  The $\alpha$-particle light response of liquid scintillators based on linear
alkylbenzene (LAB) has been measured with three different experimental
approaches. In the first approach, $\alpha$-particles were produced in the
scintillator via $^{12}$C($n$,$\alpha$)$^9$Be reactions. In the second
approach, the scintillator was loaded with 2% of $^{\mathrm{nat}}$Sm providing
an $\alpha$-emitter, $^{147}$Sm, as an internal source. In the third approach,
a scintillator flask was deployed into the water-filled SNO+ detector and the
radioactive contaminants $^{222}$Rn, $^{218}$Po and $^{214}$Po provided the
$\alpha$-particle signal. The behavior of the observed $\alpha$-particle light
outputs are in agreement with each case successfully described by Birks' law.
The resulting Birks parameter $kB$ ranges from $(0.0066\pm0.0016)$ cm/MeV to
$(0.0076\pm0.0003)$ cm/MeV. In the first approach, the $\alpha$-particle light
response was measured simultaneously with the light response of recoil protons
produced via neutron-proton elastic scattering. This enabled a first time a
direct comparison of $kB$ describing the proton and the $\alpha$-particle
response of LAB based scintillator. The observed $kB$ values describing the two
light response functions deviate by more than $5\sigma$. The presented results
are valuable for all current and future detectors, using LAB based scintillator
as target, since they depend on an accurate knowledge of the scintillator
response to different particles.
