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Rare earth yields in the interaction of 28-GeV protons with uranium, bismuth and gold
J. inorg,nucl.Chem., 1970,Vol.32, pp. I to 15. PergamonPress. PrintedinGreat Britain
RARE
EARTH 28-GeV
YIELDS IN THE INTERACTION PROTONS WITH URANIUM, BISMUTH AND GOLD*t
OF
KNUT BACHMANN Department of Chemistry, Brookbaven National Laboratory, Upton, New York 11973
(First received 12 March 1969; in revised form 21 May 1969) Abstract- Cross sections for the formation of a number of lanthanides by the interaction of 28-GeV
protons with uranium, bismuth, and gold were measured by a radiochemical method. Yield-vs.-mass curves and some information concerning the charge dispersion curves were deduced from the independent and cumulative cross sections. The yield-vs.-mass curve for uranium shows a rapid decrease with increasing mass in the region fromA = 131 t o A ~ 150. The total isobaric yields appear to reach a valley or flat region at A ~ 155. Examination of neutron deficient (N/Z ~< 1.38) and neutron excess products (N/Z > 1.38) indicates the observed decrease in the total yields is due primarily to a rapid decrease in yields of the neutron excess products. The charge dispersion curve for products from uranium in the rare earth region is double peaked. The valley separating the neutron excess and neutron deficient peaks is deeper in the europium region than that reported previously in the cesium region. Total isobaric yields in the mass 134-155 region from bismuth and gold targets are approximately constant at = 11 mb, a higher value than those reported for products in the mass 72-131 region from lead targets. The charge dispersion curves for rare earth products from Bi and Au show single peaks on the neutron deficient side of stability. The results are discussed in terms of fission and spallation as possible mechanism leading to products in the rare earth region. 1. I N T R O D U C T I O N
studies of the interaction of protons in the GeV range with heavy elements [1] have led[2] to the following description of the yield-vs.-mass curves: The typical double humped yield-vs.-mass curve for low energy fission of uranium disappears as the bombarding energy is increased and there is only one maximum left located near half the target mass. The height of the maximum decreases and the peak becomes broader with increasing energies. On the other hand there is a marked rise in the yield-vs.-mass curve for masses below -~40. In the case of lead, the corresponding curve at G e V energies shows a similar upturn at low masses but is rather flat in the mass range from 40 to 120. Only limited information on yields from heavy element targets is available for masses higher than ~ 130. Charge dispersion curves at high bombarding energies have been measured by several authors [2-5]. For the fission of uranium, Friedlander e t a/.[3] have RADIOCHEMICAL
* Research performed under the auspices of the U.S. Atomic Energy Commission. tPresent address: Lehrstuhl fiir Kernchemie, Eduard-Zintl-lnstitut, Technische Hochschule Darmstadt, Germany. I. For a summary of high energy reactions with an emphasis on fission, see E. K. Hyde, The Nuclear Properties of the Heavy Elements, Vol. IlI; Fission Phenemona, Prentice Hall, Englewood Cliffs, New Jersey (1964). 2. G. Friedlander, Physics and Chemistry of Fission, Vol. II, p. 265.1nternational Atomic Energy Agency, Vienna 1965; and private communication. 3. G. Friedlander, L. Friedman, B. Gordon and L. Yaffe, Phys. Rev. 129,1809 (1963). 4. S. Kaufmann, Phys. Rev. 129, 1866 (1963). 5. N. Porile, Phys. Rev. 148, 1235 (1966). l
2
K. B,~CHMANN
carried out an extensive study of the charge dispersion of products in the region of cesium as a function of the energy of the bombarding protons. These authors found a double-humped charge dispersion curve above 1 GeV, whereas the charge dispersion curve for lead[2] (which should be very similar to bismuth) was single-humped with products only on the neutron deficient side. Radiochemical studies of recoil products have been shown to be of considerable value in the clarification of high, energy mechanisms by giving information about the kinetic energies of single fragments as well as about the excitation energies of the fissioning nulcei. Sugarman et al.[6] investigated the recoil properties of alSr and iza'~33mBafrom the fission of bismuth induced by 50-MeV to 2-GeV protons. They found that 91Sr is formed by a process requiring less deposition energy than 12a'~33mBa, if the bombarding energy is higher than 450 MeV. The kinetic energy of barium decreases with increasing proton energy whereas the kinetic energy of a~Sr does not change. In a comparison of fission of bismuth and tantalum induced by 450-MeV protons, Porile and Sugarman [7] found that the kinetic energies of the fission products from tantalum are lower which is reasonable for a fissioning nucleus of lower charge. An extensive study by Sugarman and coworkers [8] of the fission Of uranium at 450 MeV led to the conclusion that the kinetic energies of the products could be accounted for in terms of a nearly constant separation distance (18.4___0.5 fermis) between the charge centers at scission. Studies by Alexander et al.[9] (uranium, 0.5-6.2 GeV), Brandt[10] (uranium, 0.55 and 18 GeV), Crespo et al.[ll] (uranium, 2.2 GeV) and Hageb¢ and Ravn[12] (uranium, 0.57 and 18.2GeV) indicated that the neutron deficient and neutron excess products from uranium had considerably different recoil properties at G e V bombarding energies. The kinetic energies of the neutron deficient products are considerably lower than those of the neutron excess products. The neutron deficient nuclides are formed in high deposition energy processes whereas the neutron excess products are formed in low deposition energy processes. One may conclude from these results and doublehumped charge dispersion that neutron deficient and neutron excess products arise from different mechanisms. In this paper the terms neutron excess and neutron deficient are not used with their conventional meanings in which /3stability is the dividing line. The N / Z ratio which determines the separation line in our connotation lies somewhat to the neutron deficient side of/3-stability. The aim of the present work was to measure some cross sections for forming rare earth isotopes by the interaction of 28-GeV protons with uranium, bismuth, and gold. This will serve to extend information on yield-vs.-mass curves and charge dispersion curves into a little studied region. (At the present time two 6. N. Sugarman, M. Campos and K. Wielgoz, Phys. R ev. 101, 388 (1956). 7. N.T. Porile and N. Sugarman, Phys. R ev. 107, 1410 (1957). 8. N. Sugarman, H. Miinzel, J. A. Panontin, K. Wielgoz, M. V. Ramaniah,G. Lange and E. LopezMenchero, Phys. Rev. 143, 952 (1966). 9. M. Alexander, C. Baltzinger and M. F. Gazdik, Phys. Rev. 129, 1826 (1963). 10. R. Brandt, Physics and Chemistry of Fission, Vol. I1, p. 329. International Atomic Energy Agency, Vienna, 1965. 11. V. P. Crespo, J. B. Cumming and A. M. Poskanzer, Phys. Rev. 174,1455 (1968). 12. E. Hageb¢ and H. Ravn, J. inorg, nucl. Chem. 31, 2649 (1969).
Rare earth yields
3
other research groups are investigating this same region [ 13, 14]). As a companion to the present work, some recoil properties of rare earth nuclides from uranium and gold targets were also measured [ 15]. 2. E X P E R I M E N T A L
2.1 Irradiation The irradiations were performed in the internal beam of the Brookhaven Alternating Gradient Synchrotron (AGS) at a proton energy of 28 GeV. The duration of the irradiations varied from 20 min to 2 hr. The target was kept in a shielded position during each acceleration cycle until the protons reached full energy. Then the target was raised to a position near the beam and the magnetic field was changed in order to decrease the radius of the circulating beam until the protons hit the target. The beam intensity was sufficiently constant during irradiations so that corrections in the calculation of the cross sections were not necessary. The targets of uranium and gold were bombarded with about 1012 protons per pulse. With the bismuth targets it was necessary to decrease the intensity because of the low melting point to 4.1011 protons per pulse. The integrated circulating beam was about 1015 protons per irradiation. The true intensity seen by the targets is higher due to the multiple traversals. We performed two experiments with uranium and gold, respectively, and one experiment with bismuth. 2.2 Target Figure 1 shows the target arrangement. The target consisted of a stack of rectangular foils (1 x 2 in.) arranged in the following order: 0.001 in. of aluminum guard, 0.00l in. of aluminum monitor~ 0.001 in. of aluminum catcher, 0.001 in. of aluminum monitor, 0-001 in. of aluminum guard. The T: TARGET C: CATCHER FOILS MONITOR FOILS
M:
BEAM• 1
G: GUARD FOILS
G M
C
T
CMG
Fig. 1. Schematic diagram of the target arrangement. thick target-thick catcher arrangement was used because we wanted to determine the ranges[15] in the same irradiation. The foils were aligned such as to superimpose exactly and bolted into a target-holder. The error introduced by misalignment can be estimated from the agreement of the upstream and downstream monitor foils to be less than 2 per cent. The target foils were commercially available uranium, bismuth, and gold. Bismuth and gold were analyzed spectrographically. There was no detectable impurity present which could interfere. The foils were cleaned with nitric acid prior to irradiation. Their thickness was determined by weighing known areas of the foils. After the bombardment an area of about 1 x 2 cm was cut out. 2.3 Chemical procedure The target foils were dissolved in a mixture of nitric acid and hydrochloric acid containing known amounts of each of rare earth elements and yttrium. Scandium, barium, and zirconium carriers were also added. The separation of the lanthanide group was carried out by a modification of procedures described in NAS-report 3020116]. The bulk of uranium and gold respectively, and all the 13. Y. Y. Chu, E. M. Franz, G. Friedlander, E. Hechtl and P. Karol,154th International Meeting of the ACS, Chicago, Illinois, September 10-15, 1967; Abstract R 50 and G. Friedlander, private communication. 14. J. Alstad, private communication. 15. K. Biichmann andJ. B. Cumming, Unpublished. 16. The radiochemistry of the rare earths, scandium, yttrium and actinium. Monograph Series on the Radiochemistry of the Elements, Nuclear Science Series NAS-3020.
4
K.B.KCHMANN
anionic products were separated by means of an anion exchange column (Dowex-1). The rare earths were purified by a series of fluoride- and hydroxide-precipitations. Cerium was oxidized in 10 N HNO3 by a KBrO3 solution and extracted with 25% di(2-ethylhexyl) acid orthophosphate dissolved in n-heptane. The Ce 4+ in the organic phase was reduced with H202 and back extracted into 1 N-HNOa. The other rare earths were mixed with ca. 1 ml of cation exchange resin (Dowex 50W-X12,200400 mesh) and after standing long enough to attain equilibrium, the resin was put on the top of a column. The separation was carded out at 80°C using a-hydroxyisobutyricacid (a-HIBA) (0-13 M and pH = 4.35) as eluent. The flow rate was kept constant at 3 drops per rain. The separation took about 16 hr, but most of this time was required for the separation of the light lanthanides. The series from lutetium to europium was separated in about 3 hr. After 3 hr the concentration of the a - H I B A was increased to 0.4 M. Fractions of the eluate were collected using an automatic fraction collector and their activities were measured in a well-type scintillation counter. Figure 2 shows the activity distribution of one of the separations. The activity distribution does not allow true evaluation of I
z
§
I
I 106 ~
I
I
i
't/v
t 20
// I" 40
I
Nd i
Dy/Y
[.o
iO 2 I
I
I 60 NUMBER
Eo
v
I I 80 I00 OF T H E FRACTION
n
..If I~ 120
~--
Fig. 2. Activity distribution in atypical rare earth separation. the contamination factors of adjacent rare earths elements, because some of the activity found in the valleys may be due to the growing-in of daughter activities during the separation procedure. The fractions around the maxima were combined and used to prepare samples for the activity measurements by coprecipitation with barium oxalate. Barium did not interfere in the subsequent analysis of the rare earth by the arsenazo method. Impurities in these samples coming from neighboring lanthanides never exceeded 1 per cent. However, it was not possible to separate yttrium and dysprosium quantitatively. 2.4 Beam intensity The beam intensity was measured using two monitor foils arranged upstream and downstream of the target. The results of the two monitors were the same within 2 per cent. For monitoring either the reaction 27Al(p, 3pn)24Na or the reaction 27Al(p, 3p3n)22Na was used. In the first case the activity of the 15 hr Na-24 was assayed with a calibrated endwindow proportional counter. In the second case the annihilation radiation was measured by a NaI-crystal. The cross section for the formation of Na-24 at 28-GeV was assumed to be 8-9 mb[17]. For the cross section of Na-22, a value of 10 mb was taken[17]. Subtractive corrections were made for the production of 24Na in the monitor foils be secondary reactions (mainly 2¢Al(n, a)2~Na). The correction was assumed to be 3 per cent[18]. 2.5 Measurement o f the activity The activity of the samples was followed as a function of time. Four different counting devices were used: Endwindow proportional counters for the measurement o f / 3 - - or /3+-radiation, two NaI-crystals in connection with a coincidence unit and single channel analyzers for the detection 17. J. B. Cumming, Ann. Rev. Nucl. Sci. 13, 261 (1963). 18. J. R. Grover, Phys. Rev. 126, 1540 (1962).
Rare earth yields
5
of fl+-radiation, a NaI-crystal (2-mm thick) with a multichannel analyzer for the detection of x-rays, and a lithium drifted germanium detector in connection with a multichannel analyzer for the detection of v-rays. As most of the rare earth samples contained a number of active isotopes with measurable half-lives, the lithium-drifted germanium detector was the most important detector. The crystal was calibrated with a set of standards (IAEA) in order to get a photopeak-yield curve as a function of energy and geometry. The -/-spectra were taken as a function of time in order to check the identity of questionable `/-lines. The area under a peak was determined with the usual subtractive corrections (Compton continum and background) and the activity at the end of the irradiation and at the time of separation, respectively, calculated by a least-square computer-program [19]. The positron disintegration rates were measured by the coincidences of the two annihilation quanta detected in two 3 × 3 in. NaI-crystals which were arranged at a 180° angle. The samples were mounted between copper absorbers to insure annihilation of all the positrons. A single channel analyzer was used in connection with each detector, with the channel set to the 51 l-keV peak. The background was less than 0.2 cpm. The positron detection efficiency was determined with a calibrated 22Na source. The fraction of 22Na decaying by positrons emission was taken to be 0-898[20]. The K X-rays emitted by the lanthanides which decay by electron capture were measured with a NaI-crystal used in connection with a multichannel analyzer. The use of a multichannel analyzer has the advantage that it was not necessary to check the channel settings and to correct for possible shifts. The efficiency was calibrated by a ~ C e (33 keV) and an Z41Am(59.5 keV) source. The energies of the rare earth K X-rays observed in this work fall between ~33 keV and ~46 keV. We assumed a linear variation of detector efficiency over the energy range covered by the two standards.
2.6 Analysis of the rare earths Recovery efficiencies (chemical yields) of the lanthanides were determined photometrically by the arsenazo method[21]. In separate experiments it was shown[22] that there was no interference from the barium carrier, provided the amount of barium did not exceed 3 mg, corresponding to a ratio of 10 to 1 for barium to rare earth elements. Dysprosium and yttrium were not separated completely by the ion exchange procedure; these two elements were analyzed together. First the sum of dysprosium and yttrium was determined photometrically, secondly the ratio of the spectral yields was measured and thirdly dysprosium was analyzed by flamephotometry [22]. 3. R E S U L T S A N D D I S C U S S I O N
3.1 Calculation o f the cumulative and independent cross sections The counting rates are corrected following correction factors:
to absolute disintegration rates D O by the
(1) E f f i c i e n c y o f t h e c o u n t e r (in t h e c a s e o f t h e l i t h i u m d r i f t e d g e r m a n i u m detector, the photopeak efficiency) (2) B r a n c h i n g r a t i o s in t h e d e c a y s c h e m e (3) C h e m i c a l y i e l d o f t h e s e p a r a t i o n (4) C o r r e c t i o n f o r t h e d e c a y . For nuclides which do not require taking into account parent-daughter relationships (shielded nuclides, quasi-shielded nuclides, or cumulative yields of long-lived nuclides with short-lived precursors), the cross sections are obtained by the following equation: cr = D~°F
19. 20. 21. 22.
J. B. Cumming, Brookhaven NationalLaboratory Rep., BNL-6470. Nuclear Data Sheets, compiled by K. Way et al. J. S. Fritz, M. J. Richard andW. J. Lane, Analyt. Chem. 30,1776 (1958). E. Norton, private communication.
(1)
6
K. BACHMANN
where
D ~ = D ° ( 1 - - e -xr)
(2)
WA~4XO'A1 F - WXAAID~a
(3)
and
WA~ is the superficial density of aluminum and Wx the superficial density of the target in question. DN% is the calculated disintegration rate for the monitor as in Equation (2) either for 24Na or for 22Na. Ax and AA1 are the masses for the target and aluminum, respectively, O'AI is the cross section for the formation of 24Na or Z2Na. If parent-daughter relationships had to be taken into account the equations given in Refs [3, 23] were used. The errors become large in cases where the parent half-life is of the same order of magnitude as the separation time. It is not possible to determine the exact separation time of two adjacent rare earths on the column. As the same experiment was used to measure the ranges, it was possible to correct properly for recoil losses. Some of the recoil products are escaping from the edges of target foil. The correction for the edge effect was taken to be 0.5 per cent [8]. Some of the neutron excess products could also be formed by secondary reactions, e.g., fission by low energy neutrons which are produced in the nuclear cascade or subsequent evaporation. Friedlander et a/.[3] have found the apparent 14°Ba production cross section from uranium increases by 7 per cent for each 100 mg/cmz of target thickness at 3 GeV. In this work a value was used, which corresponded to the yield-vs.-mass curve for fast neutrons (i.e. for 144Ce 5.9% and for 147Nd 2.6% per 100 mg/cmz target thickness). No corrections were applied to cross sections obtained with the gold and bismuth targets. Measured cross sections for products of the interaction of uranium, bismuth, and gold with 28-GeV protons are compiled in Tables 1 and 2. The type of cross section is indicated in the second column. The measured radiations are tabulated in the fourth column. If there was more than one determination by different radiations, for example with T-rays and/3 +, then the most reliable value or a weighted average of the separate values is reported. The decay schemes of some of the neutron deficient nuclides are very poorly known. We have used the T-lines whose intensities seemed most reliable and which were subject to least interference by other nuclides. The errors were estimated from the uncertainties due to the statistical error of counting, the counting efficiencies, the chemical yields, the separation time and the decay schemes. Not included are the systematic errors, such as the error in the monitor cross section (___8%) and errors from erroneous decay schemes. The errors on the fl+-radiation measurements are higher than one would expect since we suspect the calibration. The errors assigned to the short-lived dysprosium and terbium products are exceptionally large due to the uncertainty in the separation times and half-lives. The unpublished results of Friedlander et al.[13] and Alstad[14] are in rather good agreement with most of our values. However, some of the cross sections are 23. N.T. Porile, Phys. Rev. 120, 572 (1960).
Rare earth yields
7
Table 1. Cross sections for forming rare earth isotopes in the interactions of uranium with 28-GeV protons
Product
Type of yield*
Cross section (mb)
Measured radiations with abundances used'~
Half life used
134Ce 139Ce ~4aCe l~Ce 144Ce 13apr ~42pr ~3aNd 14°Nd 147Nd l~pm 14spm 148pm ~Pm 151pm luSm l~Sm ~45Eu ~46Eu 147Eu ~4SEu 149Eu ~5°Eu ~snEu 157Eu ~Gd 147Gd 149Gd 14aTb 15~Tb ~52Tb ~53Tb ~Tb l~Tb ~S2Dy 153Dy ~55Dy
C I C C C I I C C C C I I C C C C C I I I I I I C C C I C C I I I I C C C
7.6 -4-_1.0 0.7___0.07 3.7±0.4 6-8___0-5 7-6___0.8 1'4 : 0"7 0.8 ---0.2 3 "4 +--0"6 3-4___0.5 1.7±0.17 0-55---0.05~ 0.004 ___0.001:~ 0.17 ---0.02~: 0.48_0.05:~ 0-24_0.04~ 0.46___0.08 0-22___0.05 4.00_+0.35 0.87 _+0.07 0.30 _+0.03 0.48---0.10 0.08-+0.02 0.02 -- 0.05 0.28-+0.04 0-20---0.03 5 - 0 6 - 0.4 3.05 -+ 0.25 1.45 -+ 0.11 3"60-+0"8 4.51_+0"5 0.85---+0.40 0-95---+0.15 0.60_+0.10 0.14_+0.02 5.6+_-2.0 6.8 -+ 2-5 7.1 __-1.7
/3+(0.61) 170keV (1.00) 1-45 keV (0.70) 290 keV (0.37) 130keV (0-24) 170 keV of ~39Ce 1570 keV (0-037) 170 keV of iaaCe /3+ of Pr-140 (0.53) 530keV (0-33) 740 keV (0.45) 1470keV (0.24) 726keV (0.34) 290 keV (0.03) 340keV (0.21) 103 keV (0.78) 203 keV (0.29) 900 keV (0-67) 750 keV ( 1.00) 200 keV (0.24) 550keV (1.00) 328keV (0.13) 330 keV (0.04)0 + (0-06) 1230keV (0-11) 413 keV (0.20) 750 keV of l~Eu 200 keV of 147Eu 300 keV (0-30) 300 keV of l~Gd 250keV (0-37) 340 keV (1-00) 210 keV (0.31) 180keV (0.15) 530 keV (0-71) 340--- keV of ~SZTb 210 keV of 15Z'Fb 227 keV (0.72)
75 h 140 d 32-5 d 33.4h 277 d 4.5 h 19.2 h 5-2 h 3.3 d 11-1 d 265 d 5.4d 41 d 53.1 h 28 h 47 h 9.4h 5.6d 4-7 d 23.6 d 54 d 106 d 12.8 h 15 d 15.2 h 50 d 39 h 9.5 d 4.1 h 18 h 17.4h 2-6d 5 d 5.1 d 2.4h 5.5 h 10 h
*C = cumulative. I = independent. tThe abundances are according to the Nuclear Data sheets [20]. Furthermore, the values are corrected for conversion, and in some cases a subtractive correction for a-decay is applied, l~Gd and ~4~Gd for example get a contribution from l~°Dy and 15~Dy. eThe promethium cross sections are only relative, because no carder was present.
K. BACHMANN Table 2. Cross sections for forming rare earth isotopes in the interaction of of bismuth and gold with GeV protons
Product
Type of yield
134Ce lasCe aaPr lagNd ~4°Nd mEu 148Eu 147Eu 14SEu ~Gd ~47Gd XOGd XS~Gd t4~I'b ~5~Tb ~52Tb ~Tb ~SSTb ~S6Tb 152Dy t~Dy ~Dy
C C,I I,C C C C I,C I,C I C C I,C I,C C C I I,C I I C C C
Cross section bismuth (mb)
Cross section gold (mb)
9.9"--2.0 2.25 (C.I)
12.2"-.2.8 1.35"-.0.6(1); 10-0_ + 1.8 (C) -1.55 +-0-8(1); 8.6 +- 1-66(C) 7.05 +- 1"5 -13"35--+3"0 -6.65 - 0"6 2.24_ 0.18(I); 8.34 +-0.6 (C) 1.74+-0.15(I); 9.14_ 0.7 (C) 1.06±0-12(I);9.56+-0.9(C) 2.66--.0.30(1); 10.86-0.9(C) 0-46 ± 0.08 1.20 +-0.25 6.1 ---0.5 7.4-+0-6 8"5---0.8 8-2±0"8 1"0-----0.1(I); 9"4 +-2.0 (C) 1-8- 0"15(I); 8"8 +- 1"6 (C) -0-7 +-0.2(I); 4.0 +-0-4 (C) 8.4 ___2-0 7.0 +- 1.6* 2.3"--0.3 3-35__.0.4 2-08---0.8 2.3+__0.8 2.14--.0.4(1);14.2"-.3.5 (C) 2.95"-.0.5(1);12.05+-3.6(C) 0.6---0-1 0.40---0.06 0.42 _-.0-07 0.05 ---0.08 -6"4 -- 3"0 12.1--.3-5 9.1--.3.6 -12.0 +_3"0 -
-
*When combined with the alpha branching ratio measured by Chu, Franz and Friedlander[24], the results of Brtminx[25] imply a cross section of 3.4 mb for this reaction while those of Franz and Friedlander[26] lead to a cross section of 4.3 mb. q u i t e different. B o t h a u t h o r s u s e a m a s s s e p a r a t o r as well as c h e m i c a l s e p a r a t i o n , a p r o c e d u r e w h i c h s h o u l d i m p r o v e r e s o l u t i o n o f the v a r i o u s isotopes. 3.2 Charge dispersion curves I n a n ideal e x p e r i m e n t , all the i n d e p e n d e n t a n d c u m u l a t i v e f o r m a t i o n c r o s s s e c t i o n s w o u l d b e m e a s u r e d to give a g e n e r a l p i c t u r e o f the d e p e n d e n c e o f t o t a l i s o b a r i c y i e l d o n m a s s n u m b e r ( m a s s - v s . - y i e l d c u r v e ) as well as the d e p e n d e n c e o f y i e l d o n n u c l e a r c h a r g e ( c h a r g e d i s p e r s i o n c u r v e ) at a p a r t i c u l a r m a s s n u m b e r . H o w e v e r , for i n t e r a c t i o n s o f h i g h - e n e r g y p r o t o n s , o n l y i n a few i n s t a n c e s c a n sufficient d a t a b e a c q u i r e d at o n e m a s s n u m b e r to define a n ideal c h a r g e d i s p e r s i o n c u r v e . I n g e n e r a l it is n e c e s s a r y to u s e d a t a f r o m a r a n g e o f m a s s n u m b e r s with c o r r e c t i o n s for t h e d e p e n d e n c e o f yield o n m a s s in e s t i m a t i n g c h a r g e d i s p e r s i o n c u r v e s . F r i e d l a n d e r [ 2 ] has d i s c u s s e d s u c h p r o c e d u r e s for p r o d u c t s o f the intera c t i o n o f u r a n i u m a n d lead with 2 8 - G e V p r o t o n s . F o r u r a n i u m , the d a t a o f F r i e d l a n d e r et aL[3] i n d i c a t e the c h a r g e d i s p e r s i o n c u r v e (plot o f i n d e p e n d e n t 24. Y. Y. Chu, E. M. Franz and G. Friedlander, Phys. Rev. 175, 1523 (1968). 25. E. Bruninx, Nucl. Phys. 64, 481 (1965). 26. E. M. Franz and G. Friedlander, Nucl. Phys. 76, 123 (1966).
Rare earth yields
9
formation cross section as a function of neutron to proton ratio (N/Z) of the product) at A = 131 has two comparable height peaks with a valley at N/Z ~ 1.42. Data of Porile[5] in the mass region near A = 109 are equally consistent with either a double or single peaked charge dispersion. Results of Kaufman[4] at A = 72 indicate a single peaked curve. Data for lead targets show single peaked charge dispersions at all these masses [2]. In Fig. 3 we have plotted the independent cross sections obtained in the present work for forming europium isotopes from uranium by 28-GeV protons. I0
I
I
I
1
I
i29Cs T
..o
E
I
~E,, @\ 14SEu \\ ~
_
Z 0 I-
_L
_
15SEu
e \
147Eu \
t0
\
/ \~9Eu
0
tx
131Cs
,~/
/
0.I
?
t50 U
0.01
I 1"3
I
|
I
1.4 N/Z, NEUTRON TO PROTON RATIO
I 1.5
Fig. 3. Comparison of isotopic yield distributions of europium and cesium produced by the interaction of 28-GeV protons with uranium. Filled circles are from the present work. Open circles are from Ref. [2].
For comparison, independent yields of cesium isotopes from the work of Friedlander[2] are shown as open circles. The solid curve through the cesium points was obtained from the double peaked charge dispersion curve reported for A = 13112] with correction f o r t h e dependence of isobaric yield on mass number. The dashed curve through the europium points serves only to indicate a general trend. The point for lS°Eu represents only one of the 15°Eu isomers and as such is a lower limit for the charge dispersion curve. Although the data in Fig. 3 are strictly speaking isotopic distributions rather than charge dispersion curves, they illustrate some of the general conclusions which may be drawn from our data: the charge dispersion curve in the rare earth region is double peaked and the valley separating the two peaks appears to be relatively deeper than in the cesium region. It is interesting that the yields of the lower-mass valley products, 139Ce
10
K. B~,CHMANN
and 142pr, (see Table 1) are only the order of a factor of 2-3 below the corresponding cesium yields. The minimum in the charge dispersion curve in the rare earth region is along with range data [ 15] additional evidence for different mechanisms of formation for the neutron excess and neutron deficient products from uranium targets by 28-GeV protons. Turning now to products from bismuth and gold targets, independent cross sections from Table 2 have been plotted in Fig. 4. There is very little difference between yields from gold and those from bismuth. The dashed curve in Fig. 4 i0
r
152TI
I
I
155Tb
d
\
1 4 6 ~ (~ \\ .D
E z 0
147Et
l--
\\
b3 ~0
148Eu
A
\~56Tb \
0 {E (J
O.l
'\ (
\ \ \ \ \
O'OI
I 1.3
1"4
I \
I 1"5
N / Z , NEUTRON TO PROTON RATIO
Fig. 4. Dependence of independent formation cross sections on N/Z of the product in the interaction of 28-GeV protons with gold (O) and bismuth (0).
is the charge dispersion curve for A = 131 given by Friedlander[2] for the interaction of 28-GeV protons with lead. Despite the scatter of our results, it is apparent that differences between the charge dispersion curve at A ---- 131 and that in the rare earth region are much smaller than were observed for uranium targets (see Fig. 3). Our failure to observe neutron excess products from Au or Bi targets is consistent with the extremely low yields of nuclides such as 14°Ba reported by Friedlander[2]. 3.3 Yield-vs.-mass curves 3.3.1 Neutron deficient products from uranium. Based on the rather clean separation of the two parts of the charge dispersion curve (Fig. 3) it seems reasonable to discuss separately the yield-vs.-mass curves for the neutron excess
Rare
earth
yields
I1
and neutron deficient products in the rare earth region. We have rather arbitrarily adopted an N/Z value of 1.38 as the dividing point. Such a division is much less justified for products with A = 131 or lower where the valley is less distinct. In Fig. 5 the cumulative isobaric yields for various neutron deficient mass chains are plotted as a function of N/Z. With the assumption that the slope of the curves
IJ
i J ~ I i i l F I i I I ; I "~3D
I0
6
,55D~'53Tb
],.~ST b
15ZDY
E
I
I0 o 8 I'-
Z
-
1
--
~46Eu
0 bJ (l)
-
o
r,-o
I
{4rGd
1
2
I0
134C
ICs
Z
(J
L3~
~7~'IBo
I
~39
f4ONd 2 i
I ~-25
I
I
I
I
I
I
I
i
l
I
I
t'30 ~'35 N/Z,NEUTRON TO PROTON RATIO
I
l F40
Fig. 5. Cumulative isobaric yields of neutron deficient products of the interaction of 28-GeV protons with uranium.
is the same in a limited mass range, we have extrapolated or interpolated to cumulative yields at N/Z = 1.38. Included in Fig. 5 are values for the A = 131 mass chain from the data of Friedlander [2]. T h e curves for higher masses appear flatter than those for masses 131 and 139. This reflects the increasing depth of the valley as seen in Fig. 3. T h e neutron deficient cumulative yields obtained by this procedure are plotted in Fig. 6. F o r the higher masses, these cumulative yields are quite insensitive to the choice of N/Z ---- 1.38 as the dividing point. F o r the lower masses, use of a higher value of N/Z, e.g. N/Z = 1-42 as suggested by the results of Friedlander et a/.[3], would increase the cumulative cross sections for the neutron deficient products and increase the rise shown in Fig. 6.
12
K. B ) k C H M A N N
15
I
I
I
I
I
I
I
I
I
I
I
I
za E
z IO
-
O I--
(/5
~5 O t~ o
o
130
140 150 PRODUCT MASS NUMBER
Fig. 6. Y i e l d - v s . - m a s s c u r v e of n e u t r o n deficient p r o d u c t s f r o m the i n t e r a c t i o n of 2 8 - G e V p r o t o n s w i t h u r a n i u m . T h e filled circle w a s o b t a i n e d f r o m F r i e d l a n d e r [2].
Fig. 6 indicates that neutron deficient yields are quite constant in the mass range 139-155 but that they rise by a factor of two in going toA = 131 and 134. This increase does not continue with further decreasing mass number. For example, out of the total isobaric yield of 27.2 mb atA = 109 reported by Porile [5] 11 mb of products have N / Z ~< 1.38. At mass 72 [4], ~ 13 mb of a total of 15 mb have N / Z <-1.38. It appears that neutron deficient products (N/Z <<-1.38) constitute a rather flat background under the fission peak observed in the overall yield-vs.-mass curve of uranium at 28-GeV. It has been proposed by Rudstam and SCrensen[27] that a spallation like mechanism is responsible for the formation of neutron deficient iodine isotopes. The probability of such a process should increase with increasing product mass. The rise in cross sections from A = 139 to A = 131 is not expected for a spallation mechanism and may indicate contributions from some other mechanism to neutron deficient product formation at A = 131. The shallower valley in the charge dispersion curve also points in this direction. The data in Fig. 6 are consistent with a slight increase in cross sections in going from A = 139 to 155, but no definite conclusion can be drawn due to the magnitude of the errors. Feeding of the various neutron deficient chains by alpha decay from still heavier nuclei is additional complication for which only partial correction has been made. 3.3.2 Neutron excess products from uranium. Substantially less information has been obtained in the present work on yields of neutron excess products and construction of a figure comparable to Fig. 5 is not possible for the neutron excess chains. A general conclusion that can be drawn from the data in Table 1 is that yields of neutron excess products are decreasing rapidly with increasing mass in the rare earth region. Cumulative yields of selected neutron excess products observed in previous experiments[2, 5, 27] are plotted in Fig. 7 together with results from the present work. Open circles at A -----141, 143, and 144 are the observed cumulative yields of the cerium isotopes. These are lower limits for 27. G . R u d s t a m a n d G . S C r e n s e n , J. inorg, nucl. Chem. 28, 771 (1966).
Rare earth yields
I
I
I
13
I
I
20
E
z O w
O n~
IIO
130
150
MASS NUMBER
Fig. 7. Yield-vs.-mass curve of neutron excess products of the interaction of 28-GeV protons with uranium. Points are from, Porile[5] (I-q), Rudstam and SCrensen [27] (~), Friedlander[2] (A), and the present work ((3). Filled symbols indicate those cases which are expected to approximate the chain yield up to N/Z = 1.38.
the chain yields. T h e point of A = 147 is the cumulative yield of 14rNd with an estimated correction of 0.4 mb for 147pm. T h e points at A = 153 and A -- 156 are based on the cumulative yields of 153Sm and 156Eu with small corrections for later members of the chains. Figure 7 indicates a rapid decrease of neutron excess yields from mass 140 to 156, the same region in which the neutron deficient yields are relatively constant. N e u t r o n excess yields have virtually disappeared at mass 155 (reduced by a factor of ~ 3 0 from the peak yields at A ~ 115). T h e slope of the curve in Fig. 7 and mean range measurements[15] suggest that neutron excess products are formed by fission. 3.3.3 Total isobaric yields. In Fig. 8 the yield-vs.-mass curve for uranium irradiated with 2 8 - G e V protons is presented. T h e points and curve from A = 20 to A = 131 are basically those given by Friedlander[2] with the exception that a more recent value due to Porile [5] has been used at A = 109. Total isobaric cross sections deduced from the present experimental data are shown as filled circles. T h e s e show a rapidly decreasing cross section as the mass of product increases from A = 131 to ~ 145, and then a flattening of the curve. Despite large ( ~ 20 per cent) uncertainties in individual values, we believe it is justified to draw the yield-vs.-mass curve in Fig. 8 with zero slope at A -~ 155 based on the data presented in Fig. 6 and 7. T h a t a valley has been reached is interesting as the fission peak in the yield-vs.-mass curve is now delineated on both its low and high mass sides. W e e x p e c t that the yield-vs.-mass curve will either remain
14
K.B.~CHMANN I
I
I
I
I
I
I
I
I
I
3o .o E
z
20
m
o
o:1o
!
o 20
I
I
40
60
80 I00 120 MASS NUMBER
I
I
140
160
Fig. 8. Total isobaric yield-vs.-mass curve for the interaction of 28-GeV protons with uranium. Points are based on results o f Friedlander[2] (O), Porile [5] ([7), and the' present experiment
(0). rather flat or rise slowly for masses above ~ 155 as a consequence of increasing probability for spallation reactions. Examination of the data presented in Table 2 indicates that the differences in cross sections for products from bismuth targets and those from gold targets are smaller than the scatter of the results. As a consequence of the extremely low yields of products having N/Z > 1.4 (see Fig. 4) the observed cumulative and independent yields account for ~ 7 0 per cent of the total isobaric yield of some mass numbers in this region, We have used the data in Fig. 4 to approximate the missing yields and have plotted total isobaric yields as a function of mass number in Fig. 9. (The points at A = 151 are anomalously low in both cases, but were not in the case of u r a n i u m targets). T h e s e results are consistent with a constant cross section of -~ 11 mb per mass number in the rare earth region. T h e yield-vs.-mass curve for lead as drawn by Friedlander [2] is quite fiat at a value 7 - 8 mb per mass n u m b e r for products between A = 72 to A = 131. Our data then suggests a gradual rise with increasing mass above 131. It is interesting to note that the yield-vs.-mass curves of bismuth and gold appear to cross that of uranium in the rare earth region. 4. C O N C L U S I O N S
T h e present survey study of the interaction of 2 8 - G e V protons with heavy elements targets indicates that the rare earth region is particularly interesting in the case of uranium targets. T h e valley in the charge dispersion curve separating neutron excess from neutron deficient yield is even more p r o n o u n c e d at mass ~ 150 than has been reported at lower masses. This suggests different formation mechanisms for the two classes of products. N e u t r o n excess yield from uranium decrease rapidly with increasing mass, the yield-vs.-mass curve being similar to that observed in fission induced by 200-Me protons.
Rare earth yields t
~
I
15 -+-T
I I I
B+
T
I
15
!
io
i l
'r-
s v
5
,g
i
I---+u~ u3 o
Au
+
i
T
'
~o~
i
[
!
i 5t
t t
_i 13o
I____ I
I
140 MASS NUMBER
150
Fig. 9. Total isobaric yields for selected masses in the rare earth region formed by the interaction of 28-GeV protons with bismuth (upper figure) or gold (lower figure).
Spallation has been proposed [27] on the basis of isotopic yield distributions as the formation mechanism for neutron deficient iodine isotopes from uranium at 18 G e V . The yield-vs.-mass curve (Fig. 6) for neutron deficient products is relatively fiat from A = 139 to A = 155, but appears to rise in going to lower masses. Such a trend is not expected for spallation and suggests at least some contribution from another mechanism, probably high-deposition energy fission at the lower masses. For gold and bismuth targets, only the neutron deficient peak is present in the charge dispersion curve. The yield-vs.-mass curves in the region A ~ 150 from these targets are significantly above the corresponding curve for uranium. This is reasonable for a spallation mechanism as there is less fission competition and the rare earth region lies closer to the Au and Bi targets.
Acknowledgements-The author wishes to acknowledge the hospitality of the BNL Chemistry Department during the time he spent there. He would like to express his appreciation for assistance from the following: Dr. R. W. Stoenner and Miss E. Norton who performed numerous chemical analysis; the operating staff of the Brookhaven AGS; Miss E. M. Franz and B. Neidbart who assisted in the carrying out of the experiments; Dr. J. Hudis who kindly arranged part of the irradiations. Valuable encouragement by Dr. G. Friedlander and Professor Dr. K. H. Lieser are acknowledged with particular pleasure. The author would like especially to thank Dr. J. B. Cumming for his detailed criticism and a very helpful discussion.
RARE
EARTH 28-GeV
YIELDS IN THE INTERACTION PROTONS WITH URANIUM, BISMUTH AND GOLD*t
OF
KNUT BACHMANN Department of Chemistry, Brookbaven National Laboratory, Upton, New York 11973
(First received 12 March 1969; in revised form 21 May 1969) Abstract- Cross sections for the formation of a number of lanthanides by the interaction of 28-GeV
protons with uranium, bismuth, and gold were measured by a radiochemical method. Yield-vs.-mass curves and some information concerning the charge dispersion curves were deduced from the independent and cumulative cross sections. The yield-vs.-mass curve for uranium shows a rapid decrease with increasing mass in the region fromA = 131 t o A ~ 150. The total isobaric yields appear to reach a valley or flat region at A ~ 155. Examination of neutron deficient (N/Z ~< 1.38) and neutron excess products (N/Z > 1.38) indicates the observed decrease in the total yields is due primarily to a rapid decrease in yields of the neutron excess products. The charge dispersion curve for products from uranium in the rare earth region is double peaked. The valley separating the neutron excess and neutron deficient peaks is deeper in the europium region than that reported previously in the cesium region. Total isobaric yields in the mass 134-155 region from bismuth and gold targets are approximately constant at = 11 mb, a higher value than those reported for products in the mass 72-131 region from lead targets. The charge dispersion curves for rare earth products from Bi and Au show single peaks on the neutron deficient side of stability. The results are discussed in terms of fission and spallation as possible mechanism leading to products in the rare earth region. 1. I N T R O D U C T I O N
studies of the interaction of protons in the GeV range with heavy elements [1] have led[2] to the following description of the yield-vs.-mass curves: The typical double humped yield-vs.-mass curve for low energy fission of uranium disappears as the bombarding energy is increased and there is only one maximum left located near half the target mass. The height of the maximum decreases and the peak becomes broader with increasing energies. On the other hand there is a marked rise in the yield-vs.-mass curve for masses below -~40. In the case of lead, the corresponding curve at G e V energies shows a similar upturn at low masses but is rather flat in the mass range from 40 to 120. Only limited information on yields from heavy element targets is available for masses higher than ~ 130. Charge dispersion curves at high bombarding energies have been measured by several authors [2-5]. For the fission of uranium, Friedlander e t a/.[3] have RADIOCHEMICAL
* Research performed under the auspices of the U.S. Atomic Energy Commission. tPresent address: Lehrstuhl fiir Kernchemie, Eduard-Zintl-lnstitut, Technische Hochschule Darmstadt, Germany. I. For a summary of high energy reactions with an emphasis on fission, see E. K. Hyde, The Nuclear Properties of the Heavy Elements, Vol. IlI; Fission Phenemona, Prentice Hall, Englewood Cliffs, New Jersey (1964). 2. G. Friedlander, Physics and Chemistry of Fission, Vol. II, p. 265.1nternational Atomic Energy Agency, Vienna 1965; and private communication. 3. G. Friedlander, L. Friedman, B. Gordon and L. Yaffe, Phys. Rev. 129,1809 (1963). 4. S. Kaufmann, Phys. Rev. 129, 1866 (1963). 5. N. Porile, Phys. Rev. 148, 1235 (1966). l
2
K. B,~CHMANN
carried out an extensive study of the charge dispersion of products in the region of cesium as a function of the energy of the bombarding protons. These authors found a double-humped charge dispersion curve above 1 GeV, whereas the charge dispersion curve for lead[2] (which should be very similar to bismuth) was single-humped with products only on the neutron deficient side. Radiochemical studies of recoil products have been shown to be of considerable value in the clarification of high, energy mechanisms by giving information about the kinetic energies of single fragments as well as about the excitation energies of the fissioning nulcei. Sugarman et al.[6] investigated the recoil properties of alSr and iza'~33mBafrom the fission of bismuth induced by 50-MeV to 2-GeV protons. They found that 91Sr is formed by a process requiring less deposition energy than 12a'~33mBa, if the bombarding energy is higher than 450 MeV. The kinetic energy of barium decreases with increasing proton energy whereas the kinetic energy of a~Sr does not change. In a comparison of fission of bismuth and tantalum induced by 450-MeV protons, Porile and Sugarman [7] found that the kinetic energies of the fission products from tantalum are lower which is reasonable for a fissioning nucleus of lower charge. An extensive study by Sugarman and coworkers [8] of the fission Of uranium at 450 MeV led to the conclusion that the kinetic energies of the products could be accounted for in terms of a nearly constant separation distance (18.4___0.5 fermis) between the charge centers at scission. Studies by Alexander et al.[9] (uranium, 0.5-6.2 GeV), Brandt[10] (uranium, 0.55 and 18 GeV), Crespo et al.[ll] (uranium, 2.2 GeV) and Hageb¢ and Ravn[12] (uranium, 0.57 and 18.2GeV) indicated that the neutron deficient and neutron excess products from uranium had considerably different recoil properties at G e V bombarding energies. The kinetic energies of the neutron deficient products are considerably lower than those of the neutron excess products. The neutron deficient nuclides are formed in high deposition energy processes whereas the neutron excess products are formed in low deposition energy processes. One may conclude from these results and doublehumped charge dispersion that neutron deficient and neutron excess products arise from different mechanisms. In this paper the terms neutron excess and neutron deficient are not used with their conventional meanings in which /3stability is the dividing line. The N / Z ratio which determines the separation line in our connotation lies somewhat to the neutron deficient side of/3-stability. The aim of the present work was to measure some cross sections for forming rare earth isotopes by the interaction of 28-GeV protons with uranium, bismuth, and gold. This will serve to extend information on yield-vs.-mass curves and charge dispersion curves into a little studied region. (At the present time two 6. N. Sugarman, M. Campos and K. Wielgoz, Phys. R ev. 101, 388 (1956). 7. N.T. Porile and N. Sugarman, Phys. R ev. 107, 1410 (1957). 8. N. Sugarman, H. Miinzel, J. A. Panontin, K. Wielgoz, M. V. Ramaniah,G. Lange and E. LopezMenchero, Phys. Rev. 143, 952 (1966). 9. M. Alexander, C. Baltzinger and M. F. Gazdik, Phys. Rev. 129, 1826 (1963). 10. R. Brandt, Physics and Chemistry of Fission, Vol. I1, p. 329. International Atomic Energy Agency, Vienna, 1965. 11. V. P. Crespo, J. B. Cumming and A. M. Poskanzer, Phys. Rev. 174,1455 (1968). 12. E. Hageb¢ and H. Ravn, J. inorg, nucl. Chem. 31, 2649 (1969).
Rare earth yields
3
other research groups are investigating this same region [ 13, 14]). As a companion to the present work, some recoil properties of rare earth nuclides from uranium and gold targets were also measured [ 15]. 2. E X P E R I M E N T A L
2.1 Irradiation The irradiations were performed in the internal beam of the Brookhaven Alternating Gradient Synchrotron (AGS) at a proton energy of 28 GeV. The duration of the irradiations varied from 20 min to 2 hr. The target was kept in a shielded position during each acceleration cycle until the protons reached full energy. Then the target was raised to a position near the beam and the magnetic field was changed in order to decrease the radius of the circulating beam until the protons hit the target. The beam intensity was sufficiently constant during irradiations so that corrections in the calculation of the cross sections were not necessary. The targets of uranium and gold were bombarded with about 1012 protons per pulse. With the bismuth targets it was necessary to decrease the intensity because of the low melting point to 4.1011 protons per pulse. The integrated circulating beam was about 1015 protons per irradiation. The true intensity seen by the targets is higher due to the multiple traversals. We performed two experiments with uranium and gold, respectively, and one experiment with bismuth. 2.2 Target Figure 1 shows the target arrangement. The target consisted of a stack of rectangular foils (1 x 2 in.) arranged in the following order: 0.001 in. of aluminum guard, 0.00l in. of aluminum monitor~ 0.001 in. of aluminum catcher, 0.001 in. of aluminum monitor, 0-001 in. of aluminum guard. The T: TARGET C: CATCHER FOILS MONITOR FOILS
M:
BEAM• 1
G: GUARD FOILS
G M
C
T
CMG
Fig. 1. Schematic diagram of the target arrangement. thick target-thick catcher arrangement was used because we wanted to determine the ranges[15] in the same irradiation. The foils were aligned such as to superimpose exactly and bolted into a target-holder. The error introduced by misalignment can be estimated from the agreement of the upstream and downstream monitor foils to be less than 2 per cent. The target foils were commercially available uranium, bismuth, and gold. Bismuth and gold were analyzed spectrographically. There was no detectable impurity present which could interfere. The foils were cleaned with nitric acid prior to irradiation. Their thickness was determined by weighing known areas of the foils. After the bombardment an area of about 1 x 2 cm was cut out. 2.3 Chemical procedure The target foils were dissolved in a mixture of nitric acid and hydrochloric acid containing known amounts of each of rare earth elements and yttrium. Scandium, barium, and zirconium carriers were also added. The separation of the lanthanide group was carried out by a modification of procedures described in NAS-report 3020116]. The bulk of uranium and gold respectively, and all the 13. Y. Y. Chu, E. M. Franz, G. Friedlander, E. Hechtl and P. Karol,154th International Meeting of the ACS, Chicago, Illinois, September 10-15, 1967; Abstract R 50 and G. Friedlander, private communication. 14. J. Alstad, private communication. 15. K. Biichmann andJ. B. Cumming, Unpublished. 16. The radiochemistry of the rare earths, scandium, yttrium and actinium. Monograph Series on the Radiochemistry of the Elements, Nuclear Science Series NAS-3020.
4
K.B.KCHMANN
anionic products were separated by means of an anion exchange column (Dowex-1). The rare earths were purified by a series of fluoride- and hydroxide-precipitations. Cerium was oxidized in 10 N HNO3 by a KBrO3 solution and extracted with 25% di(2-ethylhexyl) acid orthophosphate dissolved in n-heptane. The Ce 4+ in the organic phase was reduced with H202 and back extracted into 1 N-HNOa. The other rare earths were mixed with ca. 1 ml of cation exchange resin (Dowex 50W-X12,200400 mesh) and after standing long enough to attain equilibrium, the resin was put on the top of a column. The separation was carded out at 80°C using a-hydroxyisobutyricacid (a-HIBA) (0-13 M and pH = 4.35) as eluent. The flow rate was kept constant at 3 drops per rain. The separation took about 16 hr, but most of this time was required for the separation of the light lanthanides. The series from lutetium to europium was separated in about 3 hr. After 3 hr the concentration of the a - H I B A was increased to 0.4 M. Fractions of the eluate were collected using an automatic fraction collector and their activities were measured in a well-type scintillation counter. Figure 2 shows the activity distribution of one of the separations. The activity distribution does not allow true evaluation of I
z
§
I
I 106 ~
I
I
i
't/v
t 20
// I" 40
I
Nd i
Dy/Y
[.o
iO 2 I
I
I 60 NUMBER
Eo
v
I I 80 I00 OF T H E FRACTION
n
..If I~ 120
~--
Fig. 2. Activity distribution in atypical rare earth separation. the contamination factors of adjacent rare earths elements, because some of the activity found in the valleys may be due to the growing-in of daughter activities during the separation procedure. The fractions around the maxima were combined and used to prepare samples for the activity measurements by coprecipitation with barium oxalate. Barium did not interfere in the subsequent analysis of the rare earth by the arsenazo method. Impurities in these samples coming from neighboring lanthanides never exceeded 1 per cent. However, it was not possible to separate yttrium and dysprosium quantitatively. 2.4 Beam intensity The beam intensity was measured using two monitor foils arranged upstream and downstream of the target. The results of the two monitors were the same within 2 per cent. For monitoring either the reaction 27Al(p, 3pn)24Na or the reaction 27Al(p, 3p3n)22Na was used. In the first case the activity of the 15 hr Na-24 was assayed with a calibrated endwindow proportional counter. In the second case the annihilation radiation was measured by a NaI-crystal. The cross section for the formation of Na-24 at 28-GeV was assumed to be 8-9 mb[17]. For the cross section of Na-22, a value of 10 mb was taken[17]. Subtractive corrections were made for the production of 24Na in the monitor foils be secondary reactions (mainly 2¢Al(n, a)2~Na). The correction was assumed to be 3 per cent[18]. 2.5 Measurement o f the activity The activity of the samples was followed as a function of time. Four different counting devices were used: Endwindow proportional counters for the measurement o f / 3 - - or /3+-radiation, two NaI-crystals in connection with a coincidence unit and single channel analyzers for the detection 17. J. B. Cumming, Ann. Rev. Nucl. Sci. 13, 261 (1963). 18. J. R. Grover, Phys. Rev. 126, 1540 (1962).
Rare earth yields
5
of fl+-radiation, a NaI-crystal (2-mm thick) with a multichannel analyzer for the detection of x-rays, and a lithium drifted germanium detector in connection with a multichannel analyzer for the detection of v-rays. As most of the rare earth samples contained a number of active isotopes with measurable half-lives, the lithium-drifted germanium detector was the most important detector. The crystal was calibrated with a set of standards (IAEA) in order to get a photopeak-yield curve as a function of energy and geometry. The -/-spectra were taken as a function of time in order to check the identity of questionable `/-lines. The area under a peak was determined with the usual subtractive corrections (Compton continum and background) and the activity at the end of the irradiation and at the time of separation, respectively, calculated by a least-square computer-program [19]. The positron disintegration rates were measured by the coincidences of the two annihilation quanta detected in two 3 × 3 in. NaI-crystals which were arranged at a 180° angle. The samples were mounted between copper absorbers to insure annihilation of all the positrons. A single channel analyzer was used in connection with each detector, with the channel set to the 51 l-keV peak. The background was less than 0.2 cpm. The positron detection efficiency was determined with a calibrated 22Na source. The fraction of 22Na decaying by positrons emission was taken to be 0-898[20]. The K X-rays emitted by the lanthanides which decay by electron capture were measured with a NaI-crystal used in connection with a multichannel analyzer. The use of a multichannel analyzer has the advantage that it was not necessary to check the channel settings and to correct for possible shifts. The efficiency was calibrated by a ~ C e (33 keV) and an Z41Am(59.5 keV) source. The energies of the rare earth K X-rays observed in this work fall between ~33 keV and ~46 keV. We assumed a linear variation of detector efficiency over the energy range covered by the two standards.
2.6 Analysis of the rare earths Recovery efficiencies (chemical yields) of the lanthanides were determined photometrically by the arsenazo method[21]. In separate experiments it was shown[22] that there was no interference from the barium carrier, provided the amount of barium did not exceed 3 mg, corresponding to a ratio of 10 to 1 for barium to rare earth elements. Dysprosium and yttrium were not separated completely by the ion exchange procedure; these two elements were analyzed together. First the sum of dysprosium and yttrium was determined photometrically, secondly the ratio of the spectral yields was measured and thirdly dysprosium was analyzed by flamephotometry [22]. 3. R E S U L T S A N D D I S C U S S I O N
3.1 Calculation o f the cumulative and independent cross sections The counting rates are corrected following correction factors:
to absolute disintegration rates D O by the
(1) E f f i c i e n c y o f t h e c o u n t e r (in t h e c a s e o f t h e l i t h i u m d r i f t e d g e r m a n i u m detector, the photopeak efficiency) (2) B r a n c h i n g r a t i o s in t h e d e c a y s c h e m e (3) C h e m i c a l y i e l d o f t h e s e p a r a t i o n (4) C o r r e c t i o n f o r t h e d e c a y . For nuclides which do not require taking into account parent-daughter relationships (shielded nuclides, quasi-shielded nuclides, or cumulative yields of long-lived nuclides with short-lived precursors), the cross sections are obtained by the following equation: cr = D~°F
19. 20. 21. 22.
J. B. Cumming, Brookhaven NationalLaboratory Rep., BNL-6470. Nuclear Data Sheets, compiled by K. Way et al. J. S. Fritz, M. J. Richard andW. J. Lane, Analyt. Chem. 30,1776 (1958). E. Norton, private communication.
(1)
6
K. BACHMANN
where
D ~ = D ° ( 1 - - e -xr)
(2)
WA~4XO'A1 F - WXAAID~a
(3)
and
WA~ is the superficial density of aluminum and Wx the superficial density of the target in question. DN% is the calculated disintegration rate for the monitor as in Equation (2) either for 24Na or for 22Na. Ax and AA1 are the masses for the target and aluminum, respectively, O'AI is the cross section for the formation of 24Na or Z2Na. If parent-daughter relationships had to be taken into account the equations given in Refs [3, 23] were used. The errors become large in cases where the parent half-life is of the same order of magnitude as the separation time. It is not possible to determine the exact separation time of two adjacent rare earths on the column. As the same experiment was used to measure the ranges, it was possible to correct properly for recoil losses. Some of the recoil products are escaping from the edges of target foil. The correction for the edge effect was taken to be 0.5 per cent [8]. Some of the neutron excess products could also be formed by secondary reactions, e.g., fission by low energy neutrons which are produced in the nuclear cascade or subsequent evaporation. Friedlander et a/.[3] have found the apparent 14°Ba production cross section from uranium increases by 7 per cent for each 100 mg/cmz of target thickness at 3 GeV. In this work a value was used, which corresponded to the yield-vs.-mass curve for fast neutrons (i.e. for 144Ce 5.9% and for 147Nd 2.6% per 100 mg/cmz target thickness). No corrections were applied to cross sections obtained with the gold and bismuth targets. Measured cross sections for products of the interaction of uranium, bismuth, and gold with 28-GeV protons are compiled in Tables 1 and 2. The type of cross section is indicated in the second column. The measured radiations are tabulated in the fourth column. If there was more than one determination by different radiations, for example with T-rays and/3 +, then the most reliable value or a weighted average of the separate values is reported. The decay schemes of some of the neutron deficient nuclides are very poorly known. We have used the T-lines whose intensities seemed most reliable and which were subject to least interference by other nuclides. The errors were estimated from the uncertainties due to the statistical error of counting, the counting efficiencies, the chemical yields, the separation time and the decay schemes. Not included are the systematic errors, such as the error in the monitor cross section (___8%) and errors from erroneous decay schemes. The errors on the fl+-radiation measurements are higher than one would expect since we suspect the calibration. The errors assigned to the short-lived dysprosium and terbium products are exceptionally large due to the uncertainty in the separation times and half-lives. The unpublished results of Friedlander et al.[13] and Alstad[14] are in rather good agreement with most of our values. However, some of the cross sections are 23. N.T. Porile, Phys. Rev. 120, 572 (1960).
Rare earth yields
7
Table 1. Cross sections for forming rare earth isotopes in the interactions of uranium with 28-GeV protons
Product
Type of yield*
Cross section (mb)
Measured radiations with abundances used'~
Half life used
134Ce 139Ce ~4aCe l~Ce 144Ce 13apr ~42pr ~3aNd 14°Nd 147Nd l~pm 14spm 148pm ~Pm 151pm luSm l~Sm ~45Eu ~46Eu 147Eu ~4SEu 149Eu ~5°Eu ~snEu 157Eu ~Gd 147Gd 149Gd 14aTb 15~Tb ~52Tb ~53Tb ~Tb l~Tb ~S2Dy 153Dy ~55Dy
C I C C C I I C C C C I I C C C C C I I I I I I C C C I C C I I I I C C C
7.6 -4-_1.0 0.7___0.07 3.7±0.4 6-8___0-5 7-6___0.8 1'4 : 0"7 0.8 ---0.2 3 "4 +--0"6 3-4___0.5 1.7±0.17 0-55---0.05~ 0.004 ___0.001:~ 0.17 ---0.02~: 0.48_0.05:~ 0-24_0.04~ 0.46___0.08 0-22___0.05 4.00_+0.35 0.87 _+0.07 0.30 _+0.03 0.48---0.10 0.08-+0.02 0.02 -- 0.05 0.28-+0.04 0-20---0.03 5 - 0 6 - 0.4 3.05 -+ 0.25 1.45 -+ 0.11 3"60-+0"8 4.51_+0"5 0.85---+0.40 0-95---+0.15 0.60_+0.10 0.14_+0.02 5.6+_-2.0 6.8 -+ 2-5 7.1 __-1.7
/3+(0.61) 170keV (1.00) 1-45 keV (0.70) 290 keV (0.37) 130keV (0-24) 170 keV of ~39Ce 1570 keV (0-037) 170 keV of iaaCe /3+ of Pr-140 (0.53) 530keV (0-33) 740 keV (0.45) 1470keV (0.24) 726keV (0.34) 290 keV (0.03) 340keV (0.21) 103 keV (0.78) 203 keV (0.29) 900 keV (0-67) 750 keV ( 1.00) 200 keV (0.24) 550keV (1.00) 328keV (0.13) 330 keV (0.04)0 + (0-06) 1230keV (0-11) 413 keV (0.20) 750 keV of l~Eu 200 keV of 147Eu 300 keV (0-30) 300 keV of l~Gd 250keV (0-37) 340 keV (1-00) 210 keV (0.31) 180keV (0.15) 530 keV (0-71) 340--- keV of ~SZTb 210 keV of 15Z'Fb 227 keV (0.72)
75 h 140 d 32-5 d 33.4h 277 d 4.5 h 19.2 h 5-2 h 3.3 d 11-1 d 265 d 5.4d 41 d 53.1 h 28 h 47 h 9.4h 5.6d 4-7 d 23.6 d 54 d 106 d 12.8 h 15 d 15.2 h 50 d 39 h 9.5 d 4.1 h 18 h 17.4h 2-6d 5 d 5.1 d 2.4h 5.5 h 10 h
*C = cumulative. I = independent. tThe abundances are according to the Nuclear Data sheets [20]. Furthermore, the values are corrected for conversion, and in some cases a subtractive correction for a-decay is applied, l~Gd and ~4~Gd for example get a contribution from l~°Dy and 15~Dy. eThe promethium cross sections are only relative, because no carder was present.
K. BACHMANN Table 2. Cross sections for forming rare earth isotopes in the interaction of of bismuth and gold with GeV protons
Product
Type of yield
134Ce lasCe aaPr lagNd ~4°Nd mEu 148Eu 147Eu 14SEu ~Gd ~47Gd XOGd XS~Gd t4~I'b ~5~Tb ~52Tb ~Tb ~SSTb ~S6Tb 152Dy t~Dy ~Dy
C C,I I,C C C C I,C I,C I C C I,C I,C C C I I,C I I C C C
Cross section bismuth (mb)
Cross section gold (mb)
9.9"--2.0 2.25 (C.I)
12.2"-.2.8 1.35"-.0.6(1); 10-0_ + 1.8 (C) -1.55 +-0-8(1); 8.6 +- 1-66(C) 7.05 +- 1"5 -13"35--+3"0 -6.65 - 0"6 2.24_ 0.18(I); 8.34 +-0.6 (C) 1.74+-0.15(I); 9.14_ 0.7 (C) 1.06±0-12(I);9.56+-0.9(C) 2.66--.0.30(1); 10.86-0.9(C) 0-46 ± 0.08 1.20 +-0.25 6.1 ---0.5 7.4-+0-6 8"5---0.8 8-2±0"8 1"0-----0.1(I); 9"4 +-2.0 (C) 1-8- 0"15(I); 8"8 +- 1"6 (C) -0-7 +-0.2(I); 4.0 +-0-4 (C) 8.4 ___2-0 7.0 +- 1.6* 2.3"--0.3 3-35__.0.4 2-08---0.8 2.3+__0.8 2.14--.0.4(1);14.2"-.3.5 (C) 2.95"-.0.5(1);12.05+-3.6(C) 0.6---0-1 0.40---0.06 0.42 _-.0-07 0.05 ---0.08 -6"4 -- 3"0 12.1--.3-5 9.1--.3.6 -12.0 +_3"0 -
-
*When combined with the alpha branching ratio measured by Chu, Franz and Friedlander[24], the results of Brtminx[25] imply a cross section of 3.4 mb for this reaction while those of Franz and Friedlander[26] lead to a cross section of 4.3 mb. q u i t e different. B o t h a u t h o r s u s e a m a s s s e p a r a t o r as well as c h e m i c a l s e p a r a t i o n , a p r o c e d u r e w h i c h s h o u l d i m p r o v e r e s o l u t i o n o f the v a r i o u s isotopes. 3.2 Charge dispersion curves I n a n ideal e x p e r i m e n t , all the i n d e p e n d e n t a n d c u m u l a t i v e f o r m a t i o n c r o s s s e c t i o n s w o u l d b e m e a s u r e d to give a g e n e r a l p i c t u r e o f the d e p e n d e n c e o f t o t a l i s o b a r i c y i e l d o n m a s s n u m b e r ( m a s s - v s . - y i e l d c u r v e ) as well as the d e p e n d e n c e o f y i e l d o n n u c l e a r c h a r g e ( c h a r g e d i s p e r s i o n c u r v e ) at a p a r t i c u l a r m a s s n u m b e r . H o w e v e r , for i n t e r a c t i o n s o f h i g h - e n e r g y p r o t o n s , o n l y i n a few i n s t a n c e s c a n sufficient d a t a b e a c q u i r e d at o n e m a s s n u m b e r to define a n ideal c h a r g e d i s p e r s i o n c u r v e . I n g e n e r a l it is n e c e s s a r y to u s e d a t a f r o m a r a n g e o f m a s s n u m b e r s with c o r r e c t i o n s for t h e d e p e n d e n c e o f yield o n m a s s in e s t i m a t i n g c h a r g e d i s p e r s i o n c u r v e s . F r i e d l a n d e r [ 2 ] has d i s c u s s e d s u c h p r o c e d u r e s for p r o d u c t s o f the intera c t i o n o f u r a n i u m a n d lead with 2 8 - G e V p r o t o n s . F o r u r a n i u m , the d a t a o f F r i e d l a n d e r et aL[3] i n d i c a t e the c h a r g e d i s p e r s i o n c u r v e (plot o f i n d e p e n d e n t 24. Y. Y. Chu, E. M. Franz and G. Friedlander, Phys. Rev. 175, 1523 (1968). 25. E. Bruninx, Nucl. Phys. 64, 481 (1965). 26. E. M. Franz and G. Friedlander, Nucl. Phys. 76, 123 (1966).
Rare earth yields
9
formation cross section as a function of neutron to proton ratio (N/Z) of the product) at A = 131 has two comparable height peaks with a valley at N/Z ~ 1.42. Data of Porile[5] in the mass region near A = 109 are equally consistent with either a double or single peaked charge dispersion. Results of Kaufman[4] at A = 72 indicate a single peaked curve. Data for lead targets show single peaked charge dispersions at all these masses [2]. In Fig. 3 we have plotted the independent cross sections obtained in the present work for forming europium isotopes from uranium by 28-GeV protons. I0
I
I
I
1
I
i29Cs T
..o
E
I
~E,, @\ 14SEu \\ ~
_
Z 0 I-
_L
_
15SEu
e \
147Eu \
t0
\
/ \~9Eu
0
tx
131Cs
,~/
/
0.I
?
t50 U
0.01
I 1"3
I
|
I
1.4 N/Z, NEUTRON TO PROTON RATIO
I 1.5
Fig. 3. Comparison of isotopic yield distributions of europium and cesium produced by the interaction of 28-GeV protons with uranium. Filled circles are from the present work. Open circles are from Ref. [2].
For comparison, independent yields of cesium isotopes from the work of Friedlander[2] are shown as open circles. The solid curve through the cesium points was obtained from the double peaked charge dispersion curve reported for A = 13112] with correction f o r t h e dependence of isobaric yield on mass number. The dashed curve through the europium points serves only to indicate a general trend. The point for lS°Eu represents only one of the 15°Eu isomers and as such is a lower limit for the charge dispersion curve. Although the data in Fig. 3 are strictly speaking isotopic distributions rather than charge dispersion curves, they illustrate some of the general conclusions which may be drawn from our data: the charge dispersion curve in the rare earth region is double peaked and the valley separating the two peaks appears to be relatively deeper than in the cesium region. It is interesting that the yields of the lower-mass valley products, 139Ce
10
K. B~,CHMANN
and 142pr, (see Table 1) are only the order of a factor of 2-3 below the corresponding cesium yields. The minimum in the charge dispersion curve in the rare earth region is along with range data [ 15] additional evidence for different mechanisms of formation for the neutron excess and neutron deficient products from uranium targets by 28-GeV protons. Turning now to products from bismuth and gold targets, independent cross sections from Table 2 have been plotted in Fig. 4. There is very little difference between yields from gold and those from bismuth. The dashed curve in Fig. 4 i0
r
152TI
I
I
155Tb
d
\
1 4 6 ~ (~ \\ .D
E z 0
147Et
l--
\\
b3 ~0
148Eu
A
\~56Tb \
0 {E (J
O.l
'\ (
\ \ \ \ \
O'OI
I 1.3
1"4
I \
I 1"5
N / Z , NEUTRON TO PROTON RATIO
Fig. 4. Dependence of independent formation cross sections on N/Z of the product in the interaction of 28-GeV protons with gold (O) and bismuth (0).
is the charge dispersion curve for A = 131 given by Friedlander[2] for the interaction of 28-GeV protons with lead. Despite the scatter of our results, it is apparent that differences between the charge dispersion curve at A ---- 131 and that in the rare earth region are much smaller than were observed for uranium targets (see Fig. 3). Our failure to observe neutron excess products from Au or Bi targets is consistent with the extremely low yields of nuclides such as 14°Ba reported by Friedlander[2]. 3.3 Yield-vs.-mass curves 3.3.1 Neutron deficient products from uranium. Based on the rather clean separation of the two parts of the charge dispersion curve (Fig. 3) it seems reasonable to discuss separately the yield-vs.-mass curves for the neutron excess
Rare
earth
yields
I1
and neutron deficient products in the rare earth region. We have rather arbitrarily adopted an N/Z value of 1.38 as the dividing point. Such a division is much less justified for products with A = 131 or lower where the valley is less distinct. In Fig. 5 the cumulative isobaric yields for various neutron deficient mass chains are plotted as a function of N/Z. With the assumption that the slope of the curves
IJ
i J ~ I i i l F I i I I ; I "~3D
I0
6
,55D~'53Tb
],.~ST b
15ZDY
E
I
I0 o 8 I'-
Z
-
1
--
~46Eu
0 bJ (l)
-
o
r,-o
I
{4rGd
1
2
I0
134C
ICs
Z
(J
L3~
~7~'IBo
I
~39
f4ONd 2 i
I ~-25
I
I
I
I
I
I
I
i
l
I
I
t'30 ~'35 N/Z,NEUTRON TO PROTON RATIO
I
l F40
Fig. 5. Cumulative isobaric yields of neutron deficient products of the interaction of 28-GeV protons with uranium.
is the same in a limited mass range, we have extrapolated or interpolated to cumulative yields at N/Z = 1.38. Included in Fig. 5 are values for the A = 131 mass chain from the data of Friedlander [2]. T h e curves for higher masses appear flatter than those for masses 131 and 139. This reflects the increasing depth of the valley as seen in Fig. 3. T h e neutron deficient cumulative yields obtained by this procedure are plotted in Fig. 6. F o r the higher masses, these cumulative yields are quite insensitive to the choice of N/Z ---- 1.38 as the dividing point. F o r the lower masses, use of a higher value of N/Z, e.g. N/Z = 1-42 as suggested by the results of Friedlander et a/.[3], would increase the cumulative cross sections for the neutron deficient products and increase the rise shown in Fig. 6.
12
K. B ) k C H M A N N
15
I
I
I
I
I
I
I
I
I
I
I
I
za E
z IO
-
O I--
(/5
~5 O t~ o
o
130
140 150 PRODUCT MASS NUMBER
Fig. 6. Y i e l d - v s . - m a s s c u r v e of n e u t r o n deficient p r o d u c t s f r o m the i n t e r a c t i o n of 2 8 - G e V p r o t o n s w i t h u r a n i u m . T h e filled circle w a s o b t a i n e d f r o m F r i e d l a n d e r [2].
Fig. 6 indicates that neutron deficient yields are quite constant in the mass range 139-155 but that they rise by a factor of two in going toA = 131 and 134. This increase does not continue with further decreasing mass number. For example, out of the total isobaric yield of 27.2 mb atA = 109 reported by Porile [5] 11 mb of products have N / Z ~< 1.38. At mass 72 [4], ~ 13 mb of a total of 15 mb have N / Z <-1.38. It appears that neutron deficient products (N/Z <<-1.38) constitute a rather flat background under the fission peak observed in the overall yield-vs.-mass curve of uranium at 28-GeV. It has been proposed by Rudstam and SCrensen[27] that a spallation like mechanism is responsible for the formation of neutron deficient iodine isotopes. The probability of such a process should increase with increasing product mass. The rise in cross sections from A = 139 to A = 131 is not expected for a spallation mechanism and may indicate contributions from some other mechanism to neutron deficient product formation at A = 131. The shallower valley in the charge dispersion curve also points in this direction. The data in Fig. 6 are consistent with a slight increase in cross sections in going from A = 139 to 155, but no definite conclusion can be drawn due to the magnitude of the errors. Feeding of the various neutron deficient chains by alpha decay from still heavier nuclei is additional complication for which only partial correction has been made. 3.3.2 Neutron excess products from uranium. Substantially less information has been obtained in the present work on yields of neutron excess products and construction of a figure comparable to Fig. 5 is not possible for the neutron excess chains. A general conclusion that can be drawn from the data in Table 1 is that yields of neutron excess products are decreasing rapidly with increasing mass in the rare earth region. Cumulative yields of selected neutron excess products observed in previous experiments[2, 5, 27] are plotted in Fig. 7 together with results from the present work. Open circles at A -----141, 143, and 144 are the observed cumulative yields of the cerium isotopes. These are lower limits for 27. G . R u d s t a m a n d G . S C r e n s e n , J. inorg, nucl. Chem. 28, 771 (1966).
Rare earth yields
I
I
I
13
I
I
20
E
z O w
O n~
IIO
130
150
MASS NUMBER
Fig. 7. Yield-vs.-mass curve of neutron excess products of the interaction of 28-GeV protons with uranium. Points are from, Porile[5] (I-q), Rudstam and SCrensen [27] (~), Friedlander[2] (A), and the present work ((3). Filled symbols indicate those cases which are expected to approximate the chain yield up to N/Z = 1.38.
the chain yields. T h e point of A = 147 is the cumulative yield of 14rNd with an estimated correction of 0.4 mb for 147pm. T h e points at A = 153 and A -- 156 are based on the cumulative yields of 153Sm and 156Eu with small corrections for later members of the chains. Figure 7 indicates a rapid decrease of neutron excess yields from mass 140 to 156, the same region in which the neutron deficient yields are relatively constant. N e u t r o n excess yields have virtually disappeared at mass 155 (reduced by a factor of ~ 3 0 from the peak yields at A ~ 115). T h e slope of the curve in Fig. 7 and mean range measurements[15] suggest that neutron excess products are formed by fission. 3.3.3 Total isobaric yields. In Fig. 8 the yield-vs.-mass curve for uranium irradiated with 2 8 - G e V protons is presented. T h e points and curve from A = 20 to A = 131 are basically those given by Friedlander[2] with the exception that a more recent value due to Porile [5] has been used at A = 109. Total isobaric cross sections deduced from the present experimental data are shown as filled circles. T h e s e show a rapidly decreasing cross section as the mass of product increases from A = 131 to ~ 145, and then a flattening of the curve. Despite large ( ~ 20 per cent) uncertainties in individual values, we believe it is justified to draw the yield-vs.-mass curve in Fig. 8 with zero slope at A -~ 155 based on the data presented in Fig. 6 and 7. T h a t a valley has been reached is interesting as the fission peak in the yield-vs.-mass curve is now delineated on both its low and high mass sides. W e e x p e c t that the yield-vs.-mass curve will either remain
14
K.B.~CHMANN I
I
I
I
I
I
I
I
I
I
3o .o E
z
20
m
o
o:1o
!
o 20
I
I
40
60
80 I00 120 MASS NUMBER
I
I
140
160
Fig. 8. Total isobaric yield-vs.-mass curve for the interaction of 28-GeV protons with uranium. Points are based on results o f Friedlander[2] (O), Porile [5] ([7), and the' present experiment
(0). rather flat or rise slowly for masses above ~ 155 as a consequence of increasing probability for spallation reactions. Examination of the data presented in Table 2 indicates that the differences in cross sections for products from bismuth targets and those from gold targets are smaller than the scatter of the results. As a consequence of the extremely low yields of products having N/Z > 1.4 (see Fig. 4) the observed cumulative and independent yields account for ~ 7 0 per cent of the total isobaric yield of some mass numbers in this region, We have used the data in Fig. 4 to approximate the missing yields and have plotted total isobaric yields as a function of mass number in Fig. 9. (The points at A = 151 are anomalously low in both cases, but were not in the case of u r a n i u m targets). T h e s e results are consistent with a constant cross section of -~ 11 mb per mass number in the rare earth region. T h e yield-vs.-mass curve for lead as drawn by Friedlander [2] is quite fiat at a value 7 - 8 mb per mass n u m b e r for products between A = 72 to A = 131. Our data then suggests a gradual rise with increasing mass above 131. It is interesting to note that the yield-vs.-mass curves of bismuth and gold appear to cross that of uranium in the rare earth region. 4. C O N C L U S I O N S
T h e present survey study of the interaction of 2 8 - G e V protons with heavy elements targets indicates that the rare earth region is particularly interesting in the case of uranium targets. T h e valley in the charge dispersion curve separating neutron excess from neutron deficient yield is even more p r o n o u n c e d at mass ~ 150 than has been reported at lower masses. This suggests different formation mechanisms for the two classes of products. N e u t r o n excess yield from uranium decrease rapidly with increasing mass, the yield-vs.-mass curve being similar to that observed in fission induced by 200-Me protons.
Rare earth yields t
~
I
15 -+-T
I I I
B+
T
I
15
!
io
i l
'r-
s v
5
,g
i
I---+u~ u3 o
Au
+
i
T
'
~o~
i
[
!
i 5t
t t
_i 13o
I____ I
I
140 MASS NUMBER
150
Fig. 9. Total isobaric yields for selected masses in the rare earth region formed by the interaction of 28-GeV protons with bismuth (upper figure) or gold (lower figure).
Spallation has been proposed [27] on the basis of isotopic yield distributions as the formation mechanism for neutron deficient iodine isotopes from uranium at 18 G e V . The yield-vs.-mass curve (Fig. 6) for neutron deficient products is relatively fiat from A = 139 to A = 155, but appears to rise in going to lower masses. Such a trend is not expected for spallation and suggests at least some contribution from another mechanism, probably high-deposition energy fission at the lower masses. For gold and bismuth targets, only the neutron deficient peak is present in the charge dispersion curve. The yield-vs.-mass curves in the region A ~ 150 from these targets are significantly above the corresponding curve for uranium. This is reasonable for a spallation mechanism as there is less fission competition and the rare earth region lies closer to the Au and Bi targets.
Acknowledgements-The author wishes to acknowledge the hospitality of the BNL Chemistry Department during the time he spent there. He would like to express his appreciation for assistance from the following: Dr. R. W. Stoenner and Miss E. Norton who performed numerous chemical analysis; the operating staff of the Brookhaven AGS; Miss E. M. Franz and B. Neidbart who assisted in the carrying out of the experiments; Dr. J. Hudis who kindly arranged part of the irradiations. Valuable encouragement by Dr. G. Friedlander and Professor Dr. K. H. Lieser are acknowledged with particular pleasure. The author would like especially to thank Dr. J. B. Cumming for his detailed criticism and a very helpful discussion.