscope should be carefully screened on all sides except one with lead at least one inch thick, so that the air in the electroscope will be protected from scattered radiations that would otherwise enter it. For the same reason, the windows should be as small as is conveniently possible. The ionizing radiation of which the intensity is to be measured enter the electroscope through the unscreened side. In order to minimize the effect of the absorption of the radiation by the wall of the container and by the salt itself, the measurements should be based upon the hard, penetrating radiation emitted by radium-C. For this reason it is desirable that the radiations entering the electroscope be filtered through lead at least 15 mm thick so that very little of the soft gamma radiation from radium-B enters the electroscope.

When everything is ready, all radium preparations are placed at such distances and so screened that they produce as small an ionization in the electroscope as possible – the preparations to be compared must be so placed that they produce only a negligible ionization. The insulated system is then charged and insulated, and the time required for the image of the leaf to move over a few divisions near the middle of the scale in the microscope is determined. From this, the rate of drift of the leaf, in divisions per second, when there is no radium near the electroscope, is determined.  This is called the natural, or the blank, drift. It results from imperfect insulation and slight residual ionization of the air.

Then the tube under test is placed in a suitable position, and the time required for the leaf to drift over a certain portion of the scale is determined. If the blank drift is subtracted from the rate of drift observed when the tube under test is in position, the difference will be the rate of drift due to the radiation from the tube; this is known as the corrected drift.

The tube under test is now removed to its former position where it does not affect the electroscope, and the standard tube is placed in exactly the same position previously occupied by the tube under test. Its corrected drift is determined in exactly the same way as was that for the tube under test.

The ratio of the two corrected drifts is equal to the ratio of the intensities of the two radiations; which, in turn, is equal to the ratio of the amounts of radium-C that are contained in the two tubes, provided that the absorption of the radiations by the walls of the two tubes is the same in both cases. Knowing the amount of radium-C in the standard, the amount in the tube under test can now be computed at once. Noah Dorsey

Woman at the NBS measuring source strength of a radium-226 specimen with the gold leaf electroscopeThe photograph shown here from 1923 is an unidentified member of the radium section using the NBS electroscope. Dorsey did not include a schematic of the NBS gold-leaf electroscope in his textbook, but Leon Curtiss did publish an article on a projection electroscope about two years after arriving at the Bureau.

Schematic of the NBS gold leaf electroscopeThe NBS Standard Gold Leaf Electroscope. The ionization chamber consists of a 10 cm-cube free-air volume, with walls made of 1 cm thick lead sheet with ½ cm thick aluminum lining. A gold leaf is suspended near the center of the chamber and a quartz fiber, 10 µm in diameter, at the free end of the leaf provides a fine scale for optical projection, with a magnification of 100, onto a metric scale. Transit times of the quartz fiber image between two fixed points on the scale 6 cm apart are normally measured.

By the mid twenties the hazards of working with large quantities of radium were well understood by the experts, and Curtiss designed the radium calibration range shown here in such a way that the eyepiece could be observed through a telescope, which increased the distance between the observer and the high intensity sources under test.Radium calibration range

Three decay schema from early publications by Rutherford and Soddy

In the decade that followed the pioneering work of the Curies to separate the new elements radium and polonium from pitchblende, a number of radiochemists and physicists in Europe and North America undertook intense investigations to understand the origins and nature of natural radioactivity. A description of these studies is beyond the scope of this article. By way of summary, however, we show three decay schema from early publications by Rutherford and Soddy. The most simplistic of these charts is from Rutherford’s book of 1906. By 1911, Soddy and others had identified the main decay products in three distinct decay series corresponding to uranium, actinium and thorium. The last chart shows Rutherford’s version of the uranium series in his 1936 book, The Newer Alchemy. With reference to these charts, we will now mention some of the chemical and physical properties of these decay products that are important in interpreting the response of the electroscope.


The concept of isotopes, nuclides which have the same number of protons but differ in the number of neutrons, was understood quite early.  This was essential for radium standards measurements because “mesothorium” was in fact another isotope, radium-228. Since radium-226 and radium-228 cannot be chemically separated, standards could only be prepared from ores that were rich in uranium but deficient in thorium. Both carnotite from Colorado and pitchblende from St. Joachimstal were satisfactory

since they had a low thoria content.

Radon (Radium Emanation)

Radium-226 Decay Chain: Radium-226 (1600 year half life) yields an alpha particle and Radon-222; Radon-222 (3.82 day half life) yields an alpha particle and Polonium-218; Polonium-218 (3.05 minute half life) yields an alpha particle and Lead-214; Lead-214 (26.8 minute half life) yields a beta particle and Bismuth-214; Bismuth-214 (19.7 minute half life) yields a beta particle and Polonium-214; Polonium-214 (0.16 millisecond half life) yields an alpha particle and Lead-210; Lead-210 (22 year half life) yields a beta particle and Bismuth-210; Bismuth-210 (5.0 day half life) yields a beta particle Polonium-210; Polonium-210 (138 day half life) yields an alpha particle and Lead-206; Lead-206 is STABLE.The decay chain for radium-226 is shown here. It was recognized quite early that one of the daughters in the decay chain of uranium was an unreactive gas. The half life of the “radium emanation” or radon-222 is 3.82 days. William Duane working with Marie Curie devised a radon “cow,” whereby one could separate the gaseous radon-222 from the radium-226 parent. This generator could be “milked” every 20 days or so to yield a fresh sample of radon-222 which had nearly the same activity as the parent. This radon could be used in research, but it quickly was found to be useful in therapeutic applications. Glass tubes filled with radon gas soon became widely used in the United States and Europe in intracavitary brachytherapy.

The presence of radon in the radium decay series has implications for interpreting the response of the electroscope. First, as Dorsey noted, the electroscope responded primarily to gamma rays from Radium-C. Thus, one had to ensure that both the standard radium and the radium sample under test were in secular equilibrium with radon and its daughters, at least through Radium-C. In practice, this meant that new radium test samples had to be aged a minimum of 20 days in a sealed container prior to measurement with the electroscope.

Second, one can see that the electroscope could equally well be used to measure radon samples, because Radium-C is in equilibrium with radon within a matter of hours. In fact, NBS used the electroscope increasingly for radon measurements in the 20s and 30s.

Radium-C (Bismuth-214)

Radium-C emits a gamma ray at 609 keV, which is the most abundant high energy gamma ray in the radium-226 decay series. The early investigators did not know the exact energy; they only knew that Radium-C emitted a very penetrating radiation. Dorsey listed half value thicknesses for Radium-C for a number of materials. The half value thickness for aluminum was given as 55 mm. The thick wall of the electroscope was designed to filter out softer (lower energy) gamma radiation from other nuclides in the radium series. These would be subject to more self attenuation in the sample itself.

All of these qualifications and restrictions on the use of the electrometric methods were well known to Marie Curie and Ernest Rutherford, and to Noah Dorsey and those who operated standards laboratories. Radium-226 gamma ray spectrum from high purity germanium (HPGe) detector As Dorsey noted in the quotation above, the method could only be used to measure the relative intensity of Radium-C gamma rays compared to Radium-C gamma rays from a standard of known mass. Another 30 years would pass before dosimetric standards were developed based on the quantity exposure measured in units of roentgen. Uncertainties in measurement included the mass and configuration of the radium salt, the thickness of the glass encapsulation, the positioning of the two sources (standard and test sample) relative to the electrometer, impurities in standard and test sample and the timing of the discharge of the electrometer. Dorsey describes in detail the importance of choosing two stopwatches and ensuring that they are clean and in good working order, and notes the uncertainty in timing with a stopwatch could be several fifths of a second.

1929: Marie Curie visits the Hoover White House

Marie Curie came to the United States for the second time in October of 1929.  Her purpose for the visit was the same; she was to receive a gift of radium from the people of America. This time Marie Curie needed the radium to give to a new Polish Radium Institute in Warsaw. There was considerably less fanfare on the second visit for several reasons. First, she did not receive the radium itself. She was presented with a bank draft for $50,000. Notice that in the 8 years since her previous visit the price of radium had dropped from $100,000 per gram to $50,000 per gram. This was due primarily to the introduction of commercial radium from ore deposits in Katanga in the Belgian Congo. So, the $50,000 was used to purchase radium from the Belgian chemical company that performed the separations.

Marie Curie with President Hoover at the White House in 1929Her visit to the Hoover White House was overshadowed by other events that week in the United States. She arrived in late October of 1929, two days after the stock market crash of the century.  Nevertheless, President Hoover took time to welcome her to the White House and present her with the bank draft. This report appeared from the Associated Press from October 30, 1929:

In the presence of a distinguished company of American officials and scientists Madame Curie was presented today with a bank draft for $50,000 by President Hoover to carry on her researches in the Curie-Polish Cancer Hospital and Laboratory in Warsaw. …, the President said the gift was an expression on the part of the American people of their gratitude for the “beneficient service Madame Curie has given to all mankind.” Associated Press

1937: NBS Hönigschmid standardCertificate for Hönigschmid Standard No. IV signed by Irène Joliot CurieMarie Curie and her daughter Irène in their laboratoryIn 1934, because of continuing concerns about the stability of the 1913 standards, the International Radium Standards Commission invited Hönigschmid to prepare a new set of standards. The starting material was an anhydrous radium chloride salt that Hönigschmid was using for a measurement of the atomic mass of the element. It had been highly purified such that the barium contamination was less than 0.003 atom percent. Three of the twenty 1937 standards came to North America. The NBS received Standards No. XIV and XV, which contained 38.10 mg and 20.36 mg of radium, respectively. After the death of Marie Curie in July of 1934, her daughter Irène Joliot-Curie took her place on the International Radium Standards Commission. She signed the certificates for the 1937 standards along with Stefan Meyer and Ernest Rutherford. Note the closing line that, “These statements are considered correct to 0.3 %.” This would correspond to an uncertainty of 60 µg for the mass of radium in Standard No. XV. This level of accuracy in mass measurements was readily achievable for someone like Hönigschmid.

Hönigschmid Standards of the U.S. and GermanyThree of the Hönigschmid standards are shown here. From left to right they are U.S. Standards Nos. XIV (Hönigschmid No. 5437) and XV (No. 5440) and the primary radium standard of Germany (No. 5426). The RaCl2 crystals within the tubes are clearly visible. A proper gamma-ray intercomparison required that the salt be evenly distributed in the tube, and that the tube axis be parallel to the near side of the electroscope.

1940s: NBS radon measurementsPoster of early radon-222 work at NBSPatricia Mullen with radon-222 emanation apparatusDuring the 1940s, Leon Curtiss and a number of workers in his group, including F.J. Davis, Howard Seliger, Lucy Cavallo, Leroy Stockmann and Patricia Mullen, greatly expanded the capabilities of NBS for radioactivity measurements. After World War II, the emphasis quickly changed to developing standards for fission product nuclides. In the early 1940s, Curtiss and Davis developed a radon emanation and counting system that is shown here.  Radon measurements were important because of exposures to uranium miners in Colorado.  In some cases, miners came to the Bureau and exhaled into a flask of known volume. The flask was then attached to the radon gas handling system, and the activity of the radon in their breath was measured in the pulse ionization chamber.

Pulse ionization chamber used in the 1940 version of the radon-222 emanation systemCollé has described the principles of operation of the pulse ionization chamber as follows:

Alpha particles resulting from the decay of radon and its decay progeny ionize the gas contained in the chamber. Electrons and positive ions liberated in the ionized gas are collected by means of an electric field maintained by the high voltage between the central electrode and the chamber wall. The collection of electrons on the central electrode produces a pulse of current to flow to an output resistor R. Since the voltage drop across R is proportional to the current flowing, the voltage resulting from a single alpha particle will increase and then decrease again as the current fades away. Ronald Collé

Early version of a radon-222 gas-handling systemThe ionization chambers have a volume of about 4.2 liters and operate at a potential of 1200 volts. The chambers were calibrated by introducing a known quantity of radon-222 swept from a standard solution of radium-226.

To meet the nation’s needs for low-level standards, radium-226 solutions in sealed glass ampoules have been provided since 1940. Several different series of standards were distributed, but the characterization of the 1947 series was the best documented. The radium-226 content of the ampoules ranged from 1µg to 200 µg. They were certified at NIST for radium-226 content (mass) by measurements relating them to the Hönigschmid standards.Lucy Cavallo and Leroy Stockmann preparing ampoule standards in a hood

1950s: Calorimetric comparisons of national Ra standards

NBS microcalorimeter used for radium-226 intercomparisonsIn the early 1950s, Wilfrid Mann worked with collaborators in Canada, United Kingdom and Germany to organize bilateral intercomparisons of the national standards (Hönigschmid standards) of the three countries and the United States. Most of these comparisons were done using electroscopes or other means of gamma-ray measurements. Hönigschmid had taken great care in preparing the 1937 standards and all of the exhaustive comparisons mainly served to verify his stated mass values. The only new method to be applied was a microcalorimetric technique developed by Mann. His radiation balance was a twin-cup Peltier-type microcalorimeter. The cups of the balance were of gold, and the balance was enclosed in a temperature-attenuating enclosure, which gave a precision of about ± 0.1 % in comparing the Hönigschmid standards.

The advantage of the microcalorimeter was because response depended on the power generated by the radium-226 decay, eliminating the uncertainties in the gamma-ray measurements introduced by source self-absorption and variable thicknesses in the glass walls. It was necessary, however, to account for the Radium-D (lead-210) and progeny ingrowth from 1934. Making all the appropriate corrections, and using Hönigschmid’s mass values, Mann arrived at an average power per unit mass of 165.83 µW/mg of radium element. At the conclusion of these intercomparisons, Mann and his contemporaries agreed that the set of Hönigschmid standards constituted the international standard for radium-226.Certificate for 1947 series of radium-226 solution standards signed by L.L. Stockmann

By the end of the 1950s, radium-226 standards were no longer considered separate from other standards of radioactivity. The International Radium Standards Commission had evolved into an International Committee for Radiation Quantities and Units. The task of developing and maintaining international standards of radioactivity now fell to Section II of the Consultative Committee for Ionizing Radiation Measurements.

Present status of national standards

In 1999 the U.S. National Institute of Standards and Technology (NIST) (formerly NBS) continues to provide Standard Reference Materials (SRMs) for radium-226 and radon-222 which are related to the original Hönigschmid mass standards. The three original standards, Secondary Standard No. 6 from 1913 and Standards Nos. XIV and XV from 1937, are stored in a protected lead vault in the Radiation Physics Building on the NIST campus in Gaithersburg, MD. Each source is contained in a brass cylinder which has a glass top for inspecting the integrity of the ampoule, and a valve, which is intended to allow one to remove a portion of air from the container to test for presence of radon and daughter products.

There are two interesting points to make about the present status of the standards. First, there is no evidence that the integrity of the glass ampoules has been compromised. Swipe tests of the containers in which they are stored do not show presence of radon daughters. Two possible effects have long been discussed: radiolytic damage to the glass and possible 4He buildup in the ampoules. Although, there is no evidence that these effects have led to a rupture, they certainly indicate that precautions will be needed to handle the ampoules in the future. Second, the Radium-D (22.3 year half life 210Pb) is still not in equilibrium with the parent 226Ra, but, perhaps 85 years is sufficiently long that one can calculate the daughter activities with requisite accuracy.

Over the past decades, interest in radium-226 for therapeutic applications has waned. In 1988, NIST discontinued calibrations of radium-226 sources for the quantity exposure. The M.D. Anderson Cancer Center in Houston and the University of Wisconsin in Madison continue to offer calibrations for those few medical centers still using radium-226 brachytherapy sources. Their calibrations are based on secondary standards that were previously compared to the national standards at NBS.

Radioactivity standards for radium-226 and radon-222 have been maintained, expanded and improved over the past twenty years by Ronald Collé, a radiochemist in the NIST Radioactivity Group, Ionizing Radiation Division, and his collaborators.  There is a continuing demand for radium-226 and radium-228 solution standards because of requirements to measure these nuclides in drinking water.  NIST has continued to provide SRMs based on quantitative dilutions of the 1947 material for radium-226. A sample certificate for SRM 4967 is shown here. This standard was prepared by gravimetric dilutions of the “1947″ series of radium-226 standards which were re-calibrated at NBS in 1967. The “age” of the radium, with accompanying in-growth of the lead-210 (radium-D) is at least 51 years.

Certificate for modern radium-226 solution Standard Reference MaterialAs the detail on the certificate indicates, the standard is certified in terms of the radioactivity concentration of the solution, that is Becquerels of radium-226 per gram of solution. This is computed using a conversion factor of 36.576 kBq µg-1. The standard is still based directly on the mass value from Hönigschmid, by way of a gamma-ray comparison of the 1947 standards with the Hönigschmid standards.

The needs for standards of radium-228 are driven primarily by the provisions of the Clean Water Act. Radium-228 (mesothorium) is a 5.8 year half life beta-particle emitter in the thorium-232 decay series. It is rather difficult to standardize for activity because of the necessity for chemical separations and the complex equilibria of the daughter products. To begin, one must separate radium from thorium, but pure radium-228 is immediately contaminated with daughter products including the 6.1 hour actinium-228 and 1.9 year thorium-228. Solution standards certified for the radium-228 content have been issued periodically over the past 10 years. The most recent standards were prepared by Larry Lucas in the NIST Radioactivity Group.

Modern radon-222 gas handling facilityRadon-222 standards are required for home radon testing. NIST has worked over the past decade with other national standards laboratories and with the Environmental Protection Agency and the Department of Energy’s Environmental Measurements Laboratory in New York to transfer the U.S. national standards to secondary calibration laboratories. This work is described in a series of papers by Collé and coworkers. This is a modernized version of the radon emanation and counting system from the 1940s.

Schematic of modern radon-222 gas handling apparatusSchematic of the NIST primary radon-222 measurement system showing the gas handling and purification manifolds and the pulse ionization chambers.

Collé has compared the radium calibrations based on radioactivity measurements of the entire decay chain with those based on the Hönigschmid mass standards and the two methods agree to within 0.4 percent. Thus, we find that even after 62 years it is difficult to improve on the accurate mass measurements of Hönigschmid.  For that reason, the U.S. national standards for radium are still based on the international standards introduced by Marie Curie in 1912.

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