2NCorr SPDP Introduction

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The calculation of angular correlation

The neutron efficiency and acceptance of our detector array varies highly non-uniformly over the range of two-neutron opening angles ([math]\theta_{nn}[/math]). This is due to the detector array's non-spherically symmetric geometry, and to its varying neutron detection efficiency as a function of both position and energy (see figure).

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  Measured opening angle distribution of neutrons from the spontaneous fission of Cf252.

For this reason, every measured distribution is made meaningful by normalizing against a "control" distribution. This is done by diving the values of the measured distribution by the control distribution on a bin-by-bin basis.

two-neutron opening angle

For [math]\theta_{nn}[/math], the control distribution is made by looking at the events from pairs of two pulses, such that the the two pulses occurred within a few 100 ms of each other. If there is a neutron event in both of the pulses from a given pulse pair, then the opening angle between the two events is calculated. Since no information can be shared between the neutrons of different pulses, the control distribution must be free of any two-neutron correlations. However, the control distribution is still subject to the exact same effects of detector geometry and efficiency as the distribution from truly correlated neutron pairs. So what is meant by "angular correlation" here, is the measured [math]\theta_{nn}[/math] rate relative to a totally uncorrelated neutron source. The plot below is the same data from the plot above, but after normalization to an uncorrelated source.

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  Angular correlation of neutrons (E>1.5 MeV) from the SF of Cf252. In this case, instead of separate pulses, the normalizing distribution is taken from neutron pairs formed across separate fission event triggers.

single-neutron angle relative to photon beam

When constructing a distribution from neutron singles, the method of using uncorrelated neutron pairs from different pulses no longer works, because while the neutrons in a pair are uncorrelated with each other, they are both correlated with the direction of the incident photon beam. For this reason, neutron singles from the spontaneous fission (SF) of Cf252 are used for normalization. This choice leads to several issues, because there are differences in experimental conditions when measuring photo-neutrons in the presence of a beam, vs measuring neutrons from the SF of Cf252. The energy spectrum of neutrons from Cf252 differ from that of our photo-neutrons. During the Cf252 measurement, the detectors don't experience the same level of dead-time caused by the detection of photons. Also, the noise profiles are very different. Each of these effects must all be removed or neglected with various corrections and assumptions, resulting in a analysis procedure which is less robust than the same-pulse different-pulse method.





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