THE REACTION 25Mg (p,y) 26A1 (I Experimental)

Transcription

1 Kluyver, J. C. van der Leun, C. Endt, P. M Physica XX THE REACTION 25Mg (p,y) 26A1 (I Experimental) by J. C. KLUYVER, C. VAN DER LEUN and P. M. ENDT Physisch laboratorium der Rijks-universiteit te Utrecht, Nederland Synopsis Energies and intensities have been measured of 7-rays produced in the 2SMg(p, V)26A1 reaction at six resonances in the region Ep = MeV. Tl~in enriched 2SMg targets were bombarded with protons from a Cockroft-Walton generator, and v-rays were detected with a scintillation spectrometer. The resulting pulse spectra were analyzed with a differential discriminator and photographed on an oscilloscope screen. The resonances investigated here could be assigned to 2SMg by comparison with runs on enriched 24Mg and 26Mg t~rgets. The 25Mg resonances are found at 321,395, 441, 501, 518, 580, 607, 667 and 688, all + 15 kev. The 501 and 518 kev resonances could not be resolved completely, but they show almost identical y-ray spectra. The resonances at 667 and688 key have not been investigated in detail. The six resonances investigated in detail show complicated y-ray spectra, different from resonance to resonance. A list of v-ray energies and intensities is given in Tables II and III. From absolute v-ray yield measurements the radiation widths of all resonances (multiplied by a statistical factor) could be determined. 1. Introduction. The interesting properties, connected with isobaric spin, of the self-conjugated odd-odd nuclei 6Li, l B, t4n, 18F, 2aNa, 26A1, 30p, 34C1 and 38K have only been realized recently 1) 2). Assignments of isobaric spins to the lower states of 6Li, l B, 14N, 22Na and 34C1 are now well established 8) ), but in spring 1954 the isobaric spin properties of 18F, 2SA1, 30p and 3SK were still largely unknown. In two short previous communications 5) 6) preliminary results were given, obtained from the 25Mg(p, y)26a1 and 29Si(p, y)30p reactions, concerning the positions and isobaric spins of the ground state and first excited state in 26A1 and 30p. Since then many more y-rays have been found from the 2SMg(p, y)2sa1 reaction, and a description of this work, together with a more detailed account of the older work, will be given in the present paper. The situation in 2SA1 has been cleared up appreciably in the mean time by B r 0 w n e's ~) measurements, who found several a-groups from the 28Si(d, a)26al reaction, and who measured their Q-values by accurate magnetic

2 1288 J. C. KLUYVER, C. VAN DER LEUN AND P. M. ENDT analysis. He located the 26A1 ground state at MeV above the 26Mg ground state, which is in good agreement with the value of 3.96 q MeV found from the 2SMg(p, >')26A1 reaction 5). He observed excited states in 26A1 at E x = 0.418, 1.052, 1.750, (1.846) and MeV from Q-values with 8 kev errors. The first level had also been found from 2SMg(p, >')26A1, at MeV 5). All states observed by Browne have necessarily isobaric spin T = 0, if the isobaric spin conservation rule still holds good at Z = 13, which, however, is very probable. This disproved especially our tentative assumption, that the 0.4 MeV level would be the lowest T =- 1 state in 26A1. It is shown in this paper, that this T = 1, J = 0 + state actually is situated at an excitation energy of MeV. It decays by 8 + emission, with the long known half life of 6.6 sec. In Part I of the present paper a description will be given of the experimental method ( 2), of the determination of magnesium resonant energies and their assignment to the correct isotope ( 3),,of the measurements of >,-ray energies ( 4) and intensities ( 5), and finally of the computation of the radiation widths of the observed resonances ( 6). Part II contains the conclusions to be drawn from'these measurements regarding the excitation energies of 26A1 levels ( 7), and their spins, parities, and isobaric spins ( 8). The first 2SA1 level decays by a r+ transition, and makes possible an accurate determination of the Fermi coupling constant ( 9). 2. Experimental method. A proton beam of up to 6/~A after 30 magnetic deflection was provided by the 700 kev Cockroft-Walton generator of this laboratory. A narrow slit of 1 mm width defining the proton beam was used in front of the target during the experiments to determine the proton resonant energies, and a wider slit of 3 mm during the experiments on >'-ray energies. Targets of electromagnetically separated magnesium isotopes (24Mg, 25Mg and 26Mg), of 20 and 80/~g/cm 2 evaporated on 0.5 mm copper, were obtained from the Atomic Energy Research Establishment, Harwell, England. Gamma rays were detected with" a scintillation spectrometer. The crystal surface was brought as close to the target as possible, the distance amounting to about 5 mm (see Fig. 1). A 1 cm lead shield surrounded the crystal and the lower end of the evacuated target holder. Although the shielding reduced the background (mainly X-rays from the acceleration tube) considerably, it was still difficult to measure >'-ray energies lower than about 200 kev. A NaI (T 1) crystal of 31 x mm 3, packed in a MgO reflector, served as scintillator, and was mounted on an EMI 6260 photomultiplier tube. The pulse-height distribution obtained after amplification was examined both with a single-channel pulse analyzer, monitored by an ordinary dis-.criminator, and with a photographic method. Usually the ordinary discriminator was fixed on a voltage corresponding to about I MeV and the

3 THE REACTION 2SMg(p, y)26ai. I 1289 number of pulses in a'given 2 V channel of the prise-height analyzer was counted for a predetermined number of counts of the discriminator. For the photographic method the pulses were applied to the vertical deflection plates of a Tektronix 51 1 AD oscilloscope with the horizontal sweep (sweep speed 2.5 cm//~sec) being triggered by the pulse. A 1 cm wide vertical strip of the oscilloscope screen showing the pulse maxima only, is photographed by a Zeiss-Ikon camera on Ilford HP 3 plates of 9 12 cm 2. By displacing the lens five exposures may be made on the same plate. According to the intensity of the resonance studied, the gain of the amplifier, and the setting of the iris diaphragm, exposure times varied from 5 minutes to 2 hours. Peaks in the photographic density are found by moving the plate through a Moll micro-densitometer and examining the densitogram. The pulse distributions Proton beam Rubber ring~ Paper cover ~uclte --..larget ~ "~J_ead Fig. 1. Schematic drawing of target assembly. The lucite insulates the target holder electrically from the lead shield. The black paper cover shields the multiplier optically. of known y-rays are used for calibration both in the photographic method and in the pulse analyzer method. The energy resolution (peak width at half maximum over peak energy) amounted to 13.5% at E~, = 0.5 MeV, and to 7% in the E~, = 4-6 MeV region. 3. Resonances. When this investigation was start.ed, it was not well known to which of the three magnesium isotopes the observed Mg(p, y)al resonances had to be assigned. These resonances may be observed either by y-ray detection or by/3 + detection. Whereas 2~A1 is a stable nuclide, 25A1 and

4 12'90 J. C. KLUYVER, C. VAN DER LEUN AND P. M. ENDT 26A1 have half-lives,which are nearly equal viz.7.6 sec and 6.6 sec.the problem of identification is enhanced by the occurrence of 2SMg(p, y)26a1 resonances with high y-ray and low fl+ yield, because in these cases the decay proceeds mainly to the longlived 26A1 ground state (see 8). Two strong resonances in 26Mg further hamper the observation of the ZSMg and 24Mg resonances in natural magnesium targets. For these reasons the use of separated isotopes in the investigation of Mg(p, 7) resonances is imperative. To assure correct assignments the y-ray yield was measured as a function of the proton energy with electromagnetically separated thin 2qV[g, 2SMg and 700 N,7 I 600 I 24Mg(p,~) 2SAI \ _...~Ep I I I t I kev Fig. 2. Gamma-ray yield from proton bombardment of 24Mg. 26Mg targets. The results are given in Figs The resonances at Ep = 321, 395, 441,501,518, 580, 607, 667 and 688 kev can be assigned to 2SMg, and those at Ep = 340 and 458 kev to 2SMg. A more detailed analysis allocates also the weak resonance at Ep = 300 kev to 26Mg, and the resonances at Ep = 224 and 421 kev to 24Mg. From an intensity comparison of the same resonance observed from different targets, it may be concluded, that the separated targets are isotopically pure to a high degree. The 24Mg target contains about 3% 2SMg and 2.5% 2SMg, the 2SMg target less than 6% 24Mg and about 3% 2SMg and finally the 26Mg target less than 5% 24Mg and 3% 2SMg. The natural composition of magnesium is 77.4% 24Mg, /o 2SMg and 11.1% 26Mg. Especially the 24Mg targets should be free from carbon con-

5 THE RE.-XCTm,'," 2~Mg(p, 7)26A1. I 1291 tamination, which is deposited slowly on the target during bombardm.ent, and which exhibits a broad t2c resonance at 450 kev and a strong broad 13C resonance at 554 kev. A weak 14N resonance at 277 kev has been observed, but it does not interfere with the present measurements. An incidental advantage of the work with separated isotopes is, that chemical impurities and target backing will give, if any, the same spurious resonances from all three targets. One is safeguarded hereby against incorrectly assigning them to the nuclide under investigation N.~ I I I = I ~SMg (p,~,)2% i i ,/ I Fig. 3. Gamma-ray yield frona proton bombardment of 25Mg. lh Ep kev The slit width of 1 mm corresponds to 1 kev at Ep = 200 kev and to 3.5 kev at Ep = 700 kev. The target thickness varies from 7.5 to 4.5 kev over the same range. The proton energy is measured by the current through a 1200 NX2 resistor parallel to the generator. The energy scale is found by calibration with four well known 19F(p, ~7)z60 resonances 8). Unfortunateh, the value of the resistance is temperature dependent, and the resistors are heated by the current during bombardments. Therefore a systematic error of about I 0 kev in our results may not be excluded, although an attempt was made to repeat the same operating conditions during the measurements and the calibration runs. Ill view of this uncertainty and the existence of various

6 1292 j. C. KLUYVER, C. VAN DER LEUN AND P. M. ENDT other investigations on the exact values of resonant energies, no effort was made to obtain a better accuracy than 15 kev. With the limited resolution available the resonances at E~ = 501 and 518 kev are not fully separated, and a weak resoriance at about 530 kev cannot be excluded. Table I gives a summary of all studies about the resonant energies in the 2SMg(p,?)26A1 reaction. As at Ep = 430 kev 2SA1 is produced mainly in the ground state, T a n g e n 8) concluded from the near-absence of positrons, that this resonance belonged to 26Mg. Only the work by G r o t d a let al. z2) with separated 24Mg targets made it "possible to choose between Z~Mg and o 14 0 N~, I I I. I i t4n I I kev Fig. 4. Gamma-ray yield from proton bombardment of 2SMg. 2SMg for all other resonances. In the first investigation of H u n t and J o n e s 9) no identification was attempted. The study of H u n t et al. 10) with separated isotopes gave correct assignments, but the 441 kev resonance was missed here, as in the search for positrons by T a y 1 o r et al. 11). 4, Gamma-ray energies. The main experimental part of this investigation has been the determination of the y-rays produced at the different resonances of Table I. Several amplifier gains were used to study the different energy regions of the pulse spectrum. For calibration the following y-rays were used: g~ = 6.13 MeV from t9f(p, a?)160, E~, = 4.44 MeV from 9Be(a, ny)12c

7 THE REACTION 2SMg(p, 7)26Al. I TABLE I Authors : Detection by: Target: Proton resonant energies in kev A B C D F G H I J T a n g e n ~) and fl+ natural Mg 310 ± ± ± Resonances in the =SMg.(p, ~,) =SA1 reaction Hunt and Jones ) natural Mg ~ 1.0 Hunt etal 1 ) Taylor et al H) this work separated =SMg separated tsmg separated =SMg ± ± 0.6 5J ± 0.7 all lotol all 4-15 (a-particles from a.po-be source), Ev = 2.37 MeV from 12c(p, 7)13N, Er = MeV from Z~Na, E~, = MeV from 137Cs and Ev = MeV (annihilation radiation) from Z2Na. Also complete pulse spectra were measured of these 7-rays for aid in the identification of peaks in the pulse spect'rum as photo-, pair- or (pair MeV)-peaks. The stray magnetic field caused by the beam analyzing magnet at the photomultiplier tube influences the collection of electrons from the photocathode on the first dynode. Hence the calibration of the scintillation spectrometer in Volts/MeV depends slightly on the current through this magnet. A small correction for this effect was applied, when necessary. To average out the slight drift of the scintillation spectrometer, always calibrations were taken both before and after a run. The drift was generally less than 1%/hour. The analysis of a pulse distribution into its constituent 7-rays was generally a difficult task, due to the many 7-rays present, and to the limited accuracy of every run caused by the countingstatistics. Especially in the region from 1.5 to 3 MeV, where photo peaks, pair peaks and (pair MeV) peaks are of comparable intensities, the identification of 7-rays was sometimes not unambiguous. On the other hand the analysis of different runs and densitograms of the spectrum of the same resonance made it possible to distinguish between real peaks and false peaks caused by bad statistics. Confidence in the reliability of our conclusions was stimulated by the splendid agreement of the positions of levels in 26Al derived from our 7-ray spectra with those found by B r o w n e 7) from the 28Si(d, a)26a1 reaction. In a later phase the knowledge of well established levels aided in choosing between possible 7-rays. The incomplete resolution of the resonances D and E made it difficult to distinguish between them in the experiments with the thick (80 Fg/cm 2) target. From photographs of the pulse spectrum taken at Ep = 500 and 520

8 1294 J. C. KLUYVER, C. VAN DER LEUN AND P. M. ENDT kev under identical conditions no significant deviations were found. Probably these resonances decay with identical 7-spectra. Table n lists all the 7-rays found at each resonance. Uncertain identifications of observed peaks are put in brackets. Some values deviate slightly from those previously reported, as they are found by averaging old and new resurs. There are certainly more 7-rays present of low intensity, and sometimes,trrays of intensities comparable to those listed may have been missed, when they fell in an unfavourable region. TABLE I [ Gamma rays produced at six resonances of the reaction SSMg (p, 7) '6A1 Resonance A B C D G H E x (,Me V) Gamma-ray energy in MeV ± ) ~ :!: ' ~ ~- 0. I ~ ) ~ ± :::E " 0.06 (2.41 ± i _ 0. I ± 0. I ± ± ± in kev J Atall m ' resonances annihilation radiation (E~, = MeV) was observed. 995 ± ± 7 The resonances at 667 and 688 kev have not been investigated in detail, but only a rapid photographic survey of the high-energy pulse spectrum has been made..it can only be concluded at present that the pulse spectra are markedly different from those at the other resonances. These resonances are at the high voltage limit of the generator and can only be investigated under good atmospheric conditions. Resonance A (Ep = 320 kev) has been investigated making use of the H~ beam at 640 kev. This beam contained also a small admixture of 640 kev deuterons, which are responsible for an observed broadening of the 0.82 MeV photo peak, and for a pronounced shift of this peak to higher energy, through the occurrence of the reaction 160(d, p7)170 (E:, = 870 kev). The 7-ray energy, given in Table n (o.836 MeV) has not been corrected for this effect. In Figs. 5 and 6 a representative run with the differential discriminator and a densitogram of a photograph of the same spectrum are shown.

9 THE REACTION 25Mg(p, y)26a1. I 1295 In 7 it will be shown how the y-rays of Table,II can be fitted into the 26A1 level scheme. I 1 I I i - ~' A B C D E" P G 4OO 2OO 0 ~ I I Mev Fig. 5. High-energy part of the pulse spectrum of resonance G, as measured with the differential discriminator. Peaks C and A are the photo peak and pair peak of E v = 4.27 MeV. The (pair MeV) peak of this 7-ray is not resolved from the pair peak of the weaker Ev = 4.92 MeV. Peaks G, F and E are the corresponding peaks of Ev = 6.44 MeV, where F is in the same way broadened by the pair peak of the weak E r = 6.80 MeV. The bump D indicates the presence of the pair peak of another weak 7,-ray, viz. E v = 5.82 MeV. 5. Gamma-ray intensities. For an assignment of multipole order and character (electric or magnetic) to the many observed y-rays, it is useful to measure their intensities as well as their energies. Intensity measurement~ could only be obtained from the differential discriminator runs, as reliable intensity measurements from photographed spectra are made difficult by the non-linear density vs. intensity curve of the photographic plate. To convert the observed height of a photo- or pair peak into the corresponding y-ray intensity, one has to know the efficiency of the NaI crystal for photo-peak or pair-peak formation, as a function of y-ray energy. It is also necessary to know the total detection efficiency as a function of energy. The total efficiency can be computed from the known cross sections for photo- and Compton effect and pair formation. The photo-peak efficiency

10 1-296 J. c. KLUYVER, C. VAN DER LEUN.AND P. M. ENDT can be computed from the total efficiency and from the measurements of the ratio of the number of pulses in the photo peak to the number of pulses in thetotal pulse spectrum for monoenergetic calibration sources. The pairpeak efficiency can be found in exactly the same way. The photo-peak efficiency can also be found from measurements with sources emitting two y-rays of known intensity ratio 18). The same sources, which were used for energy calibration (see 4), were also suitable for intensity calibration. In all these calibrations the sources were put at the same distance from the crystal surface, as the detection efficiency depends on this distance 14)..] A BC EV Mev Fig. 6. Densitogram of the pulse spectrum of resonance G, photographed simultaneously with the discriminator run of Fig. 5. The pulse spectrum below 1 MeV is suppressed, the peak at 0 MeV is a zero energy mark. In Table III a list is given of observed intensities of r-rays with Ev < I MeV and > 4 MeV. The assignments of upper and lower levels, given ill column 2, will be discussed in 7. Although also a number of y-rays has been observed with energies between 1.0 and 4 MeV, they were not very intensive, and their assignment suffers from the difficulties mentioned in 4. An exception is the 1.34 MeV y-ray, which is particularly strong at resonance A. The intensities given in Table III are expressed as the number of y-rays emitted of one particular energy, per unit y-ray with an energy larger than 1 MeV. All intensities have been corrected for absorption in 1.5 mm copper between target layer and crystal, taking into account oblique incidence. This correction amounted to 13% for the 0.42 MeV y-ray. The listed.intensities may be in error by as much as a factor 1.5. Most

11 m THE REACTION ZSMg(p, y)26a1. I 1297 peaks are situated on a Compton continuum from l~igher energy y-rays, and it may be difficult to estimate accurately the amount to be subtracted for this continuum. Also it has been assumed implicitly in preparing Table hi, that all y-rays have the same angular distribution, which is certainly not true. This may involve errors of up to 25%. TABLE III Intensities of y-rays from 'SMg (p, 7) 'eal" Resonance [ A [ B [ C [ D [ G [ H Energy of Upper and Relative intensity, for 1 ~-ray with energy > 1 MeV y-ray in MeV lower level *) *) 6.2--,6.5") *) *) *) *) reson. -+ (0) reson. ~ ( 1 ) reson. ~ (2) reson. -+ (3) reson. -+ (4) reson. --~ (5) reson. ~ (6) (5) ~ (3) (3) ~ (l) annihil, tad. (2) ~ (o) t).19 *) Exact 7-ray energy depending on proton energy (see Table II). t) Possibly too high, because of contamination from tsc(p, ~,)lsn. m t) ~ Radiation wiclths. From the thick target yield of y-radiation observed at each resonance one may compute the corresponding radiation width times (2J + 1), where J is the resonance spin. The number N~, of y-rays produced per proton hitting the target can be obtained by integrating the Breit-Wigner expression for the cross section over the resonance. One thus obtains: No (2J + t) N;, -- (de/dx) (21 + 1) (2s + l) 4Mp E, /'p + F, z ' where : N o is the number of target nuclei per cm 3, de/dx is the proton energy loss in the target per cm, J is the resonance spin, I is the spin of the target nucleus, s is the proton spin, Mp is the proton reduced mass, E, is the resonance energy, F~, is the radiation width and Fp is the proton width. In general we have F~ >~ _Pv which makes it possible to replace I'fl'~/(Fp+I'~) by Fy. Physica XX

12 1298 THE REACTION 2SMg(p, 7)26A1. I By mtrltiplying Nr with the fractional solid angle (co = O/4z~) subtended by the NaI crystal times the detection efficiency (e) of the crystal, one obtains the number of pulses per proton. The quantity toe was found from a calibration with r-rays from the 340 kev 19F(p~ ar)160 resonance (where N, is known 3)), which gave cos = 2.30/0. This agrees within the experimental error with the measured value of the efficiency (see 5), and a geometrical estimate of the solid angle. TABLEIV I' ResonanceThe pr dict -r'~'(2i I + 0B6 1) f i eight C A 'il~[g(p' 0D.8 ~)i'al :8 res ilances G (it = H radiati n h) I t d wi J F~,(2J t+ I) ev In Table IV the quantity Fr(2J + 1) is given for eight resonances in the 25Mg(p, r)26a1 reaction, determined, as described above, from bombardments of the thick Z~Mg target. In this run the number of r-ray pulses was measured with energy larger than 1 MeV. Actually one should count only r-rays corresponding to transitions with the resonance level as upper level. The experimental figures may thus be somewhat high, because in cascade transitions two or more 7-rays can be produced with energy larger than I MeV. This error might amount up to 30%. Also it has been assumed that the detection efficiency of all r-rays above 1 MeV is the same as for 6.1 MeV r-rays. Actually the detection efficiency goes through a broad.minimum around 5 MeV, which makes the error introduced from this assumption reasonably small. The error resulting from assumed isotropy for all angular distributions has already been mentioned in 5. Received REFERENCES References are given at the end of part II.