Nature, 3 The g BGO bolometer 1 Photograph of the heat [g BGO] and light [Ge; =5 mm] bolometers: see Fig. 1c for description Current events: Amplification gains: 8, (heat channel) &, (light channel). The pulses passed through an analogical filter (BP: 1 Hz 3 Hz; db/octave): the low frequency cut-off explains the undershoot. The selected events (a. 9 Bi seen decaying; b. absorption of a 5 kev gamma; c. a 3. MeV event from cosmic ray interaction) underline the discrimination principles of the g BGO scintillating bolometer at the level of 9 Bi decay. Gamma ray 9 Bi decay Cosmic ray event 3 b E~5 kev a E~3137 kev c E~3. MeV 3 Output (V) 1 Output (V) 1 - - -
Nature, 3 A cascade of decays, from Ra to 1 Pb: A Ra nucleus (T ½ =3. days), a natural daughter in the 3 Th chain, is seen decaying to 1 Pb. All decays proceed at 1% by alpha emission. Such delayed events help to identify without ambiguity the contaminations lines found in the crystal, by cross-checking time intervals and amplitude ratios. Due to the very low rate of alpha events, chance coincidences are highly unlikely. Paired events might also help to understand the origin of the dispersion seen in the light channel, by allowing for geometric effects, since all these events occurred within 3 Å. Ra Rn T 1/ =55 s 1 Po T 1/ =15 ms 1 Pb.5. Q α =5789 kev Q α =5 kev Q α =97 kev Output (V) 1.5 1..5 ~ s Energy (MeV), from heat channel. -.5 -, April 9 th @ ::9 -, April 9 th @ :1:33 Energy calibration of the alpha branch: The 9g BGO bolometer a. linear assumption, using the 1 Am line b. residuals from the linear assumption and best fit for correction c. final systematic uncertainty due to the previous fit correction [Pulses and pre-trigger base lines were treated in the same way to produce the kev line] a b c E measured (MeV), E real -E measured (kev) from 1 Am line 1σ (syst.) accuracy (kev) 8 5 3 1-1 - 8 " kev" line 1 Am linear calibration Estimated error shift for the new line [-> E real =3137.3 kev].3 kev 9 Bi Q α New line E measured = 319.1 kev from previous plot 3137.3 kev, from previous plot Rn 1 33 Am U 1σ (stat.) new line... E real nd ORDER WEIGHTED LEAST SQUARES FIT 1 Po, from tables TO IDENTIFIED LINES (X) 1 Am linear calibration E measured, from 1 Am E estimated, from fit 8 Energy of alpha event (kev)
Nature, 3 3 Background registered during the 1 Am calibration (alphas & gammas branches): A strong suppression of 7 Bi lines is noticeable (<1/15), when compared to the spectrum of Fig. b. 1 Annih. 1 Am 11 h Counts / bin [1 bin=1 kev] 1 3 1 1 1 7 Bi K 8 Tl 9 Bi 1 1 3 5 7 E (MeV) Quenching factors: Added notes and figures Quenching factor (QF) 15 1 5 ζ=recoils QF=_γ / _ζ ζ=alphas at E γ = E ζ g BGO 9g BGO 9 Bi Q α..5 1. 1.5..5 3. 3.5 Energy of occuring event E ζ (MeV), from heat channel A strong quenching effect is measured from kev to 3.5 MeV in the 9g BGO bolometer: events from 3. MeV gammas, or cosmic rays, give much more light than 9 Bi alpha
Nature, 3 decays of equal energy. The quenching factor found for alphas (QF α 5.8) is significantly higher than a previous result on BGO at low temperature (QF α. at 1 mk) S1, whereas a quenching factor QF α 3.±.3 is reported at 3K S. The energy range of this figure is limited at low energy by dead time considerations which pushed us to raise the trigger threshold, and by saturations of the light channel at high energy. The trend noted increasing QF α with decreasing energy reflects the curvature of the alpha branch shown in Fig. 3. Inverting the traditional QF definition, we can also state from the data that 9 Bi decay events give no more light than less energetic 58±35 kev gammas (see Current events above). Important practical consequences could follow, if this factor were to remain the same at room temperature, as it is observed in NaI S3 : as long as no active discrimination is available, the detection of 9 Bi decay in simple BGO scintillators will be extremely difficult for having to face the high gamma background around 5 kev (see Background spectrum above). In particular, the 511 kev positron annihilation line, commonly found, will probably mask the decay. Conversely, an irreducible event rate of.13 counts / mn per kg of BGO is predicted in all BGO-based PET (Positron Emission Tomography) scan installations. The quenching factor for nuclear recoils, measured during a 5 Cf fast neutron irradiation of the g BGO bolometer, is also given for information. For lack of any absolute reference in energy on the recoil branch, it has been assumed an equal thermal conversion efficiency for recoils and gammas (note that a 1.7 ratio is found for alphas with respect to gammas). The two curves seem to join asymptotically at low energy. Background suppression in the g BGO bolometer: Contour lines in units of confidence levels for the / signals ratio of nuclear recoils induced by the ambient neutron flux (bottom events) and gammas (top events). Prominent lines at 88 kev and 15 kev are due to EC decay of 7 Bi. A normalisation to 1 has been applied at the level of the 88 kev line (y axis is 1/QF). Existence zones were defined during dedicated short runs, with strong sources: Co (gammas) and 5 Cf (neutrons). These data taken at IAS; altitude m indicate that BGO bolometers could play a significant role in experiments that search for direct Dark Matter detection, where the best separation at low energy is desirable. A
Nature, 3 5 separation of nuclear recoils (and potentially of Dark Matter events) from gammas is guaranteed in BGO with a confidence level CL 9 % (resp. 99.99 %) for E 3 kev (resp. E 51 kev). In underground sites or thermalised neutrons environments, alphas could probably be separated from ambient gammas with comparable efficiency (see quenching factor curves above). Tabulated Q α ( 9 Bi)=3137 ±.9 kev: 9 Bi decay is pined out at this value with a 88 % influence weight in the sense of the Atomic Mass Evaluation program (AME) by a careful determination of the gammas following the radiative capture of thermal neutrons by bismuth ( 9 Bi(n,γ) 1 Bi reaction) S. Courtesy: G. Audi. Estimation of the partial half-life of 9 Bi decay to the first excited state of 5 Tl at kev: While the overlap of the wave functions is a little larger for this decay mode compared with decay to the ground state and the spin change smaller (L=3, mostly), a much stronger effect is expected from the lower energy available to the alpha: computations point to a 1/ reduction in P α. By analogy with the known alpha decays of 11 Bi and 13 Bi, which present the same J π configuration as 9 Bi, we reckon to be committing a less than 5% error by assuming that the branching ratio of the decay follows the P α ratio of levels. Hypotheses made for calibration A negligible energy loss in the source itself, as well as the equality of the thermal responses to recoils and alphas are the only assumptions made in this work. They are strongly supported a posteriori by the fact that we do find the line at the expected position, after an unbiased calibration with well known identified lines. An unfortunate compensation of effects from both hypotheses cannot be excluded completely. The following arguments, even if not definitive ones, support the close to 1 equality of thermal responses (the question raised being how much of the kev recoiling 5 Tl energy is converted in heat with respect to the 377 kev emitted alpha? ): 1. a similar experiment performed with our CaWO 5g bolometer indicates a ratio of thermal conversion efficiency ζ= recoils / alphas =.9 ±.5 (8% CL) Co 3 5.3 5. 5.5 5. MeV Counts / bin Counts / bin 1 1 Po internal Q α E α 1 Am + µm mylar 1 Am 1 Po 1 Am 1 3 5 Energy (MeV), from external source [ 1 Am; E α of main line]
Nature, 3 This spectrum was taken during the calibration phase of our 5g CaWO bolometer, whose internal alpha lines are now well understood. We used in this experiment the same 1 Am alpha source as in the BGO experiment, but partially masked by a µm Mylar foil. The goal was to surround the unknown line, and it explains the slowed down events seen between.5 MeV and.9 MeV. An external Co source was also present, and its associated well known peaks at 1.17 MeV and 1.33 MeV were detected, slightly shifted here in energy because we used the 1 Am peak at E α =5.85 MeV for calibration. No sign of significant non-linearity was found during this experiment (a high polarisation power was used). The intent of this experiment was to understand the nature of the more prominent alpha contamination found in this crystal, which turned to be internal 1 Po (Q α =5.75 MeV). Data were fitted by gaussian profiles, and the grey areas indicate ±1 σ uncertainty on the line positions. The ratio η= E α ( 1 Am) / Q α ( 1 Po) of the two alpha peaks is expected at η calculated =1.1... from tabulated values, and we find it η measured =1.15±.1 (8% CL) from the position of the centroids of the lines. The thermal conversion efficiency given above, unpublished, is then easily calculated from this number The fact that we found ζ close to 1 with the same 1 Am source, but in CaWO, strongly supports the assumption of negligible energy loss of the alphas in the source itself for the BGO calibration.. similar results have been observed for other materials: Crystal ζ Recoils / alphas thermalisation efficiency Calculation Place where recoils occur Reference Ge 1. ±. from satellite peaks near surface S5 Si.99 ±.7 from satellite peaks near surface 1 TeO.93 ±.3 from satellite peaks bulk 15 TeO 1.5 ±. coincidence experiment near surface S CaWO.9 ±.5 from external source bulk see above Table S1: Thermalisation efficiency of recoils versus alphas in matter at low T Due to the physics of energy losses for alphas and recoils, experiments that involve bulk recoil events (internal contaminants) may be of more concern for our estimation of Q α ( 9 Bi) than experiments that involve surface ones (via implanted sources): the heat conversion of recoils at surface might be strongly affected by the surface state itself Moreover, alphas from external sources are probably much less sensitive to surface effects than recoils as can be deduced from their range values, Bragg s curves, or dedicated SRIM-3 simulations which validates afterwards our calibration process.
Nature, 3 Supplementary References 7 S1. Meunier, P. et al. Discrimination between nuclear recoils and electron recoils by simultaneous detection of phonons and scintillation light. Appl. Phys. Lett. 75 (9), 1335-1337 (1999) S. Dlouhý, Z. et al. The response of BGO scintillation detectors to light charged nuclei. Nucl. Instrum. Methods A 317, - (199) S3. Birks, J. B. The Theory and Practice of Scintillation Counting. pp. 59-9 (Pergamon Press, New York, 19) S. Tsai, J. S., Kennett, T. J. & Prestwich,W. V. 9 Bi(n,γ) 1 Bi reaction. Phys. Rev. C 7 (5), 397- (1983) S5. Alessandrello, A. A., Camin, D. V., Fiorini, E. & Giuliani, A. Construction of a massive germanium thermal detector for experiments on rare decays. Phys. Lett. B, 11-1 (1988) S. Alessandrello, A. et al. The thermal detection efficiency for recoils induced by low energy nuclear reactions, neutrinos or weakly interacting massive particles. Phys. Lett. B 8, 5-8 (1997)