Volume 166B, number 4 PHYSICS LETTERS 23 January 1986

A NEW S O U R C E OF TUNEABLE M O N O E N E R G E T I C GAMMA-RAYS

F. Z I J D E R H A N D and C. VAN DER LEUN

Fysisch Laboratorium, Rijk~suniversiteit Utrecht, P.O. Box 80000, 3508 TA Utrecht, The Netherlands

Received 29 October 1985

The energy of a primary gamma-ray from a broad proton-capture resonance can be tuned by adjusting the proton energy. The usefulness of such a source of tuneable gamma-rays is demonstrated in an experiment in which 17 MeV gamma-rays from a broad riB(p, "y)~2C resonance are resonantly absorbed by a narrow level of 9Be.

Photoexcitation has proven to be a fruitful method in nuclear structure investigations. Several different gamma-ray sources are in use, which provide either a white energy-spectrum (bremsstrahlung) or monoener- getic gamma-rays (radioactive sources, nuclear reac- tions). For the investigation of resonant phenomena like scattering, fluorescence and absorption or trans- mission, the use of tuneable monoenergetic gamma- rays is highly preferable. It increases the signal-to. noise ratio, which is often crucial in the presence of strong non-resonant processes [1 ].

The two best-known sources of tuneable gamma- rays are both based on the Doppler effect. The first consists of a strong gamma-ray source mounted on a spinning wheel or centrifuge [2]. By changing the ro- tational speed, one changes the observed gamma-ray energy. The second is a short-lived charged-particle capture resonance [3,4]. The gamma-rays are emitted before the compound nucleus slows down and the ob- served gamma-ray energy thus depends on the detec- tion angle. By changing this angle one changes the gamma-ray energy, typically over a range of a few tens of keV. In most cases proton-capture resonances have been used, but a recent experiment [5] shows that deuteron-capture resonances can also be used. A re- view of the many results obtained with this method has been presented by Smith [6].

This letter deals with another way of producing tuneable gamma-rays. If one neglects minor correc- tions like Doppler shift and recoil-energy losses, the energy of a primary gamma-ray from a proton-capture

0370-2693/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

resonance is given by the relation

E~ = Q + [mt/(m t + rap)] Ep - Exf ,

where Q is the Q-value of the (p, q~) reaction, mt and mp the mass of the target nucleus and the proton, re- spectively, Ep the proton energy and Exf the excita- tion energy of the final state of the primary.

At a broad resonance one thus can vary E~ by vary- Lug Ep over the resonance. The range available for en- ergy variation is of the order of the resonance width. The latter may be as high as several MeV, which is considerably larger than the range mentioned above for the classical method. The energy spread of the gamma-rays produced is determined by the target thickness and by the spread in the Doppler shift due to the finite solid angle covered by the detector. The latter effect may be minimized by placing the detec- tor in the forward direction, at 0 = 0 °. In most practi- cal cases the spread will be essentially due to the first factor, but target thicknesses of less than 1 keV can be readily achieved.

The experimental set-up is extremely simple; see fig. 1. Only for small target-to-detector distances a col. limator will be required to reduce the solid angle sub- tended by the detector. In most cases shielding will not be necessary. The monitor detector checks the tar. get quality.

The feasibility of this technique has been demon- strated by resonant absorption by 9Be ofE. r ~ 16.98 MeV gamma-rays produced via the reaction 11 12 r an B(p, 7) C over the proton-ene gy r ge Ep =

389

Volume 166B, number 4 PHYSICS LETTERS 23 January 1986

TARGET

PROTON BEAM

ABSORBER N.I I OETECTOR I

Fig. 1. Experimental set-up.

1065-1085 keV, i.e. in the low-energy wing of the Ep = 1388 keV resonance, which has a total level width PCM = 1100 keV [7]. The protons are acceler- ated with the Utrecht 3 MV Van de Graaff, which pro- vides proton-beam currents of up to 150 #A on target with an energy resolution of better than 200 eV [8].

Two measurements have been performed with dif. ferent targets and absorbers; the parameters are listed in table 1. The absorbers consist of 99% pure 9Be blocks kindly provided by the IRI in Delft. The detec- tor is a 13 cm X 13 cm ~ NaI crystal. The Ge monitor detector allows a detailed check of the emitted gamma- ray spectrum. It shows that the contents of the E~ = 1 3 - 1 7 MeV window are proportional to the yield of the ground-state transition after a minor correction for the cosmic-ray background. The same windows have been applied to the NaI spectra.

Figs. 2 and 3 show the normalized transmission curves. The dip areas, listed in table 1, are calculated by numerical integration of the data points. The back- ground line is determined from a fit to the off-reso- nance data points; the slope (<1%) might be due to a

Table 1 Parameters and results of the 9Be(% X) measurements.

Parameters Measurement

1 2

absorber length (cm) 23 46 target thickness Cug/cm 2) ~ 15 ~50 measuring time (11) 120 140 total collected proton

charge (C) 40 50 dip width (keV) 2.2 + 0.2 3.1 + 0.4 dip area(eV) 270 + 30 530 + 80 I'3,o(eV) 15 + 2 18 + 3 (r',ro)(eV) 16 ± 2 16 ± 2

8

c (Q L

0

t

1.0(

0.91

0.90

i i i

I I I

16970 16975 16980 = Ey (keY)

i I i

1065 1070 1075 EplkeV)

Fig. 2. Resonant absorption by a 23 cm long Be absorber of ~ 16.98 MeV gamma-rays produced in the reaction (p, 7)12C. The broken line indicates the region of the

data points not included in the fit of the background line; the dotted line is drawn to guide the eye.

shift o f the beam spot ( < 1 ram) as a function of Ep. The r'~o values are deduced from these areas and from the known [5] level width r = 530 + 50 eV, following

8

c

x )

0

1,0

0.9

0.6

(r I f - / 46 cm al:~orber

,, I i I

16970 16980 16990 , ~ E yl(keV)

10165 1070 1075 1080 1085 Ep (keY)

Fig. 3. As fig. 2 but with a 46 cm long Be absorber and a thicker 11B target. The dip width (~3 keV) and shape are es- sentially due to proton energy loss and stxa~g~ling in the target, respectively.

390

Volume 166B, number 4 PHYSICS LETTERS 23 January 1986

the analysis described in reL [5]. The weighted mean value of the ground-state transition width is FTo = 16 -+ 2 eV, which clearly supports our previous reso- nant absorption data [5] and those of ref. [9], but which is in agreement with the values P~o = 8.6 -+ 0.9 and 11.5 + 1.4 eV deduced from (e, e') experiments reported in ref. [10] and ref. [11], respectively.

The present data combined with those from previ- ous experiments [5,7], also allow the calculation of the widths (or upper limits) of the particle-decay chan- nels of the second T = 3/2 state ofgBe a t e x = 16.98 MeV. The very small widths ( 1 0 - 4 - 1 0 -6 of the Wigner limit) for all channels but the only isospin-allowod 8Li + p decay (6% of the Wigner limit), show that iso- spin is an excellent quantum number in light nuclei, even at excitation energies as high as 17 MeV. A more detailed discussion of this conclusion will be presented Ln ref. [5].

Summarizing, we conclude that the gamma-rays of variable energy produced in the simple set-up described above, are a useful tool in photoexcitation experiments. It is a promising technique since many broad proton- capture resonances are known [12,13], which can serve as sources of tuneable gamma-rays covering an energy range from keV's to MeWs and have an energy resolu- tion of better than 1 keV. The yield of these broad resonances is usually lower than that of the narrow resonances applied in previous resonant absorption ex- periments. This is largely compensated for by the fact that collimating slits are superfluous in the present set-up. It might be added that thicker targets may be used for the investigation of broad levels, thereby in-

creasing the gamma-ray yield proportionally. Since the area of the absorption dip remains constant under changing experimental conditions, one should for ana- lytical purposes aim at dips of minimal width and thus maximum depth. In selected cases where thin targets are necessary, gas-jet targets may provide targets thinner than 500 eV.

References

[1] F.1L Metzger, in: Progress in Nuclear Physics, ed. O.R. Frisch, Vol. 7 (Pergamon, London, 1959) p. 54.

[2] P.B. Moon and A. Stormste, Prec. Phys. Soc. London A66 (1953) 585.

[3] S.S. Hanna and L. Meyer-Schiitzmeistor, Phys. Rev. 108 (1957) 1644.

[4] P.B. Smith and P.M. Endt, Phys. Rev. 110 (1958) 397. [5] F. Zijderhand, S.W. Kikstra, S.S. Hanna and C. van der

Leun, NucL Phys., to be published. [6] P.B. Smith, Am. Inst. Phys. Conf. Prec. 125 (1985)

192. [7] F. Ajzenberg-Selove, NucL Phys. A433 (1985) 1. [8] J.W. Maas, E. Somotjai, H.D. Graber, C.A. van den

Wijngaart, C. van der Leun and P.M. Endt, Nucl. Phys. A301 (1978) 213.

[9] H.G. Cleft, K.J. Wetzel and E. Spamer, Phys. Lett. 20 (1966) 667.

[10] H. Theissen, in: Springer Tracts in Modern Physics, ed. G. HoMer, Vol. 65 (Springer, Berlin, 1972) p. 1.

[11] J.C. Bergstrom et aL, Phys. Roy. C7 (1973) 2228. [12] F. Ajzenberg-Selove, Nucl. Phys. A360 (1981) 1; A375

(1982) 1; A392 (1983) 1; A413 (1984) 1; A433 (1985) 1.

[13] P.M. Endt and C. van der Leun, NucL Phys. A310 (1978) 1.

391