Nuclear Chemistry in the Chemistry Department.

The National Laboratories were spawned from the Manhattan Project. Not coincidentally, nuclear chemistry and nuclear physics burgeoned after World War II, and they played a major role in the Physics and Chemistry Departments of the Laboratories (including Brookhaven).

What differentiates nuclear chemistry from nuclear physics? A very subjective definition. One difference is that (at least in the early years), physicists used physical methods; they measured physical properties. Chemists, on the other hand, used chemical methods. They used chemical reactions to separate elements and isotopes. They used chemistry to determine properties of new isotopes and to identify new elements

Recognizing the special place nuclear chemistry occupies in the Department's history, the following is the first installment of the story of nuclear chemistry in the Chemistry Department at Brookhaven.

High-Energy Proton Interactions with Complex Nuclei
BNL Chemistry Department 1947-1966: Rapid Growth

Background: When protons were first accelerated to hundreds of MeV after the end of World War II, it became apparent that an incident particle could no longer be captured by a target to form a Compound Nucleus. Instead of the expected peak in yields well below the target mass, a broad distribution extending down from the target was observed, a process termed Spallation*.  Nucleons, alpha particles, and other light Evaporated Particles were also present. A peak due to Binary Fission, well separated from spallation residues and evaporated particles, was observed from heavy-element targets. A two-step picture was developed to account for these observations. In an initial prompt IntraNuclear Cascade (INC), the projectile interacts with individual target nucleons in a series of quasi-free collisions, ejecting some. The excited residual nuclei subsequently decay by particle evaporation, with fission competing in the case of heavy elements.  Detailed calculations using this Cascade plus Evaporation/Fission model were not practical prior to the advent of high-speed digital computers.     

Introduction: A new energy region opened in 1952 when protons were accelerated to 2.2 GeV at the BNL Cosmotron. The energy range was extended to 28 GeV at the AGS in 1960. Studies of the interactions of high-energy protons with complex nuclei expanded during this period to become a major component of research in the BNL Chemistry Department. Radiochemical techniques were particularly well suited to study the broad distributions in charge and mass encountered in high-energy interactions. While emphasis was on the unique BNL facilities, university and other national laboratory facilities were employed where appropriate. Collaborators such as J. M. Miller from Columbia, N. Sugarman and A. Turkevich from Chicago, and L. Yaffe from McGill brought their experience from lower-energy synchrocyclotrons to the fledgling BNL group. Initial experiments [2,3] at 2.2 GeV led to research in several areas:

Light Fragment Production:  Production of nuclides such as ¹⁸F and ²⁴Na from targets from copper to uranium is a characteristic high-energy process. Cross sections increase by about two orders of magnitude between 0.4 and 6 GeV [4], then become energy independent above 10 GeV [5]. In the case of copper, the rise can be accounted for as a broadening of the spallation distribution. However, the observed copious yields of such light fragments from lead targets at GeV energies were not anticipated from known mechanisms and a new process, Multifragmentation, was proposed [6]. Energy and angular distributions of ²⁴Na fragments [7] appeared to be inconsistent with the slower time scale expected for evaporation or fission. Follow-up studies sought to clarify the contribution of evaporation and this new mechanism to forming specific light fragments. Some of these employed non-radiochemical techniques. For example, ⁸Li fragments were studied using nuclear emulsions [8], and cross sections for ⁹Li, ¹⁶C, and ¹⁷N were measured by counting their beta-delayed neutrons [9]. This early BNL work triggered world wide experimental and theoretical studies of multifragmentation. The commonly accepted picture is that the process involves statistical fluctuations during the expansion of the initial hot nuclear system. Various mass clusters form, interact, and dissolve during the expansion. At some low density, interactions cease (freeze out) to give excited prefragments. These are further accelerated by Coulomb forces and may evaporate additional particles to give the observed light fragments.

Fission/Spallation Competition: The spallation and binary fission mass distributions from heavy element targets are separated by a deep gap at energies up to 0.5 GeV. This gap has disappeared and the distributions overlap at 2.2 GeV [3].  The energy dependence of cross sections in this region of fission-spallation competition (cesium through the rare earth elements), was examined using radiochemical techniques combined with mass spectrometry [10].  This showed that the charge dispersion curve is double peaked at high energies. Spallation adds a neutron-deficient component to the neutron-rich fission products. Spectra and angular distributions [11] support this two component picture. Energy spectra of ¹⁴⁰Ba are narrow with high means typical of fission, while those of ¹³¹Ba are broader and shifted to lower energy. The angular distribution of ¹⁴⁰Ba peaks at a sideward angle as expected for a low-deposition-energy fission product at high bombarding energies, but that for ¹³¹Ba is forward peaked.

Absolute Cross Sections: Cross sections for high-energy reactions are normally measured relative to some “monitor” reaction. Foil monitor techniques were also important for diagnostic measurements of external beams at the Cosmotron and AGS. BNL research played a major role in establishing a consistent set of monitor cross sections. Absolute cross sections were determined for the primary standard ¹²C(p,pn)¹¹C [12-14], and values for secondary standards such as ²⁷Al(p,3pn)²⁴Na [15] and ¹⁹⁷Au(p,X)¹⁴⁹Tb [16] were  established by relative measurements. The high threshold for the latter makes it of particular use where a background of low-energy particles is present.

Theoretical Calculations:  The Los Alamos MANIAC high-speed digital computer and Monte Carlo techniques were used to model proton-nucleus interactions as a series of quasi-free scatterings [17, 18].  Many simplifying assumptions required by earlier hand calculations were avoided in these calculations. This work provided the first extensive theoretical predictions for comparison with experimental data at energies up to 1.8 GeV. Features such as the empirical correlation between excitation energy and momentum transfer appeared as a natural consequence. Of particular interest, the rapid rise of many cross sections above 0.5 GeV was shown to correlate with the onset of meson production and reabsorption.

Evaporative decay of excited nuclei, such as those resulting from high-energy proton-nucleus interactions, was also examined by Monte Carlo techniques [19].  Predictions of these calculations are sensitive to nuclear properties, and comparisons with experimental excitation functions [20] obtained at the BNL 60-inch Cyclotron were valuable in defining parameters such as level densities and paring energies. 
____________________

* The first publication [1] from the BNL Nuclear Chemistry Group on this topic, a collaboration between G. Friedlander and J. M. Miller of Columbia, described a spallation study at the 0.4-GeV Nevis synchrocyclotron. This collaboration would continue in subsequent research at BNL and elsewhere. Their text, “Nuclear and Radiochemistry”, 2^nd Edition, by Friedlander, Kennedy, and Miller, is known worldwide as the source book for the field. 

References
High-Energy Proton Reaction Studies 1947-1966

1. Yields of Iron Isotopes in High Energy Nuclear Reactions.
J. M. Miller and G. Friedlander, Phys. Rev. 84, 589 (1951).

 2.  Nuclear Reactions of Copper with 2.2 GeV Protons.
G. Friedlander, J. M. Miller, R. Wolfgang, J. Hudis, and E. Baker, Phys. Rev. 94, 727 (1954). 

 3.  Disintegration of Bismuth by 2.2 GeV Protons.
N. Sugarman, R. B. Duffield, G. Friedlander, and J. M. Miller, Phys. Rev. 95, 1704 (1954).

 4.  Production of ¹⁸F and ²⁴Na in Irradiations of Various Targets with Protons between 1 and 6 GeV.
A. A. Caretto, J. Hudis, and G. Friedlander, Phys. Rev. 110, 1130 (1958).

 5.  Production of ⁷Be, ²²Na, and ²⁴Na Fragments from Heavy Elements at 3, 10, and 30 GeV.
J. Hudis and S. Tanaka, Phys. Rev. 171, 1297 (1968).

 6.  Radiochemical Studies of the Interaction of Lead with Protons in the Energy Range 0.6 to 3.0 GeV.
R.  Wolfgang,  E. W. Baker, A. A. Caretto, j. B. Cumming, G. Friedlander, and J. Hudis,  Phys. Rev. 103, 394 (1956).

 7.  Study of a Fragmentation Reaction by Thin-Target Recoil Techniques; Production of ²⁴Na from Bismuth by 2.9-GeV Protons.
J. B. Cumming, R. J. Cross, Jr., J. Hudis, and A. M. Poskanzer, Phys. Rev. 134, B167 (1964).

 8.  Emission of  ⁸Li Fragments from Cu, Ag, and Au by High-Energy Protons.
S. Katcoff, E. W. Baker, and N.T. Porile, Phys. Rev. 140, B1549 (1965).

 9.  Cross Sections for the Production of  ⁹Li, ¹⁶C, and ¹⁷N in Irradiations with GeV-Energy Protons.
I. Dostrovsky,  R. Davis Jr.,  A. M. Poskanzer, and P. l. Reeder, Phys. Rev. 139, B1513, ( 1965).         

 10.  Excitation Functions and Charge Dispersion in the Fission of Uranium by 0.1- to 6.2-GeV Protons.
G. Friedlander, L. Friedman, B. Gordon, and L. Yaffe, Phys. Rev. 129, 1809 (1963).

 11.  Fission of  ²³⁸U by 2.2-GeV Protons.
V. P. Crespo, J. B. Cumming, and A. M. Poskanzer, Phys. Rev. 174, 1455 (1968).

12.  The ¹²C(p,pn)¹¹C Cross Section at 2 and 3 GeV.
J. B. Cumming, G. Friedlander, and C. Swartz. Phys. Rev. 111, 1386 (1958). 

 13.  ¹²C(p,pn)¹¹C Cross Section at 28 GeV.
J. B. Cumming, G. Friedlander, and S. Katcoff, Phys. Rev. 125, 2078 (1962).

 14.  The ¹²C(p,pn)¹¹C Cross Section at 1.0 GeV.
A. M. Poskanzer, L. P. Remsberg, S. Katcoff, and J. B. Cumming, Phys. Rev. 133, B1507 (1964).

 15.  The ²⁷Al(p,3pn)²⁴Na/C(p,pn)¹¹C Cross Section Ratio in the GeV Region.
J. B. Cumming, J. Hudis, A. M. Poskanzer, and S. Kaufman, Phys. Rev. 128, 2392 (1962).

 16.  Cross Section for Production of ¹⁴⁹Tb from Au by High-Energy Protons.
E. M. Franz and G. Friedlander, Nucl. Phys. 76, 123 (1966).

 17.  Monte Carlo Calculations on Intranuclear Cascades I. Low Energy Studies. 
N. Metropolis, R. Bivins, M. Storm, A. Turkevich, J. M. Miller, and G. Friedlander, Phys. Rev. 110, 185 (1958).

 18.  Monte Carlo Calculations on Intranuclear Cascades II. High-energy Studies and Pion Processes.
N. Metropolis, R. Bivins, M. Storm, A. Turkevich, J. M. Miller, and G. Friedlander, Phys. Rev. 110, 204 (1958).

 19.  Monte Carlo Calculations of Nuclear Evaporation Processes. III. Application to Low-Energy Processes.
I. Dostrovsky,  Z. Fraenkel, and G. Friedlander, Phys. Rev. 116, 683 (1959).

 20.  Excitation Functions for Alpha-induced Reactions on Zinc-64.
N. T. Porile, Phys. Rev.  115,  939 (1959).

Last Modified: June 28, 2012

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