P

P

atomic symbol for phosphorus

P10

gas mixture consisting of 90 % argon and 10 % methane, commonly used in gas proportional counters

Pa

atomic symbol for protactinium

pair production

production of an electron and positron from a high-energy photon (>1.022 MeV) in the vicinity of a massive particle such as an atomic nucleus

parameter

quantitative or numerical property of a statistical population or of a probability distribution

parent (radionuclide)

radionuclide that produces a given nuclide in a series of one or more nuclear decays – also called ancestor (radionuclide)

Note: The term parent is often used in the context of a direct relationship involving only one decay. The term ancestor is often used when the relationship is indirect.

Pb

atomic symbol for lead

Pd

atomic symbol for palladium

peak

narrow region of elevated signal in a spectrum

percentile

If X is a random variable and p is a real number between 0 and 100, then a p^th percentile of X is a (p/100)-quantile of X. E.g., a 90^th percentile of X is a 0.9-quantile.

phosphorescence

long-lived luminescence

photoelectric effect

complete absorption of a photon by an orbital electron, resulting in the disappearance of the photon and the ejection of the electron from the atom

photoluminescence

emission of light by a material when its atoms have been excited by electromagnetic energy

photomultiplier tube (PMT)

device for detecting weak light and amplifying the resulting electric signal using a photocathode and a series of dynodes inside an evacuated tube – used in scintillation counting

photopeak

peak in a gamma-ray spectrum pro­duced by inter­actions of photons with the detector

pipet; pipette

specially designed glass tube used in the laboratory to transfer or deliver a precisely measured volume of liquid

planchet

\plan'-chit\ small metal disk, with or without a raised edge, on which radioactive material is deposited for measurement (cf. planchette)

planchette

\plan-shet'\ small triangular or heart-shaped board supported on casters at two points and a vertical pencil at a third and believed to produce automatic writing when lightly touched by the fingers [Webster’s Ninth New Collegiate Dictionary] (cf. planchet)

plasma

gas-like state of matter, consisting of a fluid of ions and electrons, whose positive and negative charges roughly balance so that the overall charge of the material tends to be neutral

plateau

1. for certain types of radiation counter, a plot of the counter’s response versus the bias voltage, so named because the graph typically has a wide, nearly level region corresponding to the range of voltages at which the counter is most stable
2. the wide, level region in such a graph

Pm

atomic symbol for promethium

Po

atomic symbol for polonium

Poisson distribution

A random variable, X, has the Poisson distribution with parameter μ if for any non­negative integer, n, the prob­abil­ity Pr(X = n) is equal to μⁿe^−μ/n! (and for any other number x, Pr(X = x) = 0). In this case the mean and variance of X are both numerically equal to μ. The Poisson dis­tribu­tion is often used to model radiation counting measure­ments, some­times incorrectly.

population

entire collection of entities about which a statistical infer­ence is to be made

power (of a statistical hypothesis test)

probability of rejecting the null hypothesis in a situation where it is false, also equal to 1 minus the prob­abil­ity of a type II error

Note: The power of a test is some­times defined as the prob­abil­ity of reject­ing the null hypothesis in any situa­tion, regard­less of whether it is true or false. The latter def­ini­tion is more appeal­ing to this author as a math­ematician, but the former is much more common.

Pr

atomic symbol for praseodymium

precision

degree to which repeated measurements of a quantity under speci­fied con­di­tions tend to agree with each other (see also repeatability and reproducibility, cf. bias)

predictor

random variable whose value is used to predict the value of another random variable (whose value may be more diffi­cult to observe)

probability

real number between 0 and 1, inclusive, asso­ciated with an event

The probability of an event E may be denoted by Pr(E). The prob­abil­ity of an event rep­re­sents either the event’s rela­tive frequency of oc­cur­rence in repeated trials or one’s degree of belief that the event occurs in a single trial.

probability density function (pdf)

If X is a random variable, then a function, f, is a probability density function for X if the integral of f over any real interval J gives the prob­abil­ity that X is in J   [X ∈ J ]. The pdf, when it exists, is the derivative of the cumulative distribution function.

(probability) distribution

The probability distribution of a random variable, X, is a function that gives the probability that X takes any specified value or belongs to a specified set of values.

proficiency testing (PT)

“means of evaluating a laboratory’s performance under controlled conditions relative to a given set of criteria through analysis of unknown samples provided by an external source” [www.nelac-institute.org]

primordial radionuclide

long-lived radionuclide, such as ²³⁸U or ⁴⁰K, that has existed since the formation of the Earth

probability mass function

If X is a random variable, then a function, f, is a probability mass function for X if the value of f at any real number, x, equals the prob­abil­ity Pr(X = x).

progeny

all the decay products of a given radionuclide, considered collectively

propagation of uncertainty

operation of mathematically combining the standard uncertainties of input estimates in a mathematical model of measurement to obtain the combined standard uncertainty of the output estimate(s) (see also uncertainty-propagation formula)

proportional counter

See gas proportional counter

proportional region

range of voltages that when applied to the electrodes of an ionization chamber produce gas amplification sufficient to cause the amplitude of the pulse generated by each ionizing event to be directly proportional to the number of ion pairs initially produced by the event

When the applied voltage increases beyond the proportional region, it enters the region of limited proportionality, where the amplitude of each pulse increases nonlinearly with increasing numbers of ion pairs.

proton number (Z)

atomic number

Pt

atomic symbol for platinum

Pu

atomic symbol for plutonium

pure substance

type of homo­geneous material, all speci­mens of which have the same com­posi­tion and the same physi­cal and chemi­cal prop­erties

Note 1: The com­po­si­tion of a pure sub­stance can­not be altered by physi­cal separations.
Note 2: Pure sub­stances are clas­si­fied as either elements or compounds.