8.4.6.4. Random Coincidences

Table of Contents

Chance overlaps in detector signals

In coincidence measurements, two or more detectors are used to decide whether events are related. A true coincidence happens when signals come from the same physical process. A random coincidence happens when unrelated events occur close enough in time that the electronics count them as if they were connected.

This is an important practical effect in radiation measurements. Even when two detectors are looking for correlated radiation, each detector may also record unrelated background counts, noise pulses, or signals from different decays. If two such independent signals arrive within the chosen timing window, the system may register a coincidence even though there is no real physical connection.

Why random coincidences happen

A coincidence system usually accepts two pulses if they arrive within a time interval called the coincidence resolving time, or coincidence window. If detector A records many events per second, and detector B also records many events per second, then purely by chance some events from A and B will fall inside the same window.

The larger the count rates are, the more often this accidental overlap happens. The wider the timing window is, the more likely it is that unrelated pulses will be accepted together. For this reason, random coincidences become especially important at high count rate or with poor timing resolution.

A random coincidence is not caused by a single shared physical event. It is an accidental time overlap of independent signals.

Basic rate formula

Suppose detector 1 has count rate $R_1$ and detector 2 has count rate $R_2$, measured in counts per second. Let the effective coincidence window be $\tau$.

For independent events, the random coincidence rate is approximately

$$
R_{\text{random}} \approx R_1 R_2 \tau
$$

in the simplest convention for the resolving time.

In some electronics conventions, the accepted window extends on both sides of a reference pulse, giving

$$
R_{\text{random}} \approx 2 R_1 R_2 \tau
$$

The factor of 2 depends on how the coincidence window is defined, so the exact expression must match the experimental setup.

Important practical rule:
$$
R_{\text{random}} \propto R_1 R_2 \tau
$$
Random coincidences increase when either detector rate increases, and when the coincidence window becomes wider.

Physical meaning of the formula

The formula can be understood in a simple way. If detector 1 produces pulses at rate $R_1$, then each pulse creates a small time interval of width $\tau$ during which a pulse from detector 2 would be accepted as coincident. Detector 2 produces pulses at rate $R_2$, so the probability of one of its pulses falling into that interval is roughly $R_2 \tau$, as long as the probability is small. Multiplying by the number of pulses from detector 1 per second gives the accidental rate.

This reasoning assumes that the two detectors are producing independent, randomly timed pulses, and that count rates are not so high that pulse pileup and dead-time effects strongly distort the statistics.

Distinguishing true and random coincidences

In a real experiment, the measured coincidence rate can contain both true and random parts:

$$
R_{\text{measured}} = R_{\text{true}} + R_{\text{random}}
$$

If random coincidences are not accounted for, the experiment may overestimate the number of physically related events. This is especially important in gamma-gamma coincidence work, positron annihilation measurements, and nuclear decay studies.

A common goal is to make $R_{\text{random}}$ much smaller than $R_{\text{true}}$.

When analyzing coincidence data, do not assume every coincidence is real:
$$
R_{\text{true}} = R_{\text{measured}} - R_{\text{random}}
$$
provided the random contribution has been estimated correctly.

Dependence on experimental conditions

The main factors affecting random coincidences are summarized below.

Quantity	Effect on random coincidences
Detector count rate $R_1$	Higher $R_1$ increases accidental overlap
Detector count rate $R_2$	Higher $R_2$ increases accidental overlap
Coincidence window $\tau$	Wider window increases accidental overlap
Background radiation	Raises singles rates, often increasing randoms
Electronic noise	Can create extra false singles and more randoms

If one doubles $R_1$, the random rate doubles. If one doubles both $R_1$ and $R_2$, the random rate becomes four times larger. If one halves the timing window, the random rate is reduced by half.

Simple numerical example

Suppose detector 1 counts at

$$
R_1 = 2000 \ \text{s}^{-1}
$$

and detector 2 counts at

$$
R_2 = 3000 \ \text{s}^{-1}
$$

with a coincidence window

$$
\tau = 1.0 \times 10^{-6} \ \text{s}
$$

Then the approximate random coincidence rate is

$$
R_{\text{random}} \approx R_1 R_2 \tau
= (2000)(3000)(1.0 \times 10^{-6})
= 6 \ \text{s}^{-1}
$$

So even if no true correlated events existed, the system would still register about 6 accidental coincidences each second.

Visual picture of accidental overlap

The idea can be pictured as unrelated pulses sometimes landing in the same allowed time interval.

Rendering scientific sketch…

Random coincidence from unrelated pulses

The pulse in detector 2 near the pulse in detector 1 is not necessarily from the same event. If it falls inside the allowed time window, the electronics may still count it as a coincidence.

How random coincidences are estimated

One common method is to measure coincidences in a delayed channel. A deliberate time shift is introduced so that true physical coincidences no longer line up, but accidental overlaps still occur at about the same rate. The delayed coincidence count then gives an estimate of the random background.

Another method is to calculate the random rate from the measured singles rates and the known resolving time using the formula above. This works best when the pulse statistics are well understood and the system behaves ideally.

Reducing random coincidences

To reduce accidental coincidences, experiments try to lower singles rates from unrelated events and narrow the coincidence timing window. Better timing electronics, lower background, shielding, and energy selection can all help. The exact detector methods belong to other topics, but the central principle is simple, make unrelated pulses less frequent and reduce the time interval in which they can be falsely matched.

To reduce random coincidences, decrease $R_1$, decrease $R_2$, or decrease $\tau$.
The most direct timing strategy is to make the coincidence window as narrow as possible while still keeping true coincidences.

Final perspective

Random coincidences are an unavoidable statistical background in coincidence experiments. They do not represent real physical correlations, but they can imitate them if timing alone is used. Their rate depends mainly on the singles count rates and the coincidence window width. Careful estimation and subtraction of this accidental background is essential for reliable coincidence measurements.

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