HEAVY ION REACTION CHANNELS DETERMINED FROM INDUCED RADIOACTIVITY MEASUREMENTS

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HEAVY ION REACTION CHANNELS

DETERMINED FROM INDUCED RADIOACTIVITY MEASUREMENTS

R. Robinson, H. Kim, J. Ford, Jr

To cite this version:

R. Robinson, H. Kim, J. Ford, Jr. HEAVY ION REACTION CHANNELS DETERMINED FROM INDUCED RADIOACTIVITY MEASUREMENTS. Journal de Physique Colloques, 1971, 32 (C6), pp.C6-265-C6-268. �10.1051/jphyscol:1971661�. �jpa-00214879�

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JOURNAL DE PHYSIQUE Colloque C6, suppl&ment au no 11-12, Tome 32, Novembre-Dkcembre 1971, page C6-265

HEAVY ION REACTION CHANNELS DETERMINED FROM INDUCED RADIOACTIVITY MEASUREMENTS

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R. L. ROBINSON, H. J. KIM and J. L. C. FORD, Jr.

Oak Ridge National Laboratory, Oak Ridge, USA

RCsumC. - Des mesures de rayonnement gamma qui suivent le bombardement par ions lourds ont BtC utilisees pour determiner les sections efficaces absolues pour differentes voies de reaction.

Les produits radioactifs ont ett: identifies par la mesure des vies moyennes des energies de transition gamma, et des intensites. Les produits de reactions formes par le bombardement des cibles de 5896oNi avec des ions ^{1 6 0}de 38 a 46 MeV ont BtB BtudiCs.

Abstract. - Gamma-ray measurements following heavy ion bombardment were used to determine the absolute cross sections for different reaction channels. The radioactive products were identified by half-life, gamma-ray energies, and intensities. Reaction products formed by the bombardment of 58960Ni targets with 38 to 46 MeV oxygen ions have been studied.

Heavy ion reactions frequently result in a large number of reaction products. There is still compara- tively little known about the absolute cross sections for different exit channels and the cross-section dependence on the type and energy of the projectile.

Yet the total reaction cross section can guide in the choice of optical model parameters and the cross section for various processes can provide insight into the reaction mechanism as well as spectroscopic information [I]-[3]. We have begun a program to determine these cross sections and report here our initial results on the reactions 58960Ni(160, X).

Since many of the emitted reaction products are radioactive, the measurement of the induced radioactivity provides a simple and rapid technique for determining cross sections for many of the large number of exit channels simultaneously. Observation of the prompt gamma rays with the target ⁽⁽in-beam ⁾⁾ complements this information.

In the present experiment reaction products formed by the bombardment of enriched 58Ni and 60Ni targets with 38 to 46 MeV oxygen ions have been studied. The 58,60Ni targets were respectively, 1.00 and 1.64 mg/cm2 thick. Recoil products were stopped in a 7 mg/cm2 gold foil placed immediately behind the target. The gamma rays from the target were observed, both during the bombardment interval and while the beam was off, by means of a 30-cc Ge(Li) detector.

In the c< in-beam ⁾⁾ study, the only identification was throughthe gamma ray energies. However, in the activation study, the half-life for the radioactivity was used along with the gamma ray energies and

(*) Research sponsored by the U. S. Atomic Energy Commis- sion under contract with Union Carbide Corporation.

intensities to establish the residual product. To emphasize products with different lifetimes, the bombarding time was varied. For the longer irradiations (more than a n hour), the target was removed from the target chamber after bombardment and gamma rays observed by a detector in a counting room. For the shorter irradiation, the scheme illus- trated in figure 1 was employed. The target was

FARLDAY CUP

CONTROLLER

T I M E R -

FIG. 1. - A schematic of the experimental apparatus.

bombarded for a given time interval. The beam was turned off for an equal length of time, the target was rotated to the detector position, and gamma ray pulses were recorded in several different groups of the analyzer for equal times. From these sequential spectra, lifetimes are ascertained. The lower limit for the duration of the cycle is about a second.

It is important that a careful record be kept of the beam current throughout the irradiations. This was done by storing current integrator pulses in a second analyzer operating in the scaler mode.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971661

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C6-266 R. L. ROBINSON, H. J. KIA 5 AND J. L. C . FORD JR.

The strongest gamma rays observed in the oxygen plus 60Ni reactions were at 511 and 634 key. The decay curves measured at 42 MeV for these transitions are shown in figure 2 and illustrate the application of

FIG. 2. - Decay curves for the 634 and 511 keV gamma rays.

The solid line through the 511 keV data points is the sum of the three components with half-lives of 16 min, 44 min and

> 10 hr.

activation analysis to the determination of cross sections. From its decay curve and energy, the 634 keV gamma ray is identified as one from the first 2' state in 74Se. The radioactive nuclei 74Br, whose decay gives rise to the 634 keV gamma ray, can be produced in two ways : (1) directly through the (160, pn) reaction and (2) by radioactive decay of 16 min 74Kr, formed by the (160, 2 n) reaction. The predicted decay curve for different ratios R of the relative cross sections of the (160, pn) and (160, 2 n) reactions were calculated. Curves for R = 2, 5 and 10 are plotted in figure 2. Comparison of these with the experimental points indicates that the (160, pn) reaction is greater than eight times the (160, 2n) reaction at 42 MeV.

Evidence for the (160, 2n) reaction is seen from the decay curve of the 51 1 keV annihilation radiation.

The curve is shown decomposed into components with half-lives of 16 min and 44 min, and a long-

lived (> 10 hr) component. The 16 min and 44 min groups are probably due to the decay of 74Kr and 74Br, respectively. No other gamma ray with a 16 min half-life was observed, indicating 74Kr decays predo- minantly to the ground state of 74Br.

The reactions observed and the methods of identi- fication are summarized in Tables I and 11. In the third column A or I indicates whether the reaction was observed by the activation technique, or during

ct in-beam >> bombardment of the target. Although several gamma rays from each product were observed, only one was used to extract the cross section. This gamma ray is given in Column 4 of Table I with the

w - Products from 58Ni(160, X ) Y Reactions

The third, fourth, and fifth columns identify the technique U- I(A = activation, I = in-beam), gamma ray transition (nucleus in which transition occurs) used to obtain the cross section, and the ratio, P, of this gamma ray intensity to the total intensity, respectively.

El, (kev) a (mb)

X Y Method (Nucleus) P at 46 MeV

- - - - -

Products .from 60Ni(160, X ) Y Reactions

The third, fourth, and fifth columns identify the technique

- - - - - - ^--

used (A = activation. I = in-beam). gamma rav transition ,,

-

(nucleus in which transition occurs) used to obtain the cross section, and the ratio, P, of this gamma ray intensity to the total intensity, respectively

E, (kev) G- (mb)

X Y Method (Nucleus) P at 46 MeV

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HEAVY ION REACTION CHANNELS DETERMINED C6-267

nucleus in which the transition occurs given in parentheses. Column 5 gives the ratio, P, of this particular gamma-ray intensity to the total intensity used in extracting the cross section. The correction due to the ground-state positron transition on the cross section determination is also included in P. This quantity, for the most part deduced from the Nuclear Data Sheets, is generally the greatest source of uncertainty in the cross section values, and one for which it is difficult to assign an error. Those values shown in parentheses are for nuclei whose positron or gamma- ray branching ratios have not been previously determined, and are estimates based on the systematics of neighboring nuclei. The last column contains the integrated cross sections measured at an incident energy of 46 MeV. Possible angular distribution effects on the cc in-beam ^>)cross section determinations have not been included. The cross-section values given in the tables and figures represent effective averages over the target thicknesses, whereas the energies given are for the incident beam.

Figures 3 and 4 show some of the measured excita- tion functions for the absolute cross sections of 58Ni and 60Ni, respectively. The error bars do not include any uncertainty in the ratio P listed in Tables I and 11.

All the cross sections rise rapidly above the Coulomb

0.01 I Î Î Î Î I

38 40 42 44 46 48

E,60 (MeV)

FIG. 3. - Excitation functions for reaction products from

1 6 0 ions incident on 58Ni. The energies are those of the incident beam, while the cross sections are effective averages over the

target thickness.

FIG. 4. - Excitation functions for reaction products from

1 6 0 ions incident on 6oNi. The energies are those of the incident beam, while the cross sections are effective averages over the

target thickness.

barrier near 38 MeV. The largest cross sections for oxygen ions incident on 58,60Ni are for 2p and pn emission, respectively, in agreement with the results of Nolte et al. [4].

The large cross sections for multiple light particle emission suggests that compound nucleus formation followed by an evaporation process is the principal reaction mode. Theoretical calculations of the expected cross sections for ¹⁶⁰

+

60Ni reactions by Doron and Blann [ 5 ] , based on the Weisskoff-Ewing form of the evaporation theory, predict that the (160, pn) reaction is dominant near 40 MeV in agreement with experiment. Other strong channels are for 2p and ax emission. In the case of 58Ni, they calculate that the 2p reaction is strongest followed by the pn and ap reactions, which again is corroborated by experiment.

However, for both 58960Fii the probability calculated for 3 nucleon emission is much smaller than found experimentally.

The large cross sections for 58Ni(160, ax) and the small cross section for the (160, a) reaction suggest that a decay takes place primarily to unbound states.

This is consistent with the a-particle energy spectra being reported for the (160, a) reaction with lighter mass nuclei.

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C6-268 R. L. ROBINSON, H. J. KIM AND J. L. C. FORD JR.

The 5 8N i (1 60 , 1 2C ) reaction, in which a n alpha particle is transferred and, therefore, which is likely t o be a direct reaction, also h a s a large cross section.

T h e 6 0N i (1 6O , 1 5N ) reaction m a y be due t o direct p r o t o n exchange which is expected to prefer p r o t o n transfer o u t of the projectile [6].

References

[1] F A L K ( W . R.), MATTER (U.), HUBER (A.), BENJAMIN

(R. W.) and MARMIER (P.), Nucl. Phys., 1968, A 117, 353.

[2] FALK (W. R.), HUBER (A.), MATTER (U.), BENJAMIN

(R. W.) and MARMIER (P.), Nucl. Phys., 1970, A 140, 548.

[3] ZIONI (J.), JAFFE (A. A.), FRIEDMAN (B.), H A I K (N.)

and SCHECTMAN (R.), Nuclear Reactions In- duced by Heavy Ions, ed. by R. Bock and W. R.

Hering (North Holland Publishing Company, Amsterdam), 1970, 693.

[4] NOLTE (E.), KUTSCHERA (W.), SHIDA (Y.) and M O R I -

NAGA (H.), Phys. Letters, 1970, 33B, 294.

[5] DORON (T. A.) and BLANN (M.), Nucl. Phys., 1971, A 161, 12.

[6] DIAMOND (R. M.), POSKANZER (A. M.), STEPHENS (F. S.), SWIATECKI (W. J.) and WARD (D.).

Phys. Rev. Letters, 1968, 20, 802.