# The Helios OSL reader: a portable system for dating and dosimetry applications

Renata Majgier, Krzysztof Maternicki, Arkadiusz Mandowski, Piotr Moska, Magdalena Biernacka, Sebastian Kreutzer

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Renata Majgier, Krzysztof Maternicki, Arkadiusz Mandowski, Piotr Moska, Magdalena Biernacka, et al.. The Helios OSL reader: a portable system for dating and dosimetry applications. Geochronometria, 2025, 52, 10.20858/geochr/208873. hal-05204928

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# THE HELIOS OSL READER: A PORTABLE SYSTEM FOR DATING AND DOSIMETRY APPLICATIONS

RENATA MAJGIER<fnref n="1" />,<fnref n="2" />, KRZYSZTOF MATERNICKI<fnref n="2" />, ARKADIUSZ MANDOWSKI<fnref n="1" />,<fnref n="2" />, PIOTR MOSKA<fnref n="3" />, MAGDALENA BIERNACKA<fnref n="4" />,<fnref n="5" />
and SEBASTIAN KREUTZER<fnref n="4" />

Received 28 March 2025 Accepted 1 August 2025

## Abstract

Optical Stimulated Luminescence (OSL) screening has emerged as a significant advancement in the field of luminescence dating applications. The employed portable luminescence readers offer practical and efficient tools for on-site measurements. In contrast to traditional luminescence dating in the laboratory, which often involves analyzing enriched quartz or feldspar mineral separates, portable OSL readers typically measure infrared (IR) or blue post-IR OSL signals from unprocessed bulk material. Here, we present a new device series that can be used as a portable OSL reader for dating purposes and luminescence screening: the OSL Helios reader. The reader has already been used in luminescence laboratories as a bench-top device for dosimetry research for almost twenty years. It recently received significant upgrades for better versatility. Our contribution demonstrates the application of the OSL Helios reader for luminescence screening on a loess profile, where luminescence signal intensities were assessed for specific sedimentological layers. The profile was measured using different versions of the OSL Helios reader, and the results were compared to those of the SUERC reader, which is commonly used as a portable reader in applications. Additionally, standard passive dosimeters (Al2O3:C and BeO), as well as detectors considered for emergency dosimetry (NaCl), were measured to determine the sensitivity of Helios devices. We conclude that the Helios reader performs similarly to the SUERC reader in most standard situations and can be considered an additional option for portable luminescence reader application.

## Keywords

optically stimulated luminescence, portable OSL reader, luminescence dating, luminescence profiling

## 1. Introduction

According to the IPCC Sixth Assessment Report, further climate warming is expected to Optically stimulated luminescence (OSL, Huntley et al., 1985) is a dosimetric technique that determines the time that has passed since a sample was last exposed to light or heat. It quantifies the

release of energy in the form of light (luminescence) from dosimeters, such as natural minerals like quartz or feldspars. OSL measurements are typically performed with protocols that employ laboratory-based luminescence readers in a time- and resource-intensive procedure. As a result, many luminescence dating studies typically feature a relatively smaller number of ages, much less than would be desirable under ideal circumstances (Munyikwa et al., 2021). An alternative and supplementing approach to dating utilizes portable luminescence readers, e.g., which offer potential as an age profiling tool for sediments in a variety of depositional settings (Gray et al., 2018). The

<footnote n="1">¹Institute of Physics, Faculty of Science and Technology, Jan Długosz University, Al. Armii Krajowej 13/15, 42-200, Czestochowa, Poland</footnote>

<footnote n="2">²Zero-Rad Sp. z o.o., ul. Czartoryskiego 3/5, 42-200 Czestochowa, Poland</footnote>

<footnote n="3">³Institute of Physics, Division of Geochronology and Environmental Isotopes, Silesian University of Technology, Konarskiego 22B str., 44-100, Gliwice, Poland</footnote>

<footnote n="4">⁴Institute of Geography, Heidelberg University, Im Neuenheimer Feld 348, 69120 Heidelberg, Germany</footnote>

<footnote n="5">⁵Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Toruń, Poland</footnote>

<footnote>Corresponding author: R. Majgier
e-mail: renata.majgier@ujd.edu.pl</footnote>
portability of these devices enhances their usefulness, especially in field settings where preliminary analysis may be conducted immediately after sample collection. Consequently, in recent years, we have seen an increasing number of applications in dating and ionizing radiation dosimetry studies, as the following examples show. Portable OSL readers are used to measure luminescence signal intensities (e.g., Gray et al., 2019; Rizza et al., 2024), which allows contextualization of the sediment stratigraphy (e.g., Gray et al., 2022) and interpretation of sediment processes dynamics to shed light on the depositional histories (Sanderson and Murphy, 2010; Muñoz-Salinas et al., 2011). Additional examples of applications using portable luminescence readers include studies of coarse age assessments in various settings (e.g., Srivastava et al., 2023; Nitundil et al., 2023; Turner et al., 2021; Stone et al., 2019, 2024; Polar et al., 2019; Brill et al., 2016; Ben-Melech et al., 2024; Euzen et al., 2024).

Most studies seem to employ variants of the SUERC portable OSL reader (Sanderson and Murphy, 2010). However, the first portable OSL reader was introduced by Poolton et al. (1994) for in field OSL measurements. Although other devices have been presented as well (e.g. Kook et al., 2011), dating and screening application studies are limited. In a different dosimetric context, portable OSL readers are indeed interesting in case of radiological and nuclear emergency scenarios that require rapid action and on-site radiation dose assessment. Studies for this purpose were conducted with the SUERC (Karampiperi et al., 2024), Helios (Mrozik et al., 2017) or other, e.g. TL/OSL reader for on-site use in a large-scale radiological accident (Kim et al., 2024). In medical dosimetry (like patient dosimetry, quality control of radiation therapy), portable OSL devices are already commonly used, mainly devices such as the microStar (Landauer Inc, Glenwood, USA), which is used with carbon-doped aluminium oxide detectors (Perks et al., 2008) or myOSL (RadPro International GmbH, Remscheid, Germany) with a beryllium oxide detectors (Richter et al., 2018). Other mobile readers were developed for, e.g., thermoluminescence (Ikeya et al., 1990) and electron-spin resonance (ESR) applications (Ikeya and Furusawa, 1989).

In general, portable readers offer several benefits, including easy transport to field sites, the ability to take measurements concurrently with sample collection, and simplicity in the measurement process. They can readily be applied outside the laboratory directly to bulk, unprocessed sediment, which significantly shortens the analytical process compared to traditional OSL dating. Samples are usually collected under a light-tight cover to prevent exposure to sunlight, typically by quickly inserting special opaque containers or tubes directly into the sediment or rock. Such samples are then either transported to the laboratory or placed directly into a portable reader and measured on-site under low-intensity amber or red light (e.g., using a headlamp). Additionally, most common simple OSL devices allow for the analysis of larger sample

quantities at a much faster rate and at a lower cost by using simple and quick protocols. However, the drawback for the portability is that those devices come without an ionizing radiation source and cannot perform routine dating protocols, such as the single aliquot regenerated (SAR, Murray and Wintle, 2000) dose protocol for OSL on quartz. Consequently, portable devices cannot readily be employed to generate chronometric ages in accordance with established protocols and are typically regarded as supplementary tools to conventional laboratory readers for preliminary analysis of sediments (Munyikwa et al., 2021). Nevertheless, these devices have substantial potential, and further advancements in the development of portable luminescent devices are worth pursuing.

The aim of this work is to present the new portable OSL Helios reader for coarse dating applications and to compare its performance with the SUERC reader used in field measurements. The SUERC reader was selected for this comparison because it is widely used in the trapped-charge-dating community and hence defines our benchmark. The OSL Helios reader has already been successfully employed in dosimetry research (e.g. Marczewska et al., 2016; Mrozik et al., 2017; Marczewska et al., 2019; Mrozik and Bilski, 2021). In this paper, we detail its usefulness in coarse screening procedures by measuring the luminescence response of a loess profile. Additionally, we report on the sensitivity of the device for standard dosimeters used in personal ($Al_2O_3$; C, BeO) and retrospective (NaCl) dosimetry.

# 2. Materials and methods

## 2.1. Measurement equipment

Two types of portable OSL readers were used in our study: (1) Helios (Zero-Rad, 2025, Mandowski et al., 2010), in three variants, and (2) a SUERC portable OSL reader (Sanderson and Murphy, 2010); short: SUERC reader (see Table 1 for an overview). A photo of the SUERC and Helios readers is shown in Fig. 1a. Additionally, a schematic diagram of the Helios reader is included in the supplement, Fig. S1.

The Helios reader family comprises several types of devices that can be adapted upon order for specific research or luminescent materials. The basic version of the reader (Helios-Basic) is equipped with one stimulation source and a sensitive detector operated in photon counting mode. It can perform basic OSL measurements, e.g. CW-OSL (continuous wave optically stimulated luminescence). The advanced version (Helios-Ex) facilitates one or two stimulation sources, extended detection also in the UV range (quartz windows) and various stimulation and detection modes. The measurements in this paper were performed using devices in the following versions: two Helios-Ex versions equipped with one stimulation source, green (Helios GR) and infrared (Helios IR), respectively, and one Helios-Ex version with two stimulation sources – blue and infrared (Helios BL/IR). The detailed stimulation
<table><caption>**Table 1.** Overview equipment used for our experiments; further details see main text. A diagram of the stimulation and detection ranges of the devices is provided in Supplementary Fig. S2.</caption><thead><tr><th rowspan="2">TYPE</th><th rowspan="2">SUERC*</th><th colspan="3">Helios family</th></tr><tr><th>Helios-Ex<br>(Helios BL/IR)</th><th>Helios-Ex<br>(Helios GR)</th><th>Helios-Ex<br>(Helios IR)</th></tr></thead><tbody><tr><td rowspan="2">Stimulation LED</td><td>470 nm</td><td>470 nm (± 20) nm</td><td>-</td><td>-</td></tr><tr><td>-</td><td>-</td><td>525 (± 30 nm)</td><td>-</td></tr><tr><td></td><td>880 nm</td><td>850 (± 20) nm</td><td>-</td><td>850 (± 20) nm</td></tr><tr><td>Stimulation filters</td><td>Schott GG420<br>(2 mm) /<br>RG830 (2 mm)</td><td>Schott GG445<br>(2 × 3 mm) /<br>RG715 (3 mm)</td><td>Schott GG495 (3 mm)<br>+ OG515 (3 mm)</td><td>Schott RG715 (3 mm)</td></tr><tr><td>Detection<br>filters</td><td>Schott UG11<br>(12 mm)</td><td>Schott UG11-IRB (2 mm)<br>+ UG11 (3 mm)<br>+ UG1 (3 mm)</td><td>Schott UG11-IRB (2 mm)<br>+ UG11 (3 × 3 mm)</td><td>Schott BG39 (3 mm)</td></tr><tr><td>Detector</td><td>ETL PMT **</td><td>Hamamatsu H11870-02-Y005<br>with a quartz window</td><td>Hamamatsu H7360-02<br>with a quartz window</td><td>Hamamatsu H7360-02<br>with a quartz window</td></tr></tbody></table>

and detection configuration for the different versions used is described below. Each Helios reader includes control electronics, a data interface, software for measurement control and data analysis, and an additional photosensor (Si photodiode S2386-5K, Hamamatsu, Japan) that controls the LED ring power density and ensures homogeneous illumination. The Helios reader is operated with the *Helios* software for Windows operating systems (see screenshot, Fig. 1b). Results are exported as ASCII files (*.osl) with a comma-separated value column structure. Data can be analysed in any standard software capable of importing comma-separated value files. Additionally, file import and analysis are supported in the R (R Core Team, 2025) package ‘Luminescence’ (Kreutzer *et al.*, 2024) through the functions *read_HeliosOSL2R()* and *analyse_portableOSL()*. Currently, Helios does not support exporting measurement data to the XLUM format (Kreutzer *et al.*, 2023) format. However, Helios .osl files can be converted easily to .xlum files using the R package ‘xlum’ (Kreutzer and Burow, 2022).

The green stimulation reader (Helios GR) consists of an illumination module containing 15 LEDs (peak wavelength at 525 nm, FWHM: 30 nm) with optical lenses and long pass filters, consisting of Schott filters GG495 and OG515, which are used to cut off the shortwave light component below 500 nm (Table 1). Luminescence detection is performed with an H7360-02 photomultiplier with a quartz window (Hamamatsu, Japan) through a Schott UG11-IRB and UG-11 filters (transmission: 240–400 nm). Samples are placed in aluminium holders with diameters ranging from 7 mm to 16 mm and a height of 4 mm (applies to all tested versions of the Helios reader).

The Helios-Ex reader with infrared stimulation (Helios IR) contains a ring with 15 LEDs (peak at 850 nm, FWHM 20 nm) and RG715 filters to block short-wave light components below 700 nm. Luminescence is detected using an

H7360-02 photomultiplier with a quartz window (Hamamatsu, Japan) and equipped with a BG39 filter, covering the wavelength range of 300–600 nm.

The reader equipped with two types of stimulation sources (Helios BL/IR) uses six blue LEDs (peak at 470 nm, FWHM 20 nm) with cut-off filters GG455 and nine infrared LEDs (peak at 850 nm) with RG715 filters. Both OSL and IRSL are detected in the UV range (280–400 nm) using the H11870-02-Y005 photomultiplier (Hamamatsu, Japan) with a quartz window and optical filters: (1) a UV filter/NIR blocker (UQG Optics, England) based on Schott UG11-IRB with a dielectric coating and UG11 (transmission: 240–400 nm), and (2) an additional UG1 filter (transmission: 280–420 nm).

The SUERC reader provides pulsed and CW mode stimulation using infrared (880 nm) and blue (470 nm) LEDs. The system most widely distributed contains blue and IR LED diodes of 25 mW and 90 mW, respectively (Alghamdi *et al.*, 2024). The infrared stimulation light is filtered with a Schott RG830 and the blue LEDs with Schott GG435 filters. Luminescence is detected through UG11 filters by an ETL photodetector module with a 25 mm diameter bi-alkali photomultiplier, integral HV generator, amplifier–discriminator, and RS232–output photon counter (Sanderson and Murphy, 2010). Data analysis of the SUERC reader is likewise supported by the R package ‘Luminescence’ (Kreutzer *et al.*, 2024) and the functions *read_PSL2R()* and *analyse_portableOSL()*. The dimensions of the sample holder are as follows: lower diameter 50 mm, upper diameter 60 mm, height 19.5 mm. To measure the loess samples in our experiment, an additional 3D printed attachment was used in which a smaller aluminium holder from Helios was placed to ensure the same sample distribution in comparative measurements. A photo of the readers with sample holders is included in Fig. S3.

<footnote>* Information from Sanderson and Murphy (2010) and available upon delivery of the particular system in Heidelberg.</footnote>

<footnote>** SUERC PPSL 2 (Sanderson and Murphy, 2010) readers use photo detector modules incorporating selected 9124B tubes with 1" photocathodes and built in HV and amplifier-discriminator systems (Carter et al., 2018).</footnote>
a)

<figure><img src="image_3.png" /></figure>

b)

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (a) Photo of SUERC (left) and Helios (right) readers for dimension comparison only; (b) Screenshot of Helios control software (v24.1). Helios's dimensions: length: 245 mm, width: 110 mm, height: 260 mm, weight: 4.8 kg.</figcaption></figure>

The stimulation intensity in the OSL Helios reader can be adjusted. The following current settings were used in our measurements: Helios BL/IR blue stimulation setting: 90 mA with a power density (irradiance at the sample position) of 1.5 mWcm⁻², infrared stimulation at 200 mA with a power density of 7.5 mWcm⁻²; Helios GR green stimulation at 80 mA with an intensity of 1.2 mWcm⁻²; and Helios IR infrared stimulation at 100 mA with an intensity of 2.0 mWcm⁻². The SUERC reader (Sanderson and Murphy, 2010) is operated with fixed values for the LED power of 25 mW and 90 mW for blue and infrared stimulation, respectively.

Measurements on all readers were performed in CW-OSL mode for 30 s for samples from the loess profile (Chap. 3.1) or 60 s (OSL detectors in Chap. 3.2) with sampling every second. For loess samples the OSL was first measured with infrared stimulation for 30s (IRSL), followed by 30 s blue-OSL or green-OSL as post-IR blue-OSL or post-IR green-OSL, respectively. It is assumed that 30 s of stimulation are sufficient for our experiments; however, in dating practices using a full protocol to determine an equivalent dose, significantly longer stimulation times are typically required to effectively erase the infrared (IR) signal from loess samples (Buylaert *et al.*, 2011). For the loess samples, the background, measured as the signal without the sample (30 s), was proportionally subtracted from IRSL or post-IRSL result. In the case of OSL detectors (Al₂O₃:C, BeO, NaCl), the background was determined as the detected light during 60 s of stimulation before any irradiation and subtracted from the measured signal after irradiation.

## 2.2. Loess samples

We selected a loess profile as a common application for portable readers in a geoscientific context. In contrast to sampling strategies with a high spatial resolution that would fully utilise the potential of portable luminescence readers, we used only stratigraphic positions already sampled for routine luminescence dating, which we deem sufficient for our comparison. The material investigated for our comparison was collected in 2023 from a newly prepared and documented loess profile at the Trzebnica brick-yards site (Fig. 2) (51°18'44" N, 17°4'14" E, 193 m a.s.l.). The site has been known since the early 20th century and is today located in the northeastern part of Trzebnica (Poland) (Winnicki, 1997). The area is part of the Trzebnica Hills, which extend in a latitudinal range of approximately 50 km in length and 5–10 km in width.

The basic lithology of the studied profile can be presented as follows: the uppermost 50 cm consists of an A horizon of brown soil, followed by horizon B extending to approximately 110 cm, characterized by numerous vertical root channels with dark colouration followed by four meters of laminated loess. Between 5.4 m and 6 m, we observed transformed loess with an accumulation of carbonate concretions. From 6 m to 7.6 m, laminated loess continues, while at the base of the profile, a deflation pavement and a clay layer are present. Samples were collected using thin-walled steel pipes from Eijkkelkamp system for sampling (Moska *et al.*, 2021). The collected loess samples were stored in the dark until measurement. Samples each weighing approximately 30 ± 3 mg were placed into the holder of the SUERC or Helios reader under a low red ambient light using a measuring cup. The samples were neither chemically nor physically processed except for gentle crushing to destroy aggregates and to achieve an even distribution. For each sampling position, the luminescence signals were always measured on three sub-samples, and
<figure><img src="image_5.png" /><figcaption>Fig. 2. Profile sketch and photo of the loess profile at Trzebnica brickyards with the sample collection points and basic lithology.</figcaption></figure>
the resulting signal is the average of these three measure-
ments. Measurement uncertainties shown in the figures
were determined as the standard deviation of the mean.

**2.3. OSL dosimeters**

The intensity of the OSL signal, and thus the detection
limit of the reader-detector system, depends on the stimu-
lations used, the detection range, and the spectral proper-
ties of the detectors. To assess the sensitivity of the readers,
passive dosimeters typically used in OSL dosimetry (for a
primer, see Yukihara *et al.*, 2022) were employed: Al₂O₃:C
(Landauer Inc., USA) with a diameter of 5 mm and a thick-
ness of 1 mm and BeO (Thermalox995, Materion, USA)
with dimensions 5×5×0.5 mm, as well as table salt pellets
similar to those considered as detectors in retrospective
and emergency dosimetry (Alghamdi *et al.*, 2022;
Karampiperi *et al.*, 2024). For Al₂O₃:C ultraviolet emis-
sions at 335 nm and 420 nm are reported (Yukihara and
McKeever, 2006), as well as a dominant OSL emission
band at ~415 nm (Denis *et al.*, 2011). The OSL emission
spectrum of BeO for stimulation with 550 nm light ranges
from 250 nm to 450 nm with two OSL emission bands at
~310 nm and ~370 nm (Yukihara, 2011). For NaCl, we
could not identify OSL emission studies, beyond reference
to several TL emission peaks, notably at ~590 nm and
~370 nm, additionally blue-OSL (stimulation: 470 nm; de-
tection: UV-range) with higher sensitivity than IRSL
(880 nm stim./orange det.) (Spooner *et al.*, 2011) and con-
firmation of OSL signal response.

Measurements using Al₂O₃:C and BeO were performed
on single samples, which were each thermally reset by an-
nealing at 500°C for 2 min, which was experimentally de-
termined as sufficient to entirely deplete the detector signal,
by placing them in the heated oven (FCF 2,5SHM, Czylok,
Poland) and quickly removing after the set time. The tem-
perature and duration of thermal resetting were determined
experimentally and assumed sufficient for the measure-
ments presented. NaCl chips of 4 mm diameter, 0.6 mm
thickness and weight of 17 ± 1 mg were obtained by hard
pressing table salt (FalksaltfintBergsalt, Sweden) with
grain sizes of 250–400 µm under a pressure of 2 t. A series
of NaCl chips were used for the measurements, i.e., each
measurement was made using a new NaCl sample. The de-
tectors were irradiated with a ⁹⁰Sr/⁹⁰Y beta source, deliver-
ing a manufacturer specified dose rate of 7.6·10⁻⁴ Gy s⁻¹.

For each administered dose, a given detector was meas-
ured once immediately after irradiation (fixed time of one
minute measured with a stopwatch). For OSL measure-
ments using the SUERC reader, samples were placed each
time on the same marked location in the holder and the
drawer was closed carefully to avoid movements to ensure
the best possible repeatability. In the Helios reader, a 6 mm
diameter holder guaranteed a repeatable sample position
every time.

The minimum detectable dose (MDD) was calculated
as the ratio of the background signal level (three standard
deviations of the background signal) to the luminescence

signal per unit radiation dose following commonly used
procedure (e.g., Bernhardsson et al., 2009, Rawat et al.,
2014). The signals from unirradiated samples were used as
background, i.e. OSL readout of 60 s was performed on a
sample that was not previously irradiated (or was zeroed).
The mean background value and standard deviation were
determined from ten measurements. The background was
determined separately for each reader-detector configura-
tion. Additional measurements were also performed with-
out any sample in the holder. These tests confirmed that
the background level was the same both with the unirradi-
ated sample and without any sample. The linear fitting pa-
rameters were used to determine the sample sensitivity
(mGy⁻¹) required to find the detection limit according to
the following procedure. The signal-to-dose coefficient
was determined as the slope of the line fitted to the dose-
response curve over the entire range of tested doses, along
with its uncertainty obtained from the fitting parameters.

All experimental parameters, including irradiation con-
ditions, waiting time, sample transfer under red light, and
the method of OSL signal readout, were consistently main-
tained throughout the study to ensure the highest possible
reproducibility of the results. However, it is important to
acknowledge that the waiting period between irradiation
and measurement was limited to approximately one mi-
nute and timed using a stopwatch. Although this approach
allowed for controlled timing, measuring the luminescence
signal shortly after irradiation may result in greater ob-
served signal instability, as compared to measurements
performed after a longer post-irradiation storage period
that allows transient components to decay and the signal to
stabilize.

**3. Results**

**3.1. Luminescence profile of loess samples**

The measurement results of samples from the loess profile
illustrate the dependence of the OSL signal on profile
depth (Fig. 3). The data in Fig. 3 were normalized to the
range from minimum (0) to maximum (1) to better illus-
trate the *signal pattern*. The absolute count values are
shown in Supplementary Fig. S4. Fig. 3a shows a compar-
ison of IRSL signals for the three readers used. Fig. 3b
shows the post-IR blue-OSL results obtained with the SU-
ERC and Helios BL/IR reader, and Fig. 3c additionally
shows the post-IR signal measured for green-OSL with the
Helios GR reader. For all presented signals, the overall sig-
nal pattern for the profile is similar for all readers, alt-
though the pattern obtained with SUERC is more pro-
nounced, showing clearer peaks and stronger contrast.
This likely related to the pulsed-stimulation of the SUREC
with its background subtraction in the dead-times.

For infrared stimulation, the order of magnitude for
photon counts (Fig. S4a) is similar for all readers (10⁴–10⁵
counts). For post-IR blue-OSL, photon counts are of the
order of 10⁵ counts for both Helios readers, with the SU-
ERC reader producing a significantly higher signal,
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Luminescence profile measured using SUERC and Helios readers: a) IRSL, b) post-IR blue-OSL, c) post-IR green-OSL. Luminescence intensities (normalized to the range from minimum zero to maximum one) versus profile depth are shown and refer to the measurement parameters described in the text.</figcaption></figure>

approximately twice as large. While Fig. 3b shows the general profile pattern, a direct comparison of the blue-OSL signals for Helios BL/IR and SUERC can be found in Fig. S4b. Fewer counts than for post-IR blue-OSL were obtained for post-IR green-OSL in the Helios GR reader (Fig. S4c), of the order of $10^4$ counts, which can be attributed to the generally lower bleaching efficiency of green light compared to blue (for quartz, e.g. Bøtter-Jensen et al., 1999; Singarayer and Bailey, 2003; Chruścińska and Przegiętka, 2005).

The repeatability of the measurement can be characterized by the relative standard deviation ($c_v$), which was determined for three natural sub-samples each (at each sampling position, luminescence was measured on three sub-samples). For the SUERC reader, the total number of averaged results are 30 (15 with blue stimulation and 15 with infrared stimulation), while for the Helios readers, it is 60 (15 with blue, 15 with green, and 30 with infrared stimulation). For all measurements, the relative standard deviation was always less than 25%. For the Helios readers, 42 out of 60 results (70%) had a $c_v$ of less than 10%, while for

the SUERC reader, 13 out of 30 (43%) had a $c_v$ of less than 10%. The values of the relative standard deviations are given in Table S1.

## 3.2. Reader sensitivity

Because the setup of the readers differs slightly, we compared the detection sensitivities of the Helios and the SUERC reader using standard passive dosimeters after irradiation with doses of different orders of magnitude: 10 mGy, 100 mGy and 1000 mGy. The obtained dose responses for Al₂O₃:C, BeO, and NaCl for blue and green stimulation are shown in Fig. 4, while the IRSL response is shown for NaCl only in Fig. 5.

The order of magnitude of the total signal from the blue stimulation is similar for SUERC and Helios BL/IR readers (Fig. 4). A higher signal was obtained using the SUERC reader for Al₂O₃:C and BeO, while for NaCl, the signal was the highest with the Helios BL/IR reader. This difference is likely due to the combined effects of the emission characteristics of each material and the optical detection components of the specific readers. In Helios readers,
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 4.** Dose dependency of the total OSL signal for a) $Al_2O_3:C$, b) BeO, and c) NaCl under blue (SUERC, Helios BL/IB) and green (Helios GR) stimulation. The continuous line indicates a fit of a straight-line function through the origin to the measurement points.</figcaption></figure>

the signal from green stimulation was lower than that from blue stimulation for all samples.

For infrared stimulation (Fig. 5), the highest number of counts was obtained using the Helios IR reader, and the lowest was for SUERC. Helios BL/IR and SUERC showed a similar order of magnitude of the signal, with Helios BL/IR providing more counts. This is due to the fact that the detection range of SUERC and Helios BL/IR is similar (about 280–400 nm), while for Helios IR, it is much wider

<figure><img src="image_10.png" /><figcaption>**Fig. 5.** Dose dependency of the total IRSL signal for NaCl measured with SUERC, Helios BL/IB and Helios IR. The solid line indicates a fit of a straight line through the origin to the measurement points.</figcaption></figure>

(300–600 nm) and thus covers more emissions from the OSL detectors. The high signal with infrared stimulation was obtained for NaCl, which is related to the spectroscopic properties of the material and the range of stimulation to which the material is sensitive (Spooner et al., 2011, Rodriguez-Lazcano et al., 2012, Hunter et al., 2012). In the case of $Al_2O_3:C$ and BeO, infrared stimulation is not effective, which is in accordance with the characteristics of the dosimeters (Bulur et al., 1998, Umisedo et al., 2010, Yukihara et al., 2016). Therefore, the IRSL results for these materials are not shown in this study.

For all tested detector types, a linear response was observed within the dose range of 10–1000 mGy under blue and green stimulation (Fig. 4). In the case of infrared stimulation, linearity was confirmed only for NaCl (Fig. 5). A straight line with a high $R^2$ value was fitted to the measurement points, representing the average counts per dose in the tested range and serving as a basis for determining the minimum detectable dose (MDD). Figs. 4 and 5 include linear fits for all tested detectors under blue and green stimulation, as well as for NaCl under infrared stimulation.

Since only one measurement was performed for each detector at each dose level, individual measurement uncertainties (of significance) were not directly available. Therefore, an indirect method was used to estimate uncertainties based on the residuals from a linear fit through the origin, assuming a proportional relationship between the uncertainty and dose. The calculated uncertainties for each detector type are shown in Figs. 4 and 5. For the $Al_2O_3:C$ and BeO detectors, uncertainties do not exceed 5%. Similarly, most NaCl measurements also exhibit uncertainties below 5%, except for 3 out of 18 measurements with the Helios readers, where higher uncertainties were observed.

### 3.3. Quantifying the minimum detectable dose (MDD)

The obtained MDD values are presented in Table 2. The orders of magnitude of MDD are similar for the SUERC
<table><caption>Table 2. Minimum detectable dose (MDD) values were determined for the tested readers using Al₂O₃:C, BeO and NaCl detectors.</caption><thead><tr><th rowspan="2">READER</th><th rowspan="2">STIMULATION</th><th colspan="3">MDD per DOSIMETER (µGy)</th></tr><tr><th>Al₂O₃:C</th><th>BeO</th><th>NaCl</th></tr></thead><tbody><tr><td>SUERC</td><td>blue</td><td>21.2 ± 0.2</td><td>47.1 ± 0.1</td><td>1390 ± 10</td></tr><tr><td>Helios BL/IR</td><td>blue</td><td>88.2 ± 0.1</td><td>265.0 ± 5.0</td><td>950 ± 20</td></tr><tr><td>Helios GR</td><td>green</td><td>20.4 ± 0.2</td><td>87.1 ± 0.3</td><td>2100 ± 110</td></tr><tr><td>SUERC</td><td>infrared</td><td></td><td></td><td>560 ± 10</td></tr><tr><td>Helios BL/IR</td><td>infrared</td><td></td><td></td><td>560 ± 10</td></tr><tr><td>Helios IR</td><td>infrared</td><td></td><td></td><td>510 ± 10</td></tr><tr><td>Literature data</td><td>blue</td><td>4-57<sup>*1</sup></td><td>10<sup>*4</sup></td><td>2-47<sup>5,6</sup>(14600<sup>*5</sup>)</td></tr><tr><td>Literature data</td><td>green</td><td>10<sup>*2</sup></td><td>nd.</td><td>nd.</td></tr><tr><td>Literature data</td><td>infrared</td><td>100<sup>*3</sup></td><td>nd.</td><td>340<sup>*6</sup></td></tr></tbody></table>

reader and the Helios readers. For Al₂O₃:C, blue/green stimulation yielded a MDD of several tens of µGy (~10⁻⁵ Gy), for BeO, several tenths or hundredths of µGy (~10⁻⁵ Gy – 10⁻⁴ Gy), and for NaCl, about 1–2 mGy (~10⁻³ Gy). In the case of infrared stimulation of NaCl, all readers yielded comparable results, with MDD around 0.5 mGy.

## 4. Discussion

In the study of loess profiles, the lower number of counts with blue stimulation in the Helios BL/IR reader compared to the SUERC reader likely is related to differences in the reader settings, such as the intensity of the stimulating light and slightly different detection efficiencies (filter configuration). It should be noted that the blue stimulation power in the tested Helios BL/IR reader is relatively low (~1.5 mWcm⁻²) and probably lower than in SUERC (the power density for the SUERC reader is not specified). However, the Helios reader settings can be optimized at the production stage for a given application by adjusting the number and type of LEDs (and thus the stimulation intensity), as well as the detection range (PMT and optical filters).

In Helios readers, green stimulation typically yields fewer counts than blue due to its generally lower bleaching efficiency, as also observed in our measurements of loess samples composed of feldspar and quartz. The IRSL measurements are dominated by a feldspar signal, while the post-IR blue-OSL targets the remaining quartz signal (Thomsen et al., 2008). Although the green stimulated quartz signal measured in the UV has a lower photo ionisation cross-section (Singarayer and Bailey, 2003) and is hence less efficient at stimulating quartz, it is still possible to obtain a high signal with a consistent profile (i.e. the luminescence signal maintains a stable and reproducible pattern under consistent experimental conditions).

The OSL emission spectrum of quartz and feldspars – the main components of loess samples – consists of several characteristic bands. For quartz, the dominant emission occurs at wavelengths of about 365 nm or 380 nm (e.g.,

Huntley et al., 1991, Lomax et al., 2015), while for feldspars these are bands in the range of 280–290 nm, 320–340 nm, 390–440 nm and 550–570 nm, with most sedimentary feldspars showing dominant emission in the range of 400–410 nm (e.g., Baril and Huntley, 2003, Lomax et al., 2015). This emission largely overlaps with the detection window used in the SUERC and Helios BL/IR systems (approx. 280–400 nm), and with the wider detection range of the Helios IR system (300–600 nm). Despite the wider detection range of Helios IR, no higher signal is observed for infrared stimulation compared to other readers, which is due to the lack of material emission in the range corresponding to the maximum sensitivity of the photodetector in Helios IR (i.e. above 400–500 nm).

The results for relative standard deviation of samples from loess profiles show that the measurement repeatability for both readers is comparable and considered sufficient, given the application. The obtained values of standard deviation are typical for measurements of natural samples (e.g. Thomsen et al., 2005, Guérin et al., 2015, Kinnaird et al., 2015), reflecting both experimental uncertainty and inherent variability related to material composition, geological origin, and grain heterogeneities.

The sensitivity of SUERC and Helios readers for standard OSL dosemeters was investigated, and the obtained values of MDD can be roughly compared with literature data. These data report a minimum detection limit of 10 µGy for Al₂O₃:C (Kreutzer et al., 2018) using the lexyg SMART reader (Richter et al., 2015) with green stimulation at measurement temperature of 70°C and the detection centred at 410 nm. Other MDD values ranging from 4 µGy to 57 µGy were obtained for Al₂O₃:C under blue stimulation depending on the stimulation intensity and signal integration time (Rawat et al., 2014). Additionally, doses as low as 100 µGy have been detected for Al₂O₃:C with infrared stimulation using the Risø TLDA 10 (DTU National Laboratory) reader (Bulur et al., 1998). The magnitude of the obtained results is comparable to those presented here for Al₂O₃:C, which we consider satisfactory in the absence of a dedicated study that would replicate all instrumental settings as closely as feasible.

<footnote>*¹ Rawat et al. (2014), *² Kreutzer et al. (2018), *³ Bulur et al. (1998), *⁴ Yukihara et al. (2016), *⁵ Karampiperi et al. (2024), *⁶ Alghamdi et al. (2022)</footnote>
For BeO, literature data report an MDD of 10 µGy for blue stimulation (Yukihara et al., 2016), while we obtained consistently larger values for all used equipment ranging from ca 47 µGy (SUERC) to 265 µGy (Helios BL/IR) for blue stimulation. The reasons are unknown and would require a dedicated study, which is beyond the scope of this manuscript.

Measurements of NaCl chips using the SUERC reader gave MDD in the range of 2–47 µGy (Karampiperi et al., 2024); however, it should be noted that readouts were taken for five chips simultaneously because, for less than five chips, the signal was very low and not properly distinguishable for the low dose (14.6 mGy). Likewise, Alghamdi et al. (2022) obtained MDD values on NaCl of 7 µGy for blue stimulation and 340 µGy for IRSL for a comparable system but for a sachet of 1 g of NaCl. On the contrary, our measurements using SUERC and Helios readers were performed on one identical chip and gave a sensitivity of about 1 mGy for blue stimulation and 0.5 mGy for infrared stimulation. The resulting sensitivity of the Helios reader should be sufficient for dosimetry and dating applications.

## 5. Conclusions

We have shown that the portable OSL Helios reader can be used for luminescence profile screening, similar to other devices of this type, such as the SUERC reader, which we used for a benchmark comparison here.

For a more particular application that is relevant to archeology and geoscience, we sampled a loess profile and measured the corresponding signal from infrared stimulation followed by blue or green stimulation (post-IR blue-OSL and post-IR green-OSL, respectively). The pattern of the signal response through the profile obtained with the Helios reader are like those of the SUERC reader typically used for this type of application, although absolute intensities sometimes vary. The measurement repeatability of the Helios reader has a relative standard deviation ($c_v$) of <=25% for measurements using three samples of natural material. Additionally, most of the measurement points in loess profiles measured using Helios (70%) have a $c_v$ of less than 10%. Similar repeatability is observed for the SUERC reader, with a maximum $c_v$ of 23% and the majority of points (60%) having $c_v$ above 10%.

The Helios reader can also be used to measure passive detectors for ionizing radiation dosimetry. Based on the MDD results for standard OSL detectors, we conclude that the sensitivity of the Helios reader is high (e.g., for $Al_2O_3:C$ with blue light stimulation, the MDD is 0.09 mGy) and comparable to other portable OSL devices such as the SUERC reader (0.02 mGy for $Al_2O_3:C$ with blue stimulation).

The advantage of the Helios family readers is the wide selection of stimulation sources, adjustable stimulation intensity, and a compact/modular design that allows for easy device operation in non-laboratory conditions. The modular design of the family of Helios readers allows for the construction of a “tailor-made” device that fits the user’s needs regarding both the stimulation and detection module. OSL Helios readers are continually being developed to introduce further improvements and capabilities, such as battery power or heating options, to make the devices even more useful for dating and dosimetry research.

## Acknowledgements

The authors would like to express their sincere gratitude to Professor David Sanderson for his valuable consultations regarding properly setting measurement parameters of the SUERC reader.

## Supplementary material

Supplementary material containing additional **Figures S1–S4** and **Table S1** is available online at https://doi.org/10.20858/geochr/208873.

## Data availability

All data was made open access on Zenodo and is digitally identified by DOI 10.5281/zenodo.15095948

**Funding:** MB was financed by project #101107989 - Luminescence - HORIZON-MSCA-2022-PF-01. SK was financed through the DFG Heisenberg programme (#505822867). The research was partially carried out with the support of the Polish National Science Centre, contract number 2021/41/N/ST10/00169. This work was partly supported (RM and AM) by research project no. 2018/31/B/ST10/03966 from the Polish National Science Centre.

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