Sensitivity of organic liquid scintillators to magnetic fields

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(I975) 459-463;

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NORTH*HOLLAND

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SENSITIVITY OF ORGANIC L I Q U I D S C I N T I L L A T O R S TO MAGNETIC FIELDS E. J E E N I C K E , P. L 1 A U D , B. V I G N O N and R. W I L S O N

lnstitut des Sciences Nucldaires, B.P. 257, 38044 Grenoble, France* Department of Physics, Harrard Unicersity, Cambridge, Massachusetts 02138"*, U.S.A. Received 26 March 1975 We have m e a s u r e d the scintillation efficiency o f organic, mineraloil-base scintillator (NE 235) as a function o f applied magnetic fields smaller t h a n 50 G. Below a threshold o f [ G, this scintillator is insensitive to magnetic fields (less than l0 5/G change in

scintillation efficiency). A b o v e this threshold, the efficiency o f the scintillator rises rapidly, the increase in efficiency reaches 1.5% at 40 G, and saturates a r o u n d 100 G to a value o f 1.8%.

1. Introduction

located inside the soft-iron enclosure for the photomultiplier tube (PMT). This magnetic shielding is sufficient to make the PMT approximately 100 times less sensitive to magnetic fields than the scintillator.

As a prerequisite for a future high-accuracy experiment, we have evaluated the scintillation efficiency of organic mineral-oil-base scintillator (Nuclear Enterprises NE 235) as a function of magnetic field B. R. C. Johnson et al.l) discovered that anthracene has a scintillation component which depends on the magnetic field, and Faulkner and Bard 2) exhibited a similar effect in organic solutions. If organic scintillators are used in a high-precision experiment, systematic errors can become important, and their magnitude must be assessed. This was first done by Bodenstedt et al.3), who also pointed out that the scintillation efficiency, in his scintillator, depends on [B] only. Kuphal ¢) verifies the latter statement, though he confines himself to one value of the field excitation ( H = 40 Oe). The measurements of refs. 1 and 2 are limited to relatively high magnetic fields (kG). Refs. 3 and 4 are in disagreement about the magnitude of the effect (more than an order of magnitude discrepancy), and the data of Bodenstedt et al. 3) imply a discontinuity of the effect at B = 0 (see section 4), which is surprising, since the change of physical processes at B = 0 is infinitesimally small. We have therefore redetermined with considera n y higher accuracy the sensitivity to magnetic fields of the scintillator that we intend to use; i.e., organic mineral-oil base (Nuclear Enterprises NE 235).

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2. Description of the experiment Fig. 1 shows the essentials and is approximately to scale. There is no ferromagnetic part in the whole construction, with the exception of 3 Conetic shields * Supported in part by lnstitut National de Physique Nucl6aire et de Physique des Particules. ** Supported in part by the National Science F o u n d a t i o n Contract no. GP-38119.

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VIEW B Fig. 1 . T h e experimental arrangement: l) a l u m i n u m liquid scintillator t a n k ; 2) level o f liquid scintillator; 2') reduced level o f liquid scintillator; 3) reflector: aluminized Mylar; 4) beta source 198Au; 4') g a m m a source 6°Co; 5) coils; 6) lucite light pipe; 7) P M T ( R C A 4525); 8) preamplifier; 9) conetic magnetic shields; 10) soft-iron P M T enclosure.

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(This was m e a s u r e d by t u r n i n g on the magnetic field a n d by shining light from a light-emitting d i o d e into the P M T . ) The reflector located inside the scintillation liquid is m a d e o f aluminized M y l a r and i m p r o v e s the light collection r o u g h l y 40 times; it also guarantees t h a t the light collected by the P M T does not originate outside this reflector, where the magnetic field is illdefined a n d where b o u n d a r y - a s s o c i a t e d p r o b l e m s m i g h t a p p e a r (due to discontinuities o f the electron density at the a i r - l i q u i d a n d a l u m i n u m - l i q u i d b o u n daries) when the p r i m a r y i o n i z a t i o n is due to g a m m a rays. Except for one d a t a p o i n t (fig. 2b), where a 6°Co source was used (placed at l o c a t i o n 4' in fig. 1), all m e a s u r e m e n t s were done with a 20 mCi source o f 198Au [T,n,~ (beta) = 0.96 MeV, E ( g a m m a ) = 0.42 MeV, half-life 2.7 d]. This source was in the form o f a 2 m m × 2 r a m × 1/100 m m foil o f gold, s a n d w i c h e d between two aluminized m y l a r sheets 10/~m thick and 1 c m in diameter. W e preferred the b e t a source as the p r i m a r y source o f ionization, because the region o f i o n i z a t i o n is well localizable (1 cm 3 centered within the coils to better t h a n 1 cm), whereas g a m m a rays easily leave the region o f interest. The 6°Co source was used

once to verify that g a m m a s introduce no new effect. The m e a s u r e m e n t accuracy available with a g a m m a source is inherently worse, because the i o n i z a t i o n is distributed over a large v o l u m e ; hence the magnetic field is ill-defined. Some sort o f average field, smaller t h a n the p e a k field in the center o f the coils, a n d obviously difficult to evaluate, has to be inserted into the data. The efficiency o f scintillators is k n o w n to be sensitive to dissolved oxygen which quenches delayed fluorescence. After receiving the scintillator from N u c l e a r Enterprises, we proceeded with a first set o f measurements, as explained below. After one week we b u b b l e d argon for one h o u r t h r o u g h the scintillator t a n k and r e m e a s u r e d several d a t a points; no change was found. In o r d e r to extract the small change in scintillation efficiency f r o m the large fluctuation o f light caused by the limited n u m b e r of photons, the magnetic field p r o d u c e d by the two air coils was turned on a n d off at a period o f 1.6 s, and the electronics subsequent to the P M T filtered out o f the noise a square-wave signal o f the correct frequency and phase. The a m p l i t u d e o f this square wave is p r o p o r t i o n a l to the change in scintillation-light intensity.

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Figs. 2 and 3. Dependence of the scintillation efficiency on the magnetic field applied by the air coils. Solid (open) points: applied field in the (opposite to the) direction of the earth's magnetic field. Error bars are the same for points corresponding to the same field. All measurements done with a 198Au beta source, except for one point (×)done wiht a (~°Cogamma source. Fig. 3 is a blowup of the section near the origin of fig. 2.

ORGANIC LIQUID SCINTILLATORS

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Fig. 4. Determination of the unknown Bres (H=O) by reversal of Happl, assuming the i vs B curve symmetric about the vertical axis (see section 3). The electronics used to process the P M T anode current consists o f a high-quality preamplifier which is ac coupled (RC = 30 s) into a lock-in amplifier. This lock-in amplifier essentially correlates (i.e., multiplies and integrates) the amplified anode current with the waveform that drives the air coils. After sufficient time has elapsed, depending on the signal-to-noise ratio, the signal emerges from the much larger r a n d o m fluctuations of the background. 3. Results The most immediate way to present the results of our measurements would be to plot "scintillation light intensity vs magnetic field at the scintillator"; i.e., i vs B. This turns out to be awkward here, because the change in light intensity is small; therefore we plot the Case (b)

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Magnetic field of the scintillator

relative change in scintillation light intensity Ai/i against the applied magnetic field H (figs. 2 and 3.) In order to correctly interpret these curves, it is important to realize that, after shutting off the applied field H switcher, i.e., switching off the current in the coils, the scintillator is still exposed to a residual field, in our case mostly the earth's magnetic field, whose effect must be assessed. The exact shape of the i vs B curve, and the residual field Bfe s were determined as follows (fig. 4): The axis of the two air coils used to produce H was aligned with the horizontal c o m p o n e n t of the magnetic field of the earth. A first set of measurements was made with the additional field H, produced by the coils, added to the earth's magnetic field. This set we call "case (a)". Then the wires on the coils were reversed, and another set of measurements was made with the applied field H subtracted from the earth's magnetic Case (b)

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Fig. 5. Magnetic field at the scintillator as a function of time.

Fig. 6. Excitations equivalent to fig. 5.

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field, "case (b)". The magnetic field at the scintillator as a function of time for the two cases is shown in fig. 5. Since the scintillation-light intensity depends, in a liquid, on B absolute only, subtracting an applied field H from the earth's magnetic field (fig. 5) is equivalent to producing a magnetic field at the scintillator according to fig. 6. The influence of the earth's magnetic field on our measurements can therefore be extracted from the two sets of measurements (figs. 2 and 3). This procedure has an additional bonus: since the two sets of measurements figs. 2 and 3 (or, equivalently, figs. 7 and 8) differ by twice the magnitude of the earth's magnetic field, which is a known quantity, the proper calibration of the horizontal axis B (magnetic field) can be verified. From figs. 2 and 3 it appears that below 1 G the sensitivity of the scintillator to magnetic fields is less than 10-5/G, but rapidly rises above 1 G. Saturation occurs around 100 G, but, according to the report of Johnson et al.l), the sensitivity should become large again (and negative) above several hundred Gauss. Fig. 3 is a blowup of the threshold region and shows the effect of the earth's magnetic field, as explained above. At 14 G, we made a measurement with a g a m m a source (6°Co); the scintillation efficiency is apparently less than for the beta source; we believe this effect is spurious and just due to the spreading of the ionization events into regions of the scintillator where the magnetic field has no more its peak value.

4. Comparison with previous work We have compared our work to the work of Bodenstedt et al.3). They do not plot i vs B or (Ai/i) vs B directly, but rather the derivative of the latter, [(Ai/i)/AB] vs B, in order to estimate the sensitivity of the scintillation light output to small changes of the magnetic field. We integrated their curve to compare it to ours (fig. 7), and we differentiated our curve to compare it to theirs (fig. 8). Our data do not display singularities (cusps or discontinuities) at the origin; these are not expected, since the change of physics at the origin is infinitesimally small. At fields of the order of the earth's magnetic -4

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field, the sensitivity is not at its maximum value, as given in ref. 3, but, on the contrary, compatible with zero. (We find that, up to 0.5 G, the upper limit for the sensitivity is 10- 5/G.)This observation is very important in practical applications of scintillators. We wish to point out, however, that Bodenstedt et al. 3) used solid (plastic) scintillator for their studies, and their evaluation was based on a light-pulse counting scheme. Such a procedure might yield different results than our work, since delayed fluorescence is included in a different way. For some time we believed that the sensitivity of the scintillator to magnetic fields was caused by a change of the Compton-electron trajectories (fig. 9). At low fields, Compton electrons leaving the scintillator will not return. At higher fields, however, Compton electrons can return to the scintillator if it is surrounded by air. (We would not expect this effect from a beta source which is entirely immersed in the scintillator.) We have, therefore, tested this hypothesis for a magnetic field of 10 G and Compton electrons produced by 1 MeV gammas (6°Co), by bringing the level of the scintillating liquid down to the height of the g a m m a source (fig. 1, view B, 2' and 4), so that the air-liquid interface was situated in a region of high field. There was no noticeable change in the efficiency of the scin-

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tillator. This hypothesis is therefore incorrect for low fields and low energy. Rather, the magnetic-field dependence of organic scintillators is intrinsic and due to their molecular structure. The effect is apparently well known to photo-physicists; an extensive review of the theory has been given by Swenberg and GeacintovS). We are very grateful to Prof. John B. Birks for pointing out to us refs. 1, 2, and 5. We thank C. Barnoux for efficient technical support. One of us (E.J.) wishes to thank Prof. Jacques Valentin for his hospitality at the ISN, where this work was completed. References 1) R. C. Johnson, R. E. Merrifield, P. Avakian and R. B. Flippen, Phys. Rev. Letters 19 (1967) 285. '~) L. 17,. Faulkner and A. J. Bard, J. Am. Chem. Soc. 91 (1969) 6495. a) E. Bodenstedt, L. Ley, H. O. Schlenz, U. Wehmann, Nucl. Phys. A137 (1969) 33. 4) E. Kuphal, Ph. D. Thesis (1971) Technische Hochschule, Darmstadt; E. Kuphal, P. Dewes and E. Kankeleit, Nuc[. Phys. A234 (1974) 308. 5) C. Swenberg and N. E. Geacintov, in: Organic molecular photophysics (John B. Birks, ed.; J. Wiley and Sons, New York, 1973) vol. 1.