Precise Determination of the 235 U Reactor Antineutrino Cross Section per Fission Carlo Giunti

Transcription

1 C. Giunti Determination of the 235 U Reactor Antineutrino Cross Section per Fission AAP Dec /10 Precise Determination of the 235 U Reactor Antineutrino Cross Section per Fission Carlo Giunti INFN, Sezione di Torino giunti@to.infn.it Applied Antineutrino Physics 2016 Liverpool 2 December 2016 Talk based on arxiv:

2 C. Giunti Determination of the 235 U Reactor Antineutrino Cross Section per Fission AAP Dec /10 Reactor Electron Antineutrino Anomaly [Mention et al (Saclay), PRD 83 (2011) ] New reactor ν e fluxes [Mueller et al (Saclay), PRC 83 (2011) ; Huber, PRC 84 (2011) ] R = N exp N cal Possible causes: Bugey 3 Bugey 4 Chooz Daya Bay Double Chooz Gosgen ILL Krasnoyarsk Nucifer L [m] Palo Verde RENO Rovno88 R = ± Short-Baseline Neutrino Oscillations: see the talk by Yufeng Li. An excess of the reactor ν e flux estimation. Rovno91 SRP

3 C. Giunti Determination of the 235 U Reactor Antineutrino Cross Section per Fission AAP Dec /10 Detection reaction: ν e p n e Experimental event rate: N a = 1 4πL 2 a N a p P a th E f a σ f,a a: experiment index Experimental cross section per : σ f,a = k f a k σ f,k k = 235, 238, 239, 241: index of the four fissile isotopes 235 U, 238 U, 239 Pu, 241 Pu Calculated cross sections per of the four fissile isotopes: Saclay (S) Huber (H) SaclayHuber (SH) uncertainty σ f, % % σ f, % σ f, % % σ f, % % We investigate which of the four fluxes could be the cause of the reactor antineutrino anomaly.

4 REACTOR ANTINEUTRINO ANOMALY PHYSICAL REVIEW D 83, (2011) C. ZHANG, X. QIAN, AND P. VOGEL PHYSICAL REVIEW D 87, (2013) TABLE II. N obs=n pred ratios based on old and new spectra. Off-equilibrium corrections have been applied when justified. The err column is the total error published by the collaborations including the error on S tot, the corr column is the part of the error correlated among multiple-baseline experiments, or experiments using the same detector. This table is used to construct the covariance matrix used in Eq. (10). TABLE I. Tabulated results of all 23 experiments. Experiments are categorized into different groups with horizontal lines. Within each group, the corr represent the correlated uncertainties among different experiments. This table is an extension of Table II of Ref. [1]. There are additional correlated uncertainties, since Double Chooz results were anchored to the Bugey-4. See the text for more explanations. Mention et al (Saclay), PRD 83 (2011) Zhang, Qian, Vogel, PRD 87 (073018) 2013 # result Det. type n (s) 235 U 239 Pu 238 U 241 Pu old new err(%) corr(%) L(m) 1 Bugey-4 3 He þ H 2O ROVNO91 3 He þ H 2O Bugey-3-I 6 Li LS Bugey-3-II 6 Li LS Bugey-3-III 6 Li LS Goesgen-I 3 He þ LS Goesgen-II 3 He þ LS Goesgen-II 3 He þ LS ILL 3 He þ LS Krasn. I 3 He þ PE Krasn. II 3 He þ PE Krasn. III 3 He þ PE SRP I Gd-LS SRP II Gd-LS ROVNO88-1I 3 He þ PE ROVNO88-2I 3 He þ PE ROVNO88-1S Gd-LS ROVNO88-2S Gd-LS ROVNO88-3S Gd-LS from R Rovno88;18 m;new ¼ 0:995 0:060ðstat þ syst þ S totþ assuming a pure 235 U spectrum. This leads to the to R Rovno88;18 m;new ¼ 0:944 0:057ðstat þ syst þ S totþ. ratios R Krasno;33 m ¼ 0:936 0:054ðstat þ syst þ S totþ and In 1991 the Rovno integral experiment [7] published a R Krasno;92 m ¼ 0:953 0:195ðstat þ syst þ S totþ, at 33 m cross section per of Rovno91 f ¼ 5:85 0:17 in units and 92 m, respectively. In 1994 two other measurements of cm 2 =, 18 m away from a nuclear core were performed 57 m from the Krasnoyarsk reactors [9]. with an average fuel composition WhiteofPaper, 235 U ¼ 61:4%, arxiv: They measured Krasno;57 m 239 Pu ¼ 27:4%, 241 Pu ¼ 3:8% and 238 f ¼ 6:26 0:26 at 57 m, and U ¼ 7:4%. They compared it to their predicted cross section of 6:33 predicted the cross section pred;old f;rovno91 ¼ 5:94 0:16 in 0: cm 2 =, based on Ref. [21] and in result Det. type units of cm 2 τn (s) 235 U 239 Pu 238 U =, and thus obtained the ratio agreement 241 Pu old new err(%) corr(%) L(m) with our reevaluation using previous reference Bugey-4 R Rovno91;old ¼ 0:985 3 HeH2O 0:037ðstat þ syst þ S totþ We recomputed the cross section per according to the we revise the ratio R Krasno;57 m ¼ 0:947 0:047ðstat þ antineutrino spectra Using the3.0 new values 3.0of Ref. 15[19] ROVNO91 3 HeH2O new antineutrino spectra and found pred;new f;rovno91 ¼ 6:223 syst þ S totþ. Bugey-3-I 6 Li-LS From the neutrino pioneering 4.8experiments led15 by F. 0:17. The new ratio is thus revised to R Rovno91;new ¼ Reines and C. Cowan [39] to the nineties, a series of Bugey-3-II 0:940 0:036ðstat 6 Li-LS þ syst þ S totþ. 889We note that0.328 the correction of the neutron 6 Li-LS mean lifetime 889contributes % 0.328to the Savannah River 0.915Plant (SRP), a14.1 U.S production 4.8 facility 95 for reactor antineutrino measurements were performed at the Bugey-3-III shift of the ratio. tritium and plutonium. For neutrino energies between 2 Goesgen-I In 1984 a 3 neutrino HeLS experiment operated at the and 8 MeV the spectrum difference between SRP and a Goesgen-II Krasnoyarsk reactors 3 HeLS [8], which 897 have an almost pure similar 0.050core with pure U fuel 6.5was estimated 6.0 to be 45less 235 U fuel composition leading to an antineutrino spectrum Goesgen-II than 0.5%. We make use of the latest results published in within 1% of pure 3 HeLS 235 U, and operate 897 over d cycles They Ref. [10]. Measurements were reported at two different measured ILL the cross 3 HeLS section per 889 at 1 two distances, baselines, m and m. The 9.5 new SRP ratios 6.0 are reevaluated to : :006ðstatÞ 5.80:0353ðsyst 4.9 þ S totþ 33and 6:30 1:28 at 92 m, in units of cm 2 =. They 1:019 0:010ðstatÞ 0:0377ðsyst þ S totþ, respectively. 9 Krasno;33 m f ¼ 6:19 0:36 at 33 m and Krasno;92 m f ¼ Krasn. I 3 HePE Krasn. compared II it to the 3 HePE predicted cross 899section of 1 6:11 0: Krasn cm III 2 =, 3 HePE based on the 899Ref. [20] U measurement instead of Ref. [21]. Correcting the neutron mean D. CHOOZ and4.9 Palo Verde lifetime SRP I and usinggd-ls the new antineutrino 887 spectra 1 we obtain Based on the good agreement 3.7 between 3.7 pred;old f 18and asrp predicted II cross Gd-LS section of887 6: cm 2 =, Bugey f obtained 1.055at Bugey [3], 3.8the CHOOZ 3.7 experiment 24 ROVNO88-1I 3 HePE ROVNO88-2I 3 HePE ROVNO88-1S Gd-LS ROVNO88-2S Gd-LS ROVNO88-3S Gd-LS # Result Detector type 235 U 239 Pu 238 U 241 Pu Ratio err (%) corr (%) L(m) P sur Year 1 Bugey-4 3 He þ H 2O ROVNO91 3 He þ H 2O Double Chooz Gd-LS Double Chooz LS (n-h) Bugey-3-I 6 Li LS Bugey-3-II 6 Li LS Bugey-3-III 6 Li LS Goesgen-I 3 He þ LS Goesgen-II 3 He þ LS Goesgen-III 3 He þ LS ILL 3 He þ LS Krasnoyarsk I 3 He þ PE Krasnoyarsk II 3 He þ PE Krasnoyarsk III 3 He þ PE SRP-I Gd-LS SRP-II Gd-LS ROVNO88-1I 3 He þ PE ROVNO88-2I 3 He þ PE ROVNO88-1S Gd-LS ROVNO88-2S Gd-LS ROVNO88-3S Gd-LS Palo Verde Gd-LS Chooz Gd-LS There are three reactor cores in the Palo Verde experiment [5]. The distances between detector and each reactor core are 750, 890, and 890 m. In calculating the average survival probability P sur, we assume that all three reactor cores have equal power. The result is compared with P 750m sur assuming full power in only the 750 m reactor and P 890m sur assuming full power in the 890 m reactors. The differences are quoted as an additional uncertainty, which is only about 5% of the total reduced experimental uncertainty. There are two reactor cores in the Chooz experiment [3,4]. The distances between the detector and each reactor core are 998 and 1115 m. A similar procedure is applied to calculate the uncertainty for the equal power assumption. The resulting uncertainty is about 6.2% of the total reduced experimental uncertainty. The fractions are assumed to be the same as those from Double Chooz [7]. We also calculated the average P sur by varying these fractions. The differences are negligible. The Double Chooz experiment is conducted at the same location as Chooz. With a single detector, the recent rateonly analyses of the data from delayed neutron capture on gadolinium (n-gd) and delayed neutron capture on hydrogen (n-h) reported the value of sin ¼ 0:170 0:052 [7] NO! and sin ¼ 0:044 0:060 [8], 1 by anchoring to the short-baseline Bugey-4 results [16], respectively. Although the measured flux normalized to the prediction of Ref. [2] has not been reported, we can deduce such ratios using the reported fractions [7], the reported values of sin [7,8], and the Bugey-4 results. The reduced err is dominated by the uncertainties of reported sin , with additional uncertainties coming from the equal power assumption. The reduced corr are calculated from the reduced experimental uncertainty reduced err from Bugey-4. Furthermore, there are additional correlated uncertainties between the n-h and n-gd measurements due to the equal power assumption. The final covariance matrix W using reduced uncertainties is shown in Fig. 1. The 2 function used in this analysis is constructed as follows: Rescaling from Saclay to SaclayHuber ratios: 2 ðr;sin Þ k R exp a,sh = f Rexp k aσs f,k a,s k f k aσsh f,k ¼ ðr ~P surðsin Þ ~RÞ T W 1 ðr ~P surðsin Þ ~RÞ þ ðsin :089Þ 2 0:011 2 : (4) 1 To be consistent with other experiments, we choose the rateonly sin results, which represent simple measures of the disappearance in the total number of events. Table C. XXI. Giunti Nobs/Npred Determination ratios based old and newofspectra. the Off-equilibrium 235 U Reactor corrections Antineutrino have been applied Cross Section per Fission AAP Dec /10

5 C. Giunti Determination of the 235 U Reactor Antineutrino Cross Section per Fission AAP Dec /10 a Experiment f235 a f238 a f239 a f241 a R exp a,sh σa exp [%] σa cor [%] L a [m] } 1 Bugey Rovno } 18 3 Rovno88-1I Rovno88-2I Rovno88-1S Rovno88-2S Rovno88-2S Bugey Bugey Bugey Gosgen Gosgen Gosgen ILL } Krasnoyarsk Krasnoyarsk Krasnoyarsk Krasnoyarsk SRP SRP Nucifer Chooz Palo Verde Daya Bay RENO Double Chooz

6 Theoretical ratios: R th a = k f a Least-squares function: χ 2 = a,b k r kσ SH f,k k f k aσsh f,k Unknowns: r 235, r 238, r 239, r 241 ( ) Rb th Rexp b,sh ( (V Ra th Ra,SH) exp 1 ) ab χ r 235 r 238 r 239 r % 99% 95.45% 90% 68.27% r k r 235 = ± Precise determination of the 235 U cross section per : σ f,235 = (6.35 ± 0.09) cm2 σ SH f, σ smaller than = (6.69 ± 0.14) cm2 Note however the unrealistic deviations of the other fluxes, e.g. r bf 239 = and r bf 241 = C. Giunti Determination of the 235 U Reactor Antineutrino Cross Section per Fission AAP Dec /10

7 In order to keep under control the values of r 238, r 239, r 241, we add a penalty term to the least-squares function: χ 2 = χ 2 ( ) 1 2 rk r k k with r 235 = r 239 = r 241 = 0.05, and r 238 = 0.1. χ r 235 r 238 r 239 r % 99% 95.45% 90% 68.27% r k r 235 = ± r 238 = ± r 239 = ± r 241 = ± Precise and reliable determination of the 235 U cross section per : σ f,235 = (6.33 ± 0.08) cm2 σ SH f, smaller than = (6.69 ± 0.14) cm2 C. Giunti Determination of the 235 U Reactor Antineutrino Cross Section per Fission AAP Dec /10

8 C. Giunti Determination of the 235 U Reactor Antineutrino Cross Section per Fission AAP Dec /10 r 238 r 235 r 239 r 235 r 241 r 235 r 239 r 238 r 241 r 238 Small anticorrelation of r 235 with r 238 and r 239. Sizable anticorrelation between r 238 and r 239. r 241 is practically uncorrelated with the other ratios. r 241 r 239

9 C. Giunti Determination of the 235 U Reactor Antineutrino Cross Section per Fission AAP Dec /10 Uncertainty due to the uncertainties of the fractions fk a? [see: Djurcic, Detwiler, Piepke, Foster, Miller, Gratta, JPG 36 (2009) ] Difficult to calculate due to the large number of experiments with mostly unknown fractions uncertainties and correlations. The most significant effect on the determination of σ f,235 could come from a non-pure 235 U antineutrino spectrum in research reactor experiments. The SRP collaboration reported that during the data collection period of this experiment, 239 Pu s constituted less than 8% of the total s and 238 U s less than 4%. [PRD 53 (1996) 6054] Considering f235 a = 0.88, f 238 a = 0.04, f 239 a = 0.08, f 241 a = 0 for the research reactor experiments (a = 14,..., 20) we obtained 43 cm2 r 235 = ± σ f,235 = (6.33 ± 0.11) 10 Result compatible with that in previous slide: 43 cm2 r 235 = ± σ f,235 = (6.33 ± 0.08) 10 Therefore, the determination of σ f,235 is robust.

10 C. Giunti Determination of the 235 U Reactor Antineutrino Cross Section per Fission AAP Dec /10 Conclusions If the reactor neutrino anomaly is due to an overestimation of the antineutrino fluxes, it is very likely that at least the calculation of the 235 U flux must be revised. This analysis does not give information on the cause of the theoretical excess for σ f,235. Speculations The theoretical excess for σ f,235 could be due to an unknown imperfection in the 1985 measurement of the 235 U electron spectrum at ILL. [Schreckenbach, Colvin, Gelletly, Von Feilitzsch, PLB 160 (1985) 325] It may be possible that the reactor antineutrino anomaly and the 5 MeV bump are somewhat related and due to the 235 U antineutrino flux. Intriguing indications: From a comparison of the NEOS and Daya Bay data P. Huber found that 235 U is the preferred source of the 5 MeV bump. [arxiv: and previous talk] RENO found that the 5 MeV bump may be correlated with 235 U fuel fraction. [Hyunkwan Seo talk]