LABORATORY OF THE MONTH
Puerto Rico Nuclear Center
Headquarters for most of the Puerto Rico Nuclear Center's programs in San Juan and Rio Piedras is the Biomedical Building. The modern facility is an integral part of the New Puerto Rico Medical Center in Rio Piedras
' T ' H E UNIVERSITY of Puerto Rico op-
erates the Puerto Rico Nuclear Center as part of the general structure of the university, under contract to the U. S. Atomic Energy Commission. The Nuclear Center has facilities a t two main locations : (1) The Reactor-Lab oratory Building adjacent to the Mayaguez campus of the University of Puerto Rico, and (2) The Biomedical Building as part of the new Puerto Rico Medical Center which will soon con tain the University of Puerto Rico, School of Medicine, Dentistry, and Public Health. This complex is close to the Rio Piedras campus of the Uni versity. The Nuclear Center also maintains a marine biology laboratory on the coast at Guanajibo Point near Mayaguez and a field station in the rain forest near El Verde. The Center, 70 A ·
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which celebrated its tenth anniversary in October 1967, now has over 300 sci entific and supporting staff. The scientific staff teaches courses in the graduate programs of the Univer sity of Puerto Rico. In addition, the Center offers special noncredit training courses for persons desiring to work with nuclear materials, such as gradu ate students, scientists, technicians, and physicians. A four-week course on radioisotope techniques is taught regu larly at the Biomedical Building. Uni versity graduate students carry out their thesis research at the Center. Many of these are fellowship students from Latin America, and the Center is bilingual in most of its activities. Visit ing scientists and professors spend varying times in the laboratories. The main experimental facility is the
swimming pool type reactor at Maya guez. The reactor a t present oper ates at a power level of 1 megawatt with a flux of 2-3 Χ 1012 neutrons/ cm 2 /sec, but will shortly be changed to a pulsed type facility, which will also operate at a higher steady power level. Two of the neutron beam tubes are uti lized for neutron diffraction studies. One of the neutron diffraction spec trometers is semi-automatic and prints out data at intervals of 0.1° of arc. This facility is at present being used for low temperature studies of the mag netic structure of hydrated salts. Tem peratures as low as 2 °K are obtained by pumping off the gas from liquid helium. Another beam tube is used for the prototype of the large spectrom eters now being used at the High Flux Reactor of the Brookhaven National
Research at the Center encompasses solid state physics, marine biology, radiation and hot-atom chemistry, and special studies of interest to the tropics. Students from Latin America take advantage of the bilingual activities to train in nuclear science
Graduate students use gas chromatographs, including radiogas chromatographs. The equipment in the foreground is for shock wave experiments
Laboratory. The spectrometer is automated so that a crystal can be oriented on three perpendicular axes following a predetermined program fed into a small computer on IBM cards. The data are recorded on IBM cards. Samples can be activated in the pneumatic "rabbit" tube systems or directly in the reactor pool. A fast neutron generator is now being installed to supplement the thermal neutron flux from the reactor. The work in hotatom chemistry covers the mechanism of the chemical transformation in organometallic compounds in which the activated metal atom is attached to carbon. The object is to determine the importance of recombination and retention reactions of polyvalent atoms. Many of the compounds are unstable to air and are handled in lead-shielded glove boxes. The neutron activation facilities are also used in an extensive program of trace metal analysis in marine biogeochemistry. This program is designed to measure the distribution and movements of trace elements from a land mass into the sea, the marine organisms and marine sediments, and to relate trace element transfer to measurements of biological productivity and movements of organic components through the food chains. The principal studies have been carried out offshore from rivers draining three watersheds with different geological substrates. The field area is off the west coast of Puerto Rico, and includes the site of the BONUS (Boiling Nuclear Superheat) power reactor at Rincon. The group is now completing a similar study of the south and north coasts of the Isthmus of Darien in Panama, as part of a feasibility study for excavating a sea-level canal using nuclear explosives.
Robert Weinberg is ajdusting a crystal on two-axis neutron diffractometer VOL 40, NO. 8, JULY 1968 ·
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X-Ray spectrometer used in studies of t h e resonance in radiation effect is shown Sergio Quadri of Chile by S. H. Kamath
The samples are collected by the "Shimada," a 100-ton research vessel with diesel propulsion fitted with the standard océanographie equipment, and a laboratory for field analysis of seawater samples. The uptake of carrier-free or high specific activity radioisotopes is also being studied in marine organisms, and it has been recently found, by using U0 Ag, that silver is strongly concentrated in the hepatopancreas of the spiny lobster. A related project deals with the feasibility of increasing the teachability of mineral deposits through a contained underground nuclear explosion. The resulting fracture of the ore body will increase the permeability to the leaching solution, but the nuclear shock wave may also produce some chemical effects. The studies at present involve chalcopyrite, of which there are large deposits in Puerto Rico. The second major facility in the Mayaguez laboratory is a gamma source contained in a water-filled pool. The source consists of 12 cobalt-60 capsules with a total activity of 1100 curies. The capsules are mounted on an iris-type mechanism and can be moved to various distances from a sample placed at the center, thus permitting variable dose rates. The source is used primarily in studies related to food preservation. It has been found that the shelf-life of the Montecristo variety of banana can be extended by 5 to 6 days by exposing the green fruit to 40 kilorads of radiation. Mangoes can also be preserved, with doses of 150 Krad. At higher doses the fruit softens due to the depolymerization of the pectins. The gamma source is also used for studies in radiation chemistry. Work on gaseous hydrogen chloride has shown that hydrogen molecules are formed 72
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from both excited and thermal hydrogen atoms. This work is being extended to radiolysis using the recoil of fission fragments formed when 235TJ in the sample is bombarded with thermal neutrons in the reactor. Other studies involve the effect of shock waves on organic vapors. Research in the solid state has concentrated on ferroelectrics, and recently a series of alkali hydrogen selenites have been prepared. The sodium salt has been found to have two ferroelectric transitions and its structure is now being studied by neutron diffraction. The 2700 curie cobalt-60 source at Rio Piedras is contained in an underground shielded room. This source is being used to study the radiolysis of heterocyclic molecules in organic glasses at liquid nitrogen temperature. In addition to using conventional 2-methyltetrahydrofuran or 3-methypentanc glasses, a mixture in equal parts of Freon 11 (CC13F) and Fréon 114B2 (CF 2 Br,CF 2 Br) gives transparent glasses. The radiolysis of liquid dimethysulfoxide is also being studied. In the field of organic chemistry the mechanism of the Wolff-Kishner reaction is being studied as well as that of the oxidation of 1,1-diarylethanes. Other projects deal with aspects of the organic chemistry of sulfur. Labelled compounds are also synthesized in connection with these mechanistic studies. The effect of chemical structure on quenching of chemiluminescence is also being examined, in relation to the efficiency of liquid scintillating counting. A General Electric XRD-6 X-ray spectrometer has been utilized in radiobiochemical studies as a source of monoenergetic X-rays, since it has been found that the action of X-rays in the 5-20 KeV region may be wavelength dependent. The inactivation by mono-
energetic X-rays of metalloenzymes such as catalase, is enhanced at the absorption wavelength of the metal atom in the molecule. This radiation effect has also been found in E. coli cells treated with the thymidine analogue 5-bromouracil deoxyriboside, where the frequency of chromosome aberrations was greatest at the Κ absorption edge of bromine. In the solid state field, radiation damage in single crystals of anthracene and phenanathrene is investigated by measuring space charge limited cur rents and by delayed fluorescence. An thracene crystals respond to doses from 102-107 rads. Projects related to agriculture have involved the mass screening of sugar cane for radiation effects. The use of an AutoAnalyzer for sucrose analysis permits the selection of sections of a sugar cane stem. The rain forest project in the moun tains southeast of San Juan includes a section of virgin forest used for radio isotope uptake experiments. By using radioisotopes, it has been found that the uptake and mineral recycling through soil and litter is very slow. Tritiated water is also used to follow the displace ment of rainwater in soils. The use of gaseous nitrogen-15, has recently shown that tree leaves have symbiotic epi phytes which can fix atmospheric nitro gen and contribute to the available nitrogen supply. The Puerto Rico Nuclear Center, by maintaining a broad program of chem istry which also covers environmental studies, contributes to the technological development of Puerto Rico. In ad dition, it provides a training ground for scientists from Latin America, where they can gain additional experience under conditions similar to those which can be established in their home coun tries. Circle No. 93 on Readers' Service Card
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