Recoil ranges of Tc activities from 93Nb(4He, xn) reactions

]. inorg,nucLChem.Vol.43.pp.1739-1742,1981

0022-1902/81/081739-04502.00/0 PergamonPressLtd.

PrintedinGreatBritain.

RECOIL RANGES OF Tc ACTIVITIES FROM 93Nb(4He, xn) REACTIONS G. W. A. NEWTON, V. J. ROBINSON and E. M. SHAW Department of Chemistry,The University, Manchester M13 9PL, England (Received 6 October 1980; received for publication 22 October 1980)

Abstract--The recoil momenta of a series of technetium isotopes produced by the reactions 93Nb(4He,xn) (x = 2-5) have been measured at 38.4 and 56.8MeV, using the thick target[thickcatcher technique. The isotopes are produced by a mixture of compound nucleus (CN) and pre-equilibrium(PE) processes. The isomer ratios (of 9*l'cand 93Tc)and the recoil momenta are consistent with a picture of PE reactions which involve the emissionof one or more high energy neutrons, scattered mainly in the forward direction.This results in a net residual spin and a net recoil momentum which is lower than that predicted by the CN mechanism.

INTRODUCTION The thick target/thick catcher technique is a wellestablished method for measuring energies of radioactive recoils from nuclear reactions[l[ and it has been used in the present work to obtain the recoil energies of Tc nuclides produced in 93Nb(4He, xn) reactions. Simple nuclear reactions such as (4He, xn) have been generally assumed to proceed via the compound nucleus (CN) mechanism, but there is increasing evidence that this mechanism does not give a complete picture of these reactions [2]. For example the excitation functions often show a high energy tail which is not explicable using the CN model. Blann[2] has emphasised the importance of "pre-equilibrium" (PE) processes in 4He-induced reactions, in which nucleon emission takes place before true statistical equilibrium is attained. These PE nucleons are emitted with a higher kinetic energy than those evaporated from a statistically equilibrated nucleus, so that PE processes become dominant in the tail of the excitation function. Blanns "hybrid model" [3] which includes both CN and PE processes, gives a very satisfactory account of the shapes of excitation functions for 4He-induced reactions. We have recently measured the excitation functions for a number of Tc activities produced by the reaction 93Nb(4He, xn) (x = 1-4)[4] and compared the results with calculations using the hybrid model. All four of these Tc isotopes have well established high and low spin isomer states and in addition to the total cross sections, isomer yield ratios for all four isotopes were obtained as a function of 4He beam energy. Isomer yield ratios have often been found to be correlated with the spin of the excited precursor nucleus and they have been used to estimate the spin associated with products from a wide variety of nuclear processes such as fission, high energy proton spallation and heavy ion compound nucleus reactions. The Tc isomer ratios from the 93Nb(4He, xn) reactions all showed the same general features: the isomer ratio (expressed as high spin yield/low spin yield) increased rapidly from the threshold of the excitation function and then levelled off or even decreased in the tail. The initial rapid increase is fully in accord with expectations from the CN model. The increasing beam energy results in an increase of the mean spin of the compound nucleus and the subsequent evaporation of low energy neutrons is not able to remove I N C Vot. 43, N o . 8 - - B

much of this. Thus there is an increasing preference for the high spin isomer in the final product. The levelling off or decrease in the isomer ratio in the tail of the excitation function occurs over the same energy range in which PE processes begin to account for the majority of the yield, suggesting that the residual excited nuclei from PE reactions have a lower average spin than those formed by the CN mechanism. The hybrid model does not explicitly include angular momentum effects, but it certainly seems very reasonable that the emission of high energy PE neutrons should give a residual nucleus of lower average spin than that produced by the CN mechanism. A further consequence of the emission of high energy PE nucleons is that the recoil momentum of the residual nucleus could be quite different to that associated with compound nucleus reactions. If, as seems probable, the high energy PE nucleons are emitted predominantly in the forward direction, the net recoil momentum of PE residuals shuld be considerably less than CN residuals. The present work was undertaken to detect differences in recoil momenta associated with PE and CN reactions by examining differences in the recoil ranges of the high and low spin isomers. EXPERIMENTAL The work was carried out using the Manchester Heavy Ion Linear Acceleratorand the VariableEnergy Cyclotronat AERE, Harwell. Recoilranges were measured usingthe thick target/thick catcher technique, as indicated in Fig. 1. If it is assumed that the recoils are aligned along the beam direction, then those recoils formed with in a distance x of the rear of the target foil will escape into the catcher, where x is the range of the recoil in the target material. The range is then Targel

Catcher

,,

"2 •,I---'-- 0

ID

Fig. 1. Target catcher arrangement x = range of recoil. 1739

G.W.A. NEWTONet aL

1740

readily obtained from the ratio of the activities present in target and catcher and a knowledgeof the target foil thickness. The Nb targets were about 3 mgcm-2 and the escaping fraction about 10% so that the recoil ranges were around 300 #g cm-2. Counting and analysis procedures have been described previously[4]. It is worth noting that the range measurements were obtained by measuringthe relative activity of a particular y ray in target and catcher foil irradiated and counted under essentially identical conditions. Many sources of systematic error, such as GeLi efficiency,beam current measurements and gamma abundances are eliminated by this comparitive method. RESULTS AND DISCUSSION

Two experiments were performed, one at 38.4 MeV 4He energy at Manchester and the second at 58.6 MeV at AERE. The results are given in Tables 1 and 2. In the AERE experiment, absolute cross sections were measured for comparison with previous work [4]. (a) The range-energy relation The recoil ranges determined as described above need to be converted to recoil energies using a range-energy relation. Range-energy relations for slow moving heavy ions are subject to considerable uncertainty. It is generally believed that the relation between range and energy is linear in such cases and Harvey[5] gives the following equation: A2 (Z12/3"~ Z2213)1/2 Ro = KE -~ (A1 + A2) zig2

(1)

where subscripts 1 and 2 refer to the recoil and the stopping medium respectively. E is the recoil energy and Ro the mean range. When E is in MeV and Ro in #g cm-2, Harvey gives a value of 600 for K. This equation takes no account of range straggling and large angle scattering of the recoils, both of which are very important when, as in the present case, the masses and charges of the recoils and atoms of the stopping medium are similar. In view of the uncertainties arising from this, it was thought more satisfactory to use the basic form of eqn (1), but to obtain the value of K semi-empirically. This was done by assuming that those recoils formed via the CN mechanism have kinetic energies corresponding to full momentum transfer from the incident 4He ion. In view of what was said above, high spin isomers at or near the top of the excitation function will be produced mainly from compound nucleus decay, i.e. 94gTc at 38.4 MeV and 93STc at 58.6 MeV. The recoil energies for these two isotopes were calculated assuming that their velocities are the same as that of the 97Tc* compound nucleus, since the evaporation of neutrons is isotropic in the centre of mass system and does not affect the average recoil velocity. The results of the calculation are shown in Fig. 2: the calculated ranges are shown as a function of El, where [ is the function of masses and charges given on the right side of eqn (1). The points show that the assumption of proportionality between range and energy is valid, but the value of K obtained (328) is only just over half the value given by Harvey. The reason for this large discrepancy is not clear, but it could be a result of large angle scattering, which would reduce the effective range in the direction of the beam. Since such scattering effects should be similar for all the recoils studied in this work, a value of 328 for K has been assumed throughout in calculating recoil energies from ranges.

(b) Recoil energies and momenta The energies and hence momenta of the recoils were calculated from the data given in Tables I and 2 using the range-energy relation described above. The results are shown in Table 3. To assist in comparison of the data, Table 3 also shows the ratio of the experimental recoil momentum to the momentum calculated assuming full momentum transfer from the incoming 4He ion, i.e. the value expected for the CN mechanism. The final column in Table 3 gives an estimate of the fraction that PE reactions contribute to the total cross sections for each product. These values were obtained using Blann's code[3]. Average values only for isomer pairs are obtained, since the code does not include angular momentum effects explicitly. The results are consistent with the idea postulated earlier[4] that PE reactions produce a higher proportion of low spin isomer and also with the further hypothesis that the average linear momentum of recoils from PE reactions is lower than that of CN products at the same beam energy. At 38.4 MeV the high spin 94~Tc has the highest recoil momentum, consistent with the (4He, 3n) products being formed mainly by the CN mechanism. The lower momentum of the 94mTcis then a result of the favouring of this low spin isomer by the PE reactions, which account for about 10% of the isotopic cross section at this energy. The high spin 95~Tc (from (.4He,2n) has a lower momentum that the CN expectation value: at 38.4MeV the (4He,2n) is well past the peak of the excitation function and PE reactions account for some 40% of the total cross section. Unfortunately the low spin 95"Tc could not be detected in these experiments because its lonR half life resulted in a very low activity. At 58.6 MeV, the data allow similar conclusions to be drawn. The (4He, 2n) reactions now arise almost entirely from PE reactions and the reduction of recoil momentum below the CN expectation is even more marked than at 38.4 MeV. Similarly the (4He, 3n) reaction is accounted for mainly by PE reactions at this energy and both the 94~Tc and 94'~Tc momenta are lowered, with the low spin 94mTc showing the greatest effect. The calculations suggest that the fractional PE contribution to the (4He, 4n) reaction is negligibly small at 58.4 MeV and both isomers of 93Tc have practically the same momentum. 92Tc, the product of (4He, 5n), also has a high momentum consistent with the CN reaction dominating its production. (c) Mechanism of PE reactions It is difficult to give a quantitative treatment of the present results without a more detailed understanding of PE reactions. The (4He, 2n) and (4I-Ie,3n) reactions, which show a significant contribution from PE, could be the result of emission of one or more PE neutrons leaving the nucleus with sufficient excitation to evaporate further neutrons. Without knowing the numbers, energies and angular distributions of PE neutrons, it is not possible to calculate either the residual angular momentum of the recoil or its linear momentum. Nevertheless, all the data on isomer ratios and recoil momenta are consistent with a picture in which PE reactions are associated with high energy neutrons emitted predominantly in the forward direction, leaving the residual nucleus with a lower net spin and a lower linear momentum than it would have had if formed by the CN mechanism.

94.1 97.7 100.0

871

850

871

94mTc(WHe,3 n)

94gTc(WHe,3n)

3.

I

I

5.31 x 104

5.48 X 104

2.70 x 104

4.39 x 103

9. O0

8.71

7.50

8.10

Escape Fraction, %

:7o

Calculated from the range using eqtn.(1) with K = 328.

3.

2.18 1.96 1.92

Target thickness 2.W1 mg cm-2

76 9.82

~ 0.2 9.6

O. 79

Taken from ref.(6).

330 78.0 773 97.0 1509 100.0

92Tc(WHe,Sn)

3.91 257. 9 257. 2

.2

15.9

0.76

W. 09

16.35

18. 28

18.35

16.29 16.17

16.81

14.56 15.86 1w.39

11.34

11.85

Escape Fraction,

2.

65.8 23.8

5.9W

30.4

Apparent Cross Section, mb T~rget Catcher

Table 2 Results at 586 MeV

1.

58.2

392

1363 1520

93gTc(WHe,Wn)

99.8 97.7 100.0

94.1

93.9

aO) %

93mTc(WHe,4n)

871

703 850 871

94gTc(WHe,3n)

766

9~gTc(WHe, 2n)

9WmTc(WHe,3n)

E KeV

Isotope, Reaction

4.

5.37 x 105

5.74 X 105

3.33 x 105

4.97 x 104

|

Initial Activity, dps (2) Target Catcher

Taken from ref. (6). Absolute cross sections were not measured i n this experiment. Target thickness 2.96 mg cm -2. Calculated from the range using eqtn. (1) with K = 328.

93.9

766

195gTc(4He,2n)

1, 2.

aO) %

E KeV

Isotope, Reaction

Table 1. Results at 38.4 MeV

W25

391

405

344

273

286

Recoil (2) Range, ~g cm -2

262

222

240

Recoil (3) Rang% ~g cm -2

2.48

2.30

2.38

2.03

z. 61

1.70

Recoil (3) Energy, MeV

1.55

i. 31

1.43

Recoll (W) Energy, MeV

5

(%

Z

7"

#

o

1742

G. W. A. NEWTON et al. Table 3. Recoil energies and momenta of Tc activities

Isotope, Reaction

Recoil Momentum, PR(amu-MeV)%

Spin Parity

Recoil Energy,MeV

95gTc(4He,2n)

3÷

1.43

11.66

0.96

40%

94mTc(4He,3n)

2+

1.31

ll.lO

0.92

lO%

94gTc(4He,3n)

7+

1.55

12. O7

1.oo (3)

95gTc(4He,2n)

3+

1.70

12.71

0.85

lOo%

94mTc(4He,3n)

2+

1.61

12.30-

0.83

8o~

94gTc(4He,3n)

7+

2.03

13.81

0.93

93mTc(4He,4n)

½-

a.38

14.88

i. 01

93gTc(4He,4n)

3+

2.30

14.63

1.00 (3)

92Tc(4He,Sn)

--z/~÷

2.48

15.10

1.04

PR__R_(I) PCN

PE fractlon (2)

ta) 38.4 MeV4He

(b)

~8.6 MeV 4He

I.

3~

o%

This column gives the ratio of the experimental recoil momentum PR to the value expected from the CN model. This column gives the estimated contribution of PE processes to the total reaction cross section. See text for details. The recoil momenta of 94gTc at 38.4 MeV and 93gTc at 58.6 MeV were used to obtain the range-energy calibration, assuming these species were produced entirely by the CN mechanism.

2. 3-

5O0 Slope600/

Acknowledgements--The crews of both the Manchester Linac and the Harwell cyclotron gave their usual courteous assistance with the irradiations. The Science Research Council provided financial support. One of us (EMS) thanks The University, Manchester for a research grant.

343O REFERENCES

=L

g 2OO C

I00

00

05

1.0 L5 20 E.f. (MeV) Fig. 2. Range-energyrelation for Tc recoils in Nb.

1. J. M. Alexander, Nuclear Chemistry (Edited by L. Yaffe), Vol. 7, p. 273. Academic Press, New York (1%8). 2. M. Blann, Ann. Rev. Nucl. Sci. 25, 123 (1975). 3. M. Blann, "ALICE" code: details from M. Blann, University of Rochester, NY 14627. 4. C. L. Branquinho, S. M. A. Hoffmann, G. W. A. Newton, V. J. Robinson, H.-Y. Wang and I. S. Grant, J. lnorg. NucL Chem. 41,617 (1979). 5. B. G. Harvey, Introduction to Nuclear Chemistry and Physics, 2nd Edn, p. 327. Prentice Hall, Englewood Cliffs, New Jersey (1969). 6. C. M. Lederer (Ed.), Table of Isotopes, 7th Edn. Wiley, New York (1978).