Inches Of Lead Research Articles

SU‐E‐J‐165: Shielding Design for An MR Guided Radiotherapy (MRgRT) Suite

Purpose: An MRI on rails is being installed at our institution to provide MR‐guided brachytherapy and teletherapy (MRgRT). The facility is comprised of collinear brachytherapy, MR‐Sim, and linac suites. The MR‐on‐rails translates into the adjacent suites for imaging. An existing bunker and adjacent space were renovated to permit the MR to be moved away from the linac during irradiation. This work describes the design and radiation shielding characteristics of the linac suite within the MRgRT facility. Methods: The original linac vault was designed with a two‐corner maze design to operate at 18 MV. The maze was removed and replaced with three direct‐shielded doors: a sliding console door allow for personnel and gurney access to the bunker; and two double sliding doors through which the MRI translates. The double doors employ an S‐shaped coupling surface to minimize radiation leakage when they are closed. These doors are also RF‐shielded as part of the MR RF cage. The shielding was designed for a workload of 5 × 106 cGy per year; 6 MV beam; maximum dose rate: 1400 MU/min; shielding level 0.05 mSv/year for members of the public and 1 mSv/year for nuclear energy workers. Results: The total thickness of the linac‐MRI sliding doors was 7 inches of lead and 2 inches of stainless steel, producing an equivalent thickness of 8 inches (20.3 cm) of lead. The dose rate at the distal surface of the linac‐MR door is 0.08 mSv/year or 10.9 μSv/hr. The final design includes a copper RF shield and bladder to provide RF shielding. Conclusion: A facility comprising three collinear brachytherapy, MR‐Sim, and linac facility has been designed to meet radiation and radiofrequency shielding limits.

SU‐GG‐J‐160: Radiation Enclosure Shielding Calculations for a Laboratory‐Based Small Animal Conformal Radiation Therapy Device

Purpose: Present radiation shielding calculations associated with a new laboratory‐based small animal conformal radiation therapy device (SACRTD). Method and Materials: SACRTD uses a Seifert 225kV X‐ray tube, which produces ∼100 times more penetrating radiation (at maximum tube voltage) than a typical 120kV diagnostic X‐ray imaging tube. Shielding calculations were performed assuming beam energy of 250kV. The Seifert maximum and leakage dose equivalent rates at 1m are ∼13Sv/hr and <10 mSv/hr, respectively. The a 6×6×6ft3 shielded enclosure consist of a steel frame from which two layers of 10lb/ft2 lead blankets are hung (∼½inch lead equivalent). A lead‐lined door (∼½inch lead) was also constructed. The holding structure for all components was constructed using modular framing systems (80/20 Inc). The X‐ray source is mounted ∼40in from the floor, which is a primary barrier. Shielding calculations were performed following the NCRP Report♯49, which is still applicable to sources up to 500kV. Results: Assuming maximum occupancy and use factor, T=U=1, the thickness of the primary barrier required to shield the primary beam at 1m was determined to be ∼0.6inch of lead while for secondary and leakage radiation were ∼0.3inch and 0.17inch of lead, respectively, for a maximum allowed weekly exposure P = 0.08 R / week. The primary beam was shielded with 1 inch of lead while the secondary and leakage barriers consisted of ½ inch of lead equivalent. In addition, the therapy radiation beam is further shielded by a beam collimation attachment. Other radiation safety features (door interlock, radiation monitors and alerts) have been incorporated. A preliminary radiation survey was conducted. Conclusion: A laboratory‐based radiation shielded enclosure for a SACRTD meets or exceeds the requirements of NCRP Report♯49.

SU‐FF‐I‐13: Accurate Measurement of Lead Shielding In‐Situ

Purpose: To design a protocol for measuring or verifying the amount of lead in a wall of unknown thickness for purposes of radiation shielding in a diagnostic x‐ray room. The aim is to develop a method of accurate measure for thicknesses up to 1/8 inch of lead that can be used with a simple scintillation counter attached to a standard survey meter. Method and Materials: We employed a simple scintillation detector connected to a digital rate/meter scaler combination and a scintillation detector attached to a multi‐channel analyzer (MCA). Measurements were made for a Tc‐99m source in a vial placed in a lead cylinder in alignment with a 1 inch by 1 inch scintillation detector. Lead sheets (2′ × 2′) were placed half‐way between the source and detector. The energy spectra were obtained as well as integrated counts with the survey meter. Background was determined and subtracted from the measurements. Different regions of the spectrum were investigated by integrating over varying energy ranges (variable low energy to 195 keV). A plot of the counts as a function of lead thickness were obtained and compared to the expected attenuation curve for a good geometry. In addition we have investigated the effect of collimation and source geometry. Results: As expected the results for a window about only the photopeak produced the expected logarithmic variation as a function of thickness given sufficient count statistics. At lower energy thresholds, the scatter caused significant deviation from the expected behavior beyond 1/16 inch. An effective half‐value layer was derived as a function of lower energy threshold and thickness. Conclusion: We have developed a method to determine with reasonable accuracy the value of an unknown thickness of lead in the wall of an x‐ray room using a scintillation detector with a digital survey meter.

The development and application of a beta-gamma-gamma multidimensional spectrometer

A beta-gamma-gamma multidimensional spectrometer has been developed which employs two 12″' diameter by 8″' thick NaI(Tl) crystals with associated 4″' thick NaI light pipes and two 5″' diameter by 1 2 ″' thick gas proportional counters as principal detectors. The system utilizes two 40″' diameter by 24″' thick plastic phosphors as active anticoincidence shields in addition to four inches of borated paraffin and four inches of lead as passive shields. The detector system is coupled to two 4096 word computer memories which record gamma-ray events in coincidence with either beta detector. The gamma-ray energies identify the radionuclide while the beta detectors identify the relative position of the radioactivity in the specimen being analyzed. The background and efficiency of the system and its application to the identification of radionuclides in environmental, niological and extraterrestrial materials are discussed.

Radioactive debris over Fairbanks, Alaska, on February 18, 1965

A number of incidental detections of layers of intense radioactive debris have been reported as a result of balloon flights during the atmospheric nuclear weapon testing period 1958–1962 [Anderson, 1965]. Radioactive debris was observed in the lower atmosphere over Fairbanks, Alaska (65°N, 148°W), on February 18, 1965, in the course of a balloon flight to investigate bremsstrahlung X rays in the auroral zone. The balloon-borne detector, a Victoreen IB 85 Geiger counter shielded by 1/32 inch of lead with a narrow slit opening, recorded counting rates estimated to be over three times that from normal cosmic background radiations (see Figure 1). The debris appeared to lie in two or more layers. One layer, as indicated by the region of extremely high counting rate, extended vertically from 6 to 8 km in association with a layer of maximum wind speed. Such an association was reported previously by Mantis and Winckler [1960]. Above 8 km the counting rate is irregular but always higher than the normal rates measured on previous flights. Evidently the radioactive debris was inhomogeneously distributed in this turbulent region.

Electroproduction de muons de trés haute énergie

We evaluate the muon beam intensities which might be obtained from existing and projected high-energy electron accelerators. First we give the cross-section for the photoproduction of a μ (+ or -) in a forward cone of small angle, taking into account a nuclear form factor. The number of muons of given energy produced by an electron is then obtained by integration over the thick target gamma-spectrum. For this calculation we make several approximations, which are subsequently discussed; this leads us to suggest that a few radiation lengths of a heavy element (e.g. 1 or 2 inches of lead) will give the best results. Finally we give an estimate for the pion contamination; the ratio of pions to muons is roughly of the order of unity, provided that the angle of the production cone is taken sufficiently small (a few degrees). The results of our calculations are displayed in curves, which show the muon intensities which might be obtained from 6, 12 and 25 GeV electron accelerators; other curves show the pion contamination.

Storage facilities for radioactive materials: practical homemade units.

Because of widely varying requirements for the storage of radioactive materials, and the few types and relatively high cost of commercially available safes, we believe a description of some practical and locally constructed facilities may prove of value. In the interest of brevity, a minimum of description will be presented, since the illustrations are largely self-explanatory. General Requirements Storage facilities for radioactive materials should provide the following: 1. Adequate protection for personnel, depending upon the types and maximum amounts of radioactive materials which may be on hand at any one time. 2. Security against loss and accidental exposure to radiation. 3. Adaptability to the varying requirements of individual circumstances. 4. Economy through simple design and use of locally available materials. 5. Efficiency. Storage safes should be easily opened and closed to provide for rapid removal and insertion of radioactive materials to minimize the time during which these materials are not behind protective barriers. The barriers should be sufficiently thick to permit close proximity of storage safes, loading bench, and the transport cart. Storage Safe Construction: Photographs and construction details of a storage safe are shown in Figures 1–4. This safe was constructed for a maximum of 250 millicuries of cobalt 60 and the occasional storage of isotopes of low activity, such as iodine 131, gold 198, and phosphorus 32. The storage cavity is 4×8×4 inches deep, with a minimum of 4 inches of lead at the sides, bottom, and top. Standard lead bricks, 2×4×8 inches, were stacked so that it was not necessary to cut any of them and, after assembly, were forced into close contact. This was done by driving thin layers of Masonite between the bricks and the steel frame, but could have been accomplished by hammering a piece of wood placed against the bricks at appropriate points. The sheet of steel supporting the top was designed to be slightly higher from the table than the height of the bricks forming the walls. Thin sheets of cardboard were then placed in the bottom of the frame and a trial stack of bricks reassembled until the top surface of the wall began to engage the sheet of steel. A close approximation of the walls and the top was then obtained by filing away the bricks. The top is made of 12 bricks and weighs about 300 pounds. The ball-bearing rollers supporting the top are each 300 pounds test, permitting a weight of up to 1,800 pounds, if necessary. Security: Locking of the safe is provided for by a steel pin welded to the sheet of steel supporting the top. This pin engages a hole in the frame when the top is closed, as shown at H in Figure 3. When a padlock is fastened through the holes in the other end of the steel plate and the frame, the top is completely locked to the frame.

THE SUN AS A SOURCE OF COSMIC RAYS OF INTERMEDIATE ENERGIES

The cosmic ray intensity as measured with an extremely narrow-angle telescope, 1.2 × 10−3 steradians, and with 96 inches of lead as absorber for the period 1 January 1955 to 31 December 1958 shows an increase of 20%. This increase is attributed to particles coming from the sun. It is shown that the change in hour of maximum of the first and second harmonics can be explained by a change in the relative importance of the impact zones. This phenomenon is attributed to a change in the number and polarity of sunspots.

The part-thigh ratio: a useful aid in evaluating the uptake of I 131 by thyroidal cancer.

Radioiodine has been used at the Mayo Clinic in the treatment of a certain limited group of patients with thyroidal cancer since 1947. In the main these patients have had distant metastatic foci that selectively concentrated radioiodine. The presence of actively functioning metastatic lesions can frequently be detected by carefully scanning the body with a suitable detector, such as a Geiger counter or scintillation counter. In order to detect or evaluate suspected regions of abnormal radioactivity, however, following the administration of tracer amounts of radioiodine, it is worthwhile to determine in some fashion the distribution patterns of radioactivity to be expected in patients without thyroidal cancer. We have found the technic to be described a simple, reliable, and useful one for the in vivo determination of distribution patterns of radioactivity in patients who have received tracer doses of radioiodine. Method and Group Studied Briefly, the method consists of measuring the counts per second recorded by placing a properly collimated detector in contact with certain standard spots on the body, always including a site over the anterior lower part of the thigh. Calculations are then made of the ratio of counts per second over the part in question to the counts per second over the thigh. In our laboratory a Texas type of Geiger-Müller tube surrounded by 1 inch of lead for shielding, with a 3-cm. aperture, is used for the counting. A sensitivity curve for I131 at a distance of 10 cm. is shown in Figure 1. The instrument is placed with the collimating cone in gentle contact with the surface of the part in question and counts are obtained; it is then placed over the right thigh at the junction of the lower and middle thirds of the femur and counts are obtained there. The procedure is illustrated in Figure 2. Counts are made routinely at twenty-four and forty-eight hours after oral administration of suitable tracer amounts of radioiodine (I131). Enough counts are obtained to minimize random errors as much as is practicable. It is essential to maintain a constant detector-tube geometry and a constant spectrum of photon sensitivity in the detector for the resulting ratios to have consistent values for similar distribution patterns of radioactivity. It should be emphasized at this point that the actual numerical values obtained in different laboratories will vary from one another if identical instrumentation is not used. An illustration of this point the following short experiment was done. Ratios of part to thigh (part-thigh ratios) were obtained simultaneously with a scintillation detector and a Geiger-Müller counter on a series of 10 patients without cancer. The scintillator had a much higher counting rate, wider viewing angle, and greater sensitivity to scattered photons of reduced energies.

Attenuation of 14-Mev Neutrons in Water and in 6-In. Lead and Water

Curves of dose rate as a function of distance were prepared from measurements made with indium and phosphorus detectors in water and in water faced with 6 inches of lead. (D.E.B.)

A preliminary report on the use of gallium in clinical tracer studies.

Dudley (1–5) and King (6, 7) have reported on the distribution of radiogallium in animals and on its excretion by both man and animals, while Mulry and Dudley (8), in 18 patients with radiographic evidence of malignant bone tumors, demonstrated increased deposition of gallium72 in the affected skeletal areas in 15. Conclusions based on these studies may be summarized as follows: 1. Gallium72 is a short-half-life isotope (14.3 hours) with energetic gamma emissions (the value for Iγ is 13.3 compared with 8.47 for radium) and moderately energetic beta particles, with an average energy of 0.45 mev. 2. Biological studies indicate that gallium72 has an early deposition in growing bone and that the more rapidly growing the bone the more rapid the uptake. It is excreted almost primarily by way of the kidneys, with a very small amount in the feces. It is apparent from the data presented that this isotope might be employed as a diagnostic tracer in studying the spread and extent of certain types of bone tumor. The purpose of this report is to present the preliminary results of a project utilizing gallium72 for clinical tracer studies. The objective was to evaluate this isotope as a possible diagnostic tool (a) in the determination of the spread of primary bone tumors to areas other than the primary site and (b) in the determination of the site and extent of early bone metastasis from malignant lesions in the soft tissues. The results presented here concern the preliminary part of this project, during which 72 tracer studies were performed on 63 patients. The advanced portion of the study is now under way with a simplified and improved technic and a more careful selection of patients. It is anticipated that the results will be more conclusive than those to be reported here. A continuous study was made of a group of patients with proved cancer of a type likely to metastasize to bone. Gallium72-tracer studies were repeated at three-to six-month intervals along with routine bone surveys at corresponding periods. The final evaluation cannot be made until sufficient time has elapsed for radiographic evidence of bone metastases to develop in a significant number of the patients. When this significant number is available, an attempt will be made to correlate the gallium studies with the radiographic findings. Procedure A tracer dose (0.5 millicurie) of gallium72 citrate, prepared as described by Dudley (4), was administered intravenously. Counts were made by means of an end-window Geiger tube (1∕2-inch diameter window with 3 1∕2-mg.∕cm. window thickness) shielded by 1 inch of lead. The distance between the window and end of the lead collimator, or the sleeve, was 1 inch. With the patient supine, skin surface counts were made at thirty-seven points over the skeleton, as indicated in Figure 1.

The Penetration of Particles Associated with Cosmic-Ray Stars. II

A second series of measurements has been made at Climax, Colorado (elevation 3510 meters), of the particles associated with nuclear explosions, or stars, in a thin-walled ionization chamber. The associated particles were detected by observing coincidences between the ion chamber and appropriately placed trays of Geiger counters. The experiment was planned in such a way as to distinguish between those particles which are products of nuclear disintegrations which emerge from the ion chamber and those particles which are incident in the atmosphere in association with the particles producing the stars. Absorption measurements were made of both the low energy and penetrating product particles. There was no conclusive evidence indicating the presence of electrons or photons among the disintegration products. The absorption mean free path in lead was not exponential but increased to 190\ifmmode\pm\else\textpm\fi{}40 g/${\mathrm{cm}}^{2}$ at the maximum of eight inches of lead. Absorption measurements of the associated particles which are not disintegration products of the stars indicate that they are parts of local penetrating showers having mean free paths greater than 500 g/${\mathrm{cm}}^{2}$. Absorber placed over the ion chamber produced bursts of electrons in the chamber whose number greatly exceeded that of extensive air showers. These bursts have been tentatively identified with narrow air showers.

THE SUDDEN INCREASE IN COSMIC RAY INTENSITY, NOVEMBER 19, 1949

Cosmic ray counter telescopes were in continuous operation on November 19, 1949, at Ottawa (geomagnetic lat. 56.8°N) and at Resolute (geomagnetic lat. 83°N) in the Canadian Arctic. The two sets of apparatus were identical and consisted mainly of two counter telescopes, one with six inches and the other with 14½ inches of lead between counters. At Ottawa the counting rates of the individual counter trays were recorded as well as that of the coincidences.On November 19 at about 1048 GMT a sudden increase occurred at Ottawa. The maximum intensity reached about 70% above normal in a few minutes, then decayed slowly to normal in about four hours. The increase was similar in both counter telescopes and in the individual counter trays. At Resolute an increase was noted but of a different type. The intensity did not rise suddenly but increased slowly to about 20% above normal and returned to normal again in about three hours, centered about 1140 GMT. These results and those of other observers indicate that the beam of rays responsible for this increase must strike the earth on a limited area only. Some deductions can be made about the extent of this area.

Preliminary clinical experience with the betatron.

The betatron is primarily a physics research tool and industrial x-ray machine. We are exploring its medical possibilities and realize that it will be a long-term project. The present report high-lights some of our progress, some of the obstacles encountered, and some of the early biological effects we have observed. As its name implies, the betatron is an agency for producing high-energy electrons, i.e., beta particles. The desirability of great acceleration of electrons and the limitations of ordinary transformers for this purpose were recognized many years ago. As far back as 1922 the general idea of speeding electrons in a magnetic field was recognized as feasible. Since that time many university and industrial groups have tackled the problem, and in 1940 Kerst (1) succeeded in accelerating and guiding electrons into a useful beam of x-rays. At present we are converting these high-speed electrons into extremely powerful x-rays but, as Uhlmann and Skaggs (2) have shown, electrons can be brought out as a separate beam for future investigation. The betatron differs from other accelerators in that it is quite compact, gives continuous acceleration to the partides, and uses only light particles. In contrast, any accelerator involving the cyclotron principle gives an intermittent or discontinuous type of push to the particle. Several of the larger types of accelerators are better at moving heavy particles than electrons. The first problem one faces in housing a betatron is the protection of operating personnel and neighbors from harmful radiations. In our installation, personnel are protected by a wall of 30 inches of concrete and 4 inches of lead separating the treatment magnet from the control rack. This wall runs parallel to the main axis of the beam and keeps the radiation level below 0.3 r per week (forty-hour operating time) in the control and preparation room. Our neighbors are protected by concrete walls and the underground direction of our beam. The problem of protecting animals or patients in front of the treatment magnet from scattered electrons, scattered x-rays, and neutrons is not so easily solved. Scattered electrons are reduced by interposing blocks of lucite; scattered x-rays are reduced by the heavy laminated lead collimator through which the useful beam passes, and by a massive external lead panel (Fig. 1). The neutrons are more difficult to reduce; at present, we are eliminating all possible sources of the gamma-n reaction in the path of the main x-ray beam. The determination of quantity of radiation at any given point is not easy, because of the long forward travel of the scattered rays and the behavior of ionization devices at high-energy levels. The rather limited amount of ionization occurring in air prevents us from adhering to the strictest detail of the definition of the roentgen.

Narrow Air Showers of Cosmic Rays II. Absorption Measurements

The absorption of narrow air showers in Pb, Fe, and Al has been measured at sea level. Results indicate that there are two components in these narrow air showers, a soft component which consists of electrons and a hard component which consists of $\ensuremath{\mu}$-mesons. The soft component is practically completely absorbed in four inches of lead, whereas the hard component is not appreciably absorbed in 8\textonehalf{} inches of lead. A wooden roof of only 1 g ${\mathrm{cm}}^{\ensuremath{-}2}$ produces many local showers, which are nevertheless totally absorbed in a very thin layer of material.

The Penetration of Particles Associated with Cosmic-Ray Stars

A second series of measurements has been made at Climax, Colorado (elevation 3510 meters), of the particles associated with nuclear explosions, or stars, in a thin-walled ionization chamber. The associated particles were detected by observing coincidences between the ion chamber and appropriately placed trays of Geiger counters. The experiment was planned in such a way as to distinguish between those particles which are products of nuclear disintegrations which emerge from the ion chamber and those particles which are incident in the atmosphere in association with the particles producing the stars. Absorption measurements were made of both the low energy and penetrating product particles. There was no conclusive evidence indicating the presence of electrons or photons among the disintegration products. The absorption mean free path in lead was not exponential but increased to 190\ifmmode\pm\else\textpm\fi{}40 g/${\mathrm{cm}}^{2}$ at the maximum of eight inches of lead. Absorption measurements of the associated particles which are not disintegration products of the stars indicate that they are parts of local penetrating showers having mean free paths greater than 500 g/${\mathrm{cm}}^{2}$. Absorber placed over the ion chamber produced bursts of electrons in the chamber whose number greatly exceeded that of extensive air showers. These bursts have been tentatively identified with narrow air showers.

Production of mesons by X-rays.

At the spring, 1949, meeting of the National Academy of Sciences(l) a preliminary account was given of some observations of mesons produced by the 335 Mev x-ray beam from the Berkeley synchrotron. The present paper is a progress report of this work; no claim is made for completeness, but sufficient new data are available to make publication at this time worth while, especially since some of the numerical results given in the earlier report require revision. The x-ray beam, produced by the impact of 335 Mev electrons on a 20 mil thick platinum target, has a width at half maximum of 0.0135 radian (about 1 inch at 6 feet from the target). In all but the earliest experiments the beam was further defined by a 1 inch hole in a lead block, then passed through a piece of carbon which served as the meson source. The x-ray intensity at one meter from the target was about 3500 r per hour (measured behind 1/8 inch of lead) under the best running conditions; the average was about half this. The actual exposures at the carbon meson source (6 feet from the target) ranged from 500 to 2000 r in the later runs.more » Mesons were recorded on Ilford nuclear plates; the highest density of meson endings observed was about 100 per square centimeter in a 100 micron emulsion. In the following sections the experimental conditions and some of the results are described in more detail.« less

A Cloud-Chamber Study of Extensive Air Showers at 3260 Meters Elevation

The penetrating particles in extensive air showers have been observed in a cloud chamber under 6 inches of lead at an elevation of 3260 meters. If a group of air showers is selected in such a way that their cores mostly land between 4 and 24 meters from the chamber, the penetrating particles are seen to consist mainly of $\ensuremath{\mu}$-mesons which have been produced in the air above the apparatus. If a group of air showers is selected so that their cores are mostly 6 meters from the chamber there are again many $\ensuremath{\mu}$-mesons and in addition there is evidence for local production of penetrating particles in mixed showers and high energy nuclear disintegrations produced by both neutral and ionizing penetrating particles. Thus we have evidence that high energy neutral particles and highly interacting ionizing particles make up an appreciable portion of the penetrating particles in the cores of air showers. There are more cascades of energy >${10}^{8}$ ev under the lead shield than can be accounted for by the radiation and knock-on processes of the observed number of mesons.

Clay's work on cosmic rays

During the last thirty years the greater part of Professor J. Clay's activity in physics has been devoted to cosmic-ray research. A full account of his contributions to this comparatively young branch of physics can only be given in connection with the story of cosmic-ray physics from Victor Hess's discovery down to the present times. Yet his work may be regarded as complete in itself to such an extent as to justify an attempt to let its picture stand by itself and be the re: flection of the development of an entire era. Because of their remarkable penetrating power, cosmic rays were at first supposed to consist of electromagnetic radiations rather similar to X-rays but of much shorter wavelengths and accordingly stronger penetration. Opposed to this line of thought advocated by the Ameri can scientist Robert A. Millikan was the corpuscular concept according to which the radiation was composed of charged particles. Much dis cussion took place before this fundamental point was finally cleared up by the historic voyages of 1927 and subsequent years between the East Indies and Europe, and indeed even after. In the course of these voyages, Clay proved that the intensity of cosmic radiation at sea level between Genoa and Batavia varied according to the geomagnetic latitude, its value being smallest at the geomagnetic equator. Although his observation of the interaction of the geomagnetic field with cosmic ray particles provided conclusive evidence that they carried an electric charge, it was quite a time before his results were generally accepted, partly because it was at the time extremely difficult to take accurate long-term measurements, and partly for non-physical reasons. Subsequent expeditions to equatorial regions, the southern hemi sphere and the fjords of Norway completed the picture of the latitude effect of the two components into which the radiation could be divided, the soft component that only penetrated a few inches of lead, and the hard component that penetrated many feet and more. As the absorption suffered by the particles of the hard component in

A Study of Cosmic-Ray Bursts

A fast ionization chamber, suitable for the investigation of cosmic-ray bursts, is described. The operation of the chamber and of the associated electronic circuits is discussed. The nature of the cosmic-ray events responsible for the production of ionization bursts was studied (a) by observing coincidences between several ionization chambers, (b) by analyzing the shape of the ionization pulses, (c) by operating an ionization chamber inside a cloud chamber which was expanded whenever the ionization chamber recorded a pulse. The experimental results led to the following conclusions: (1) at mountain altitudes 98 percent of the ionization pulses in an unshielded chamber of the type used are produced by nuclear disintegrations, the remaining 2 percent by air showers; (2) in the nuclear disintegrations, heavily ionizing particles of sufficiently high energy are produced to cause coincidences between neighboring chambers; (3) nuclear disintegrations are produced mainly by non-ionizing rays, which are possibly neutrons, certainly not photons; (4) about 12 inches of lead is necessary to reduce the intensity of the radiation which produces nuclear disintegrations to $\frac{1}{e}$ times its original value.